The concept of impedance is very important in antennas and transmission lines. Remember that impedance is the ratio of voltage to current. Let's look at what happens when we apply a radio frequency voltage to one end. When the voltage is first applied, a certain amount of current will flow. What ever the ratio voltage to current turns out to be is called the characteristic impedance of the transmission line. If we apply 10 volts and find that initially .2 amps flows, then 10/. 2 = 50. The impedance of the line is 50 ohms. If the line was 186,000 miles long we would be able to measure this .2 amp current for about two seconds. After however long it takes the applied voltage and current to reach the end and be reflected this current may change. If the transmission line is connected to an impedance equal to the characteristic impedance, then there will be no reflection and the current remains at its initial value. You will also hear the term surge impedance used interchangeably with characteristic Impedance. Usually you will hear it shortened to just impedance. Most of us will be using coax for transmission line and it will probably have an impedance of 50 ohms or somewhere close to that. Coax is also frequently found with 75-ohm impedance. In order to get as much of your power as possible into the antenna so it can be radiated, you need to have the antenna impedance as close to the transmission line's impedance as possible. If the antennas impedance does not match that if the line, you can use a simple matching network. The main idea is to present a load to the transmission line so there is a minimum mismatch and minimum reflected voltage and current where the line connects to the antenna.
If there is a reflection it causes standing waves to be set up on the transmission line. Usually I like to see the standing wave ratio less than 2:1.
The higher you go in frequency, the more important it is to have a low SWR. We will discuss SWR in more detail at a later date.
Saturday, February 27, 2010
Saturday, February 13, 2010
Two halves does not always make a whole
Antennas do not have to be resonant to work well, but it does make feeding them a little less complicated in general. When learning the basics it may make it easier to study resonant antennas first. It is my intention to stick with resonant antennas for a while longer. There may be exceptions as there is with anything.
The shortest resonant antenna is a half wave long. Remember a half wave in free space is determined by the formula 492/frequency in MHz. When confined to a wire, the speed changes and there are other effects that appear to change the speed. The sum total of all these effects is to reduce the length by about 5%. So you change the formula to 468 if you are using most kinds of wire. That means that a half wave of wire is determined by dividing 468 by the frequency in MHz. So for example on 1.850 MHZ (160 meter band) the length of a half wave of wire is 468/1.85 = 252.9 feet or about 253 feet. The formula for a half wave in a wire is not exact. Although the length depends primarily on the speed of the radio signal in the wire, the type of insulation and its thickness makes a big difference. There are also effects caused by the wire being close to the ground or any other conductor, even wood or tree branches. There is an effect due to the insulator at the end of the wire. Some of these effects, especially due to the insulation, are more pronounced the higher the frequency. This formula is also not good if the antenna is more than one half wave long. The formula was reduced from 492 to 468 for a one half wavelength wire that has two ends connected to an insulator. If the antenna is say, one wavelength long, there are still only two ends, but there is effectively a half wavelength of wire in the middle that has no open ends. That has an effect on the calculation. It turns out that a large part of the reason that a half wave wire antenna is physically shorter than a half wave in free space is due to the insulators at the two ends or the “end effects”. The way I see it, the ends will either have an insulator or be hanging loose. In either case there is an end effect of one sort or another. If you connect two half wave lengths of wire together, you will find that together they are shorter than they should be for full wave resonance. The calculation of each half wave was based on each length having two ends. Remember, a full wavelength of wire will be slightly longer than two half-wave lengths added together. This is not a big deal, but something to keep in mind if you are constructing a wire antenna longer than a half wave long.
The shortest resonant antenna is a half wave long. Remember a half wave in free space is determined by the formula 492/frequency in MHz. When confined to a wire, the speed changes and there are other effects that appear to change the speed. The sum total of all these effects is to reduce the length by about 5%. So you change the formula to 468 if you are using most kinds of wire. That means that a half wave of wire is determined by dividing 468 by the frequency in MHz. So for example on 1.850 MHZ (160 meter band) the length of a half wave of wire is 468/1.85 = 252.9 feet or about 253 feet. The formula for a half wave in a wire is not exact. Although the length depends primarily on the speed of the radio signal in the wire, the type of insulation and its thickness makes a big difference. There are also effects caused by the wire being close to the ground or any other conductor, even wood or tree branches. There is an effect due to the insulator at the end of the wire. Some of these effects, especially due to the insulation, are more pronounced the higher the frequency. This formula is also not good if the antenna is more than one half wave long. The formula was reduced from 492 to 468 for a one half wavelength wire that has two ends connected to an insulator. If the antenna is say, one wavelength long, there are still only two ends, but there is effectively a half wavelength of wire in the middle that has no open ends. That has an effect on the calculation. It turns out that a large part of the reason that a half wave wire antenna is physically shorter than a half wave in free space is due to the insulators at the two ends or the “end effects”. The way I see it, the ends will either have an insulator or be hanging loose. In either case there is an end effect of one sort or another. If you connect two half wave lengths of wire together, you will find that together they are shorter than they should be for full wave resonance. The calculation of each half wave was based on each length having two ends. Remember, a full wavelength of wire will be slightly longer than two half-wave lengths added together. This is not a big deal, but something to keep in mind if you are constructing a wire antenna longer than a half wave long.
Sunday, February 7, 2010
Sketching Current on the Antenna
I may be repeating a few things as we go along, but I think the important principles of radio and antenna theory need to be reinforced. I have found over the years that current, voltage, impedance and standing waves were among the most misunderstood concepts in radio or electronics. I think everyone has heard the term SWR (Standing Wave Ratio). I have been talking about standing waves on antennas. These are both standing waves of voltage and standing waves of current.
The ratio of voltage to current is the impedance at that point. When dealing with transmission likes, the common SWR is not a voltage to current ratio, but a ratio of two voltages measured at two different places on a line (VSWR). It could also be the ratio of two currents measured at different places on the line (ISWR). In fact there are numerous methods that can be used to measure transmission line SWR. I will eventually get around to explaining that. Right now I want to get the concept of antenna current and voltage down first. That really comes first and is a direct cause of the transmission line SWR.
There are a couple things that we need to know about antennas. First we want them to radiate as high a percentage of the power we supply to them as possible. (We want the transmission line to deliver as high a percentage of the transmitter power to the antenna as possible.) In order for the antenna to be able to accept all the power from the feed line we must have an antenna whose impedance matches that of the feed line. There are numerous physical things that will determine this ‘feed point” impedance. Almost all of these things are within our control, but we must have a basic understanding of them and how they work in order to control them. Otherwise it is just by trial and error.
The most important thing is to know approximately what the current and voltages are on an antenna. In order to start we need to pick a frequency and determine the wavelength and half wavelength. I always sketch the antenna then note some ballpark information about the current standing wave at various points along the wire. What do we know about the current at an open end? (It will be zero) Then I put a dot a quarter wave back from every end. What do we know about current at this point? (Current will be a maximum.) Continue on away from the end another quarter wave (if the wire is long enough). What do we know about the current here? It will be zero or a minimum. The reason it is not zero is that some of it is radiated as it travels along the wire to and from the ends, and when it crosses “fresh” current, there is not enough of it to completely cancel out the “fresh” current.
You should be able to sketch a half wave wire and draw a smooth curve representing the current. It should look like the first half of a sine wave. The curve representing current should start at zero on one end, rise like a sine wave to a maximum in the center and down to zero at the other end. If the wire is one wave long, we draw that current curve the same for the first half wave (going from left to right) then the second half wave is drawn below the line down to a maximum negative value, then back up to zero at the end. The current above the curve represents positive current and below the line represents negative current. It does not really matter which way the first arrow goes, what does matter is that the current in the first half wave flows one way and the current in the second half wave flows the opposite way. Another way to illustrate can be seen if you draw a long line from left to right. Divide it into four equal segments. So you can follow what I am doing, number the segments left to right 1, 2, 3 and 4. Over segments 2 and 4 draw an arrow pointing right. Over segments 1 and 3 draw an arrow pointing left. The currents in segments 2 and 4 are in phase; the currents in 1 and 3 are also in phase with each other but out of phase with currents in segments 2 and 4. To start to analyze any antenna, you can draw a sketch. Divide the wire into half wave segments and draw arrows over each half-wave segment, being sure to change the direction of the arrows in each adjacent half-wave segment.
Once you have drawn the sketch, you immediately know where the current is zero (at the ends) and where the current is a maximum (between the zero points). You also know that the points where the impedance is low are at the same place where the current is a maximum. These are the most likely places that you would want to connect a coaxial feed line. (There are exceptions, but we will deal with the exceptions one at a time later. For now, be advised that there seems to always be an exception to everything)
The impedance is different at different points on the wire. The voltage to current ratio at the ends is very high. Voltage is at its highest. We say the current is zero. You cannot really divide something like 100 or 200 volts by zero. You have to divide by almost zero, and get a close answer. In math or physics we always seem to run into problems where we want to divide by zero. If you divide by zero you get infinity. Not a good answer. So what we do is usually try and figure out what the limit is as we approach zero. We divide by smaller and smaller numbers to approximate dividing by zero. You might think of it a instead of dividing by zero, dividing by 1/1000 amps or maybe 1/1,000,000 amps. Sometimes you get close enough to the right answer that no one can ever tell it is not exactly the right number! In our case it is almost always good enough to say that the impedance at the end of an antenna wire is very high and leave it at that. I do. Funny things happen in the real world when you try and work with impedances like at the end of a wire where they are real high. There are situations where things become unstable and unpredictable. There are cases where impedances rise very fast to an extremely high inductive value and then like you would flip a switch, they change to a very high capacitive value. For most of us, we need only know that such conditions do exist, recognize when they may exist, understand that it is best to avoid them! I will try and help you with that. I just rambled some. Let me back up and start again.
The impedance is different at different points on the wire. The voltage to current ratio is very high at the ends and this ratio goes down as you move away from the end. The impedance reaches a minimum at the middle of each half-wave section. You can connect cut the wire and connect a feed line anywhere. In the center you will get a good match to 50 or 75 ohm coax. Somewhere between the center and the end you may find a point that is a close match for 300-ohm line. No matter where you choose to connect a transmission line on any length wire there will be an impedance at that point that is acting like either a pure resistance, or a resistance in series with a coil (inductor) or capacitance. If the impedance is not a good match for the chosen feed line, you have several choices. First you can change the point of connection. That sometimes works. Second you can change the type of feed line. Third you can add a matching circuit or matching network to transform the impedance of the antenna to the value of the transmission line. Fourth you can change the length of the antenna (longer or shorter). Lastly, you can do nothing. That means you accept the additional loss due to the mismatch. We normally do not accept much of a mismatch when using coax. However, when using open wire or parallel wire transmission line, typically 300 or 600-ohm lines, we find that the increased losses due to a rather large mismatch are not significant and we can live with that small loss.
The ratio of voltage to current is the impedance at that point. When dealing with transmission likes, the common SWR is not a voltage to current ratio, but a ratio of two voltages measured at two different places on a line (VSWR). It could also be the ratio of two currents measured at different places on the line (ISWR). In fact there are numerous methods that can be used to measure transmission line SWR. I will eventually get around to explaining that. Right now I want to get the concept of antenna current and voltage down first. That really comes first and is a direct cause of the transmission line SWR.
There are a couple things that we need to know about antennas. First we want them to radiate as high a percentage of the power we supply to them as possible. (We want the transmission line to deliver as high a percentage of the transmitter power to the antenna as possible.) In order for the antenna to be able to accept all the power from the feed line we must have an antenna whose impedance matches that of the feed line. There are numerous physical things that will determine this ‘feed point” impedance. Almost all of these things are within our control, but we must have a basic understanding of them and how they work in order to control them. Otherwise it is just by trial and error.
The most important thing is to know approximately what the current and voltages are on an antenna. In order to start we need to pick a frequency and determine the wavelength and half wavelength. I always sketch the antenna then note some ballpark information about the current standing wave at various points along the wire. What do we know about the current at an open end? (It will be zero) Then I put a dot a quarter wave back from every end. What do we know about current at this point? (Current will be a maximum.) Continue on away from the end another quarter wave (if the wire is long enough). What do we know about the current here? It will be zero or a minimum. The reason it is not zero is that some of it is radiated as it travels along the wire to and from the ends, and when it crosses “fresh” current, there is not enough of it to completely cancel out the “fresh” current.
You should be able to sketch a half wave wire and draw a smooth curve representing the current. It should look like the first half of a sine wave. The curve representing current should start at zero on one end, rise like a sine wave to a maximum in the center and down to zero at the other end. If the wire is one wave long, we draw that current curve the same for the first half wave (going from left to right) then the second half wave is drawn below the line down to a maximum negative value, then back up to zero at the end. The current above the curve represents positive current and below the line represents negative current. It does not really matter which way the first arrow goes, what does matter is that the current in the first half wave flows one way and the current in the second half wave flows the opposite way. Another way to illustrate can be seen if you draw a long line from left to right. Divide it into four equal segments. So you can follow what I am doing, number the segments left to right 1, 2, 3 and 4. Over segments 2 and 4 draw an arrow pointing right. Over segments 1 and 3 draw an arrow pointing left. The currents in segments 2 and 4 are in phase; the currents in 1 and 3 are also in phase with each other but out of phase with currents in segments 2 and 4. To start to analyze any antenna, you can draw a sketch. Divide the wire into half wave segments and draw arrows over each half-wave segment, being sure to change the direction of the arrows in each adjacent half-wave segment.
Once you have drawn the sketch, you immediately know where the current is zero (at the ends) and where the current is a maximum (between the zero points). You also know that the points where the impedance is low are at the same place where the current is a maximum. These are the most likely places that you would want to connect a coaxial feed line. (There are exceptions, but we will deal with the exceptions one at a time later. For now, be advised that there seems to always be an exception to everything)
The impedance is different at different points on the wire. The voltage to current ratio at the ends is very high. Voltage is at its highest. We say the current is zero. You cannot really divide something like 100 or 200 volts by zero. You have to divide by almost zero, and get a close answer. In math or physics we always seem to run into problems where we want to divide by zero. If you divide by zero you get infinity. Not a good answer. So what we do is usually try and figure out what the limit is as we approach zero. We divide by smaller and smaller numbers to approximate dividing by zero. You might think of it a instead of dividing by zero, dividing by 1/1000 amps or maybe 1/1,000,000 amps. Sometimes you get close enough to the right answer that no one can ever tell it is not exactly the right number! In our case it is almost always good enough to say that the impedance at the end of an antenna wire is very high and leave it at that. I do. Funny things happen in the real world when you try and work with impedances like at the end of a wire where they are real high. There are situations where things become unstable and unpredictable. There are cases where impedances rise very fast to an extremely high inductive value and then like you would flip a switch, they change to a very high capacitive value. For most of us, we need only know that such conditions do exist, recognize when they may exist, understand that it is best to avoid them! I will try and help you with that. I just rambled some. Let me back up and start again.
The impedance is different at different points on the wire. The voltage to current ratio is very high at the ends and this ratio goes down as you move away from the end. The impedance reaches a minimum at the middle of each half-wave section. You can connect cut the wire and connect a feed line anywhere. In the center you will get a good match to 50 or 75 ohm coax. Somewhere between the center and the end you may find a point that is a close match for 300-ohm line. No matter where you choose to connect a transmission line on any length wire there will be an impedance at that point that is acting like either a pure resistance, or a resistance in series with a coil (inductor) or capacitance. If the impedance is not a good match for the chosen feed line, you have several choices. First you can change the point of connection. That sometimes works. Second you can change the type of feed line. Third you can add a matching circuit or matching network to transform the impedance of the antenna to the value of the transmission line. Fourth you can change the length of the antenna (longer or shorter). Lastly, you can do nothing. That means you accept the additional loss due to the mismatch. We normally do not accept much of a mismatch when using coax. However, when using open wire or parallel wire transmission line, typically 300 or 600-ohm lines, we find that the increased losses due to a rather large mismatch are not significant and we can live with that small loss.
Friday, February 5, 2010
Slight review and trap dipole principle
In a resonant half wave dipole, the voltage and current enter the antenna at the center, travel to the ends and get reflected. The current reflection is out of phase so the currents at the ends cancel. There is always zero current at the ends. (There is always zero current at the end of a an open wire. The voltage is also reflected but is not reversed in polarity like the current, so the voltage at the end is pretty much twice the original voltage. Due to the fact that the voltage and current vary with time, the actual currents and voltages that are set up on the antenna (these are called standing waves) varies or oscillates between two values. At the center the current varies at any given instant between a maximum positive value and a maximum negative value. As you move toward either end the current varies between two limits of decreasing value until at the end the two values go to zero. The voltage and current at the antenna terminals will be in phase. That means when the voltage peaks the current peaks. When the voltage is zero the current is zero. This is just like current and voltage in a pure resistor. When the radio frequency voltage is first applied at the transmitter end of a feed line current will flow (in a ratio of voltage to current determined by the characteristic impedance of the line) Lets look at the current first. The current reaches the terminals then flows to the end, gets reflected (out of phase) and flows back to the center. When it gets to the center it is exactly in phase with the current from the next cycle (remember this is only true for a resonant condition). These two currents then simply add together. Now lets think about the voltage. The voltage signal enters the antenna and travels down the wire to the end where it is reflected but the phase is not reversed. By the time it gets back to the terminals, the voltage is out of phase with the voltage of the next cycle. This reflected voltage tends to cancel the incoming voltage much like happens to current at the ends of the antenna. We should call the voltage and current that comes up the transmission line the initial voltage and current. We can also call it the incident voltage and current. These are the voltages and currents existing before things get muddy due to the reflections, standing waves and all that. The current and voltage waves that are set up on the antenna are standing waves of voltage and current. Initial voltage and current are in phase. Like they would be in a pure resistance. If the antenna is resonant the resulting current will still be in phase with this terminal voltage. The resonant antenna will act like a pure resistance. Now if the antenna is long or short, the resulting standing waves set up on the antenna will be a bit different and will cause the resulting current at the antenna terminals to be slightly out of phase with the applied voltage which will cause the antenna to not act like a pure resistance. The antenna will act like it has a coil or capacitor connected in series with it, even though it does not. It just acts like it does. That’s why we say the antenna is capacitive if it is short and inductive if it is long.
We can take a center fed dipole that is resonant and make it longer. We can also take a center fed dipole and add a coil to both sides. If we pick the right size coil, you will not be able to tell the difference at the antenna terminals. The same goes for making the antenna shorter or adding a capacitor. You will not be able to tell the difference. You can take a short antenna and add a coil to make it “appear” to be longer. You can take a long antenna and add a capacitor and make it appear shorter. This is the principle behind trap antennas. The trap is designed so that it makes the antenna appear to be longer or shorter so that the voltage and current at the terminals will be in phase and of a low enough impedance to be a good match to a coax transmission line.
We can take a center fed dipole that is resonant and make it longer. We can also take a center fed dipole and add a coil to both sides. If we pick the right size coil, you will not be able to tell the difference at the antenna terminals. The same goes for making the antenna shorter or adding a capacitor. You will not be able to tell the difference. You can take a short antenna and add a coil to make it “appear” to be longer. You can take a long antenna and add a capacitor and make it appear shorter. This is the principle behind trap antennas. The trap is designed so that it makes the antenna appear to be longer or shorter so that the voltage and current at the terminals will be in phase and of a low enough impedance to be a good match to a coax transmission line.
Thursday, February 4, 2010
Voltage and current in non-resonant dipole
If the antenna is a resonant half wavelength the currents and voltages are easy to figure. The voltage is normally zero every half wave. The current is also zero at every half-wave point. So the current is quite happy ☺ if its natural zeros are a half wave apart as at the two open ends of a wire. Likewise the voltage has peaks every half wavelength and is also quite happy with those peaks occurring naturally at the ends of the dipole. Under these ideal conditions all is well in the world. To top it all off the ratio of voltage to current at the center (that’s where you normally connect coax) is low. Remember that the current is as large as it will ever be at the center and the voltage is at its low point. We said zero but anytime we say zero it may actually be really small. So what is the ratio of voltage to current? Hint: V/I Another hint E/I. Remember Ohms law? R=E/I Resistance is voltage divided by current. Resistance is impedance to current flow. Resistance impedes current. Resistance is also Impedance. So if the ratio of voltage to current is low, the impedance is low. Coax cable likes to be connected to low impedances. Remember anytime anyone mentions impedance you need to simply think voltage to current ratio. High voltage and low current means high impedance. Low voltage and high current means low impedance. Impedance is not really as complicated as you might think. It is just the relationship of voltage to current.
Now what happens if the wire dipole is increased in length so that it is longer than a half wavelength. Well the current will enter the wire at the center and move toward the end where the first time it gets there it will be reflected, equal and out of phase (adds to zero at the end point). It then races toward the other end at almost the speed of light where it hits the other end. This time when it reaches the center of the antenna it is lagging behind the current from the next cycle that entered the center just before this reflected wave reached the center. In the resonant case we talked about these two currents would have been in phase at the center and simply added together. The two maximum values would have occurred at the same time. So where each current may have been 2 amps there would have been 4 at the center on the resonant antenna. Now in the non resonant case, when the reflected 2 amps passes the center the incoming current from the coax would have been 2 an instant before, but because of the increased travel time of the reflected wave, it arrives at the center a bit late and if the incoming current has decreased to 1 amp. There will only be 3 amps at the center due to the addition of these two waves. Because the first reflected wave is late it disrupts the timing of everything. It gets still worse because the next incoming wave must travel longer than in the resonant case before it is reflected and each set of reflections is late arriving back at the center and is again out of phase with the incoming current. After a few cycles of this everything settles into a grove so to speak. The end result is that the resulting standing waves of voltage and current do not have the same relationship in the non-resonant case as in the resonant case. In the resonant case the voltage to current ratio was low, maybe even 50 ohms. It would even act like it was a pure 50 ohm resistance connected to the coax. In the non-resonant (long) case, the phase relationship between the voltage and current would be shifted somewhat like it is in an inductor. The current would seem to be behind where it should be or at least where it was in the resonant case. That is why we say the long antenna is inductive or its impedance is inductive. Its voltage to current ratio is its impedance as always but the points where the voltage maximum and current maximum occur are shifted such that the current peaks later than it would have in a resonant antenna. In addition to this inductive effect, the current is lower at the center (remember the current peaks of the incoming and reflected wave are not both occurring at the center at the same time) so the voltage divided by the smaller current at the feed point gives a larger number for the impedance (voltage divided by current).
Its like (not exactly) 200 volts divided 4 is 50 ohms in the resonant case, while 200 volts divided by 3 is 66 ohms. Do not take this example to literal. The resulting voltages and currents in the non-resonant case are not easy to calculate and both voltage and current will change. They also change in different directions depending on the antenna being long or short.
Now what happens if the wire dipole is increased in length so that it is longer than a half wavelength. Well the current will enter the wire at the center and move toward the end where the first time it gets there it will be reflected, equal and out of phase (adds to zero at the end point). It then races toward the other end at almost the speed of light where it hits the other end. This time when it reaches the center of the antenna it is lagging behind the current from the next cycle that entered the center just before this reflected wave reached the center. In the resonant case we talked about these two currents would have been in phase at the center and simply added together. The two maximum values would have occurred at the same time. So where each current may have been 2 amps there would have been 4 at the center on the resonant antenna. Now in the non resonant case, when the reflected 2 amps passes the center the incoming current from the coax would have been 2 an instant before, but because of the increased travel time of the reflected wave, it arrives at the center a bit late and if the incoming current has decreased to 1 amp. There will only be 3 amps at the center due to the addition of these two waves. Because the first reflected wave is late it disrupts the timing of everything. It gets still worse because the next incoming wave must travel longer than in the resonant case before it is reflected and each set of reflections is late arriving back at the center and is again out of phase with the incoming current. After a few cycles of this everything settles into a grove so to speak. The end result is that the resulting standing waves of voltage and current do not have the same relationship in the non-resonant case as in the resonant case. In the resonant case the voltage to current ratio was low, maybe even 50 ohms. It would even act like it was a pure 50 ohm resistance connected to the coax. In the non-resonant (long) case, the phase relationship between the voltage and current would be shifted somewhat like it is in an inductor. The current would seem to be behind where it should be or at least where it was in the resonant case. That is why we say the long antenna is inductive or its impedance is inductive. Its voltage to current ratio is its impedance as always but the points where the voltage maximum and current maximum occur are shifted such that the current peaks later than it would have in a resonant antenna. In addition to this inductive effect, the current is lower at the center (remember the current peaks of the incoming and reflected wave are not both occurring at the center at the same time) so the voltage divided by the smaller current at the feed point gives a larger number for the impedance (voltage divided by current).
Its like (not exactly) 200 volts divided 4 is 50 ohms in the resonant case, while 200 volts divided by 3 is 66 ohms. Do not take this example to literal. The resulting voltages and currents in the non-resonant case are not easy to calculate and both voltage and current will change. They also change in different directions depending on the antenna being long or short.
Wednesday, February 3, 2010
Voltage on a wire (especially at the end)
Voltage on an antenna works in a similar manner to the current. When the voltage wave reaches the end of the antenna wire, it also is reflected. Unlike the current which totally reverses its direction resulting in zero total current, the voltages do not cancel but add together. A voltage wave consisting of say 50 volts will be reflected at the open end of a wire. The resultant voltage will be 50 plus 50 or 100 volts. Remember the current had to be zero by law. There is a law that says the sum of all currents, into and out of a point must be zero. This law is called Kirchhoff current law. That law does not apply to voltage. Kirchhof does have a voltage law that we, however must follow. More on that later. At this point think of the voltage wave similar to an ocean wave hitting a breakwater. Water flows in, abruptly hits the breakwater and is stopped. Piles up and bounces back away from the wall. If you have ever watched two water waves passing or crossing each other you have seen that when their peaks cross the wave rises up about twice as high as either wave alone. Voltage acts the same way. In fact so does current with the exception of the fact that when it is reflected it is reflected out of phase and there fore the two currents cancel. Voltage is not reflected out of phase from an open end and there fore the two voltages add together.
This leads us to another interesting thing about antennas. The voltage at the end will always be a maximum. The highest voltage that can exist on an antenna will be at its end. Do not ever touch the end of an antenna while it is being used for transmitting. I had a friend who had on a pair of gloves and when he touched the end of the antenna while it was transmitting on 160 meters he got bit right through the gloves.
At the center of the antenna (remember we are only talking about a half wave resonant dipole at this point. Once we understand that we will increase its length so that it is not resonant) the voltages will be out of phase and add to zero (well almost zero). The voltage at the center of a half wave resonant dipole will always be a minimum.
Lets review what we know for sure about a half wave resonant dipole. The current at both ends will always (by law) be zero. The voltage at the ends will always be a maximum. The current at the center will be a maximum, while the voltage will be a minimum. These currents and voltages are a result of the reflections (of both current and voltage) from the ends. There is a name for this kind of wave. It is a standing wave. You might say the voltage stands at a maximum at the end. The current stands at a maximum at the center. The current stands at a minimum at the end. The voltage stands at a minimum at the center. In reality these current and voltage waveforms do not actually stand still, they sort of “march in place”. At the end where the voltage is maximum, it really varies from a maximum positive voltage to a maximum negative voltage and back again. It does this at the same rate as the frequency of the radio signal. The current does the same, at the center it varies from a maximum positive value to a maximum negative value. At all points on the antenna the voltage or current will “oscillate” between a fixed maximum and minimum value. Think of what it looks like to watch some one jumping rope. The rope in fixed at both ends but the center goes up to a maximum height over the persons head then down to a lowest point almost on the ground and repeats over and over until the person jumping the rope stops transmitting…I mean stops jumping. This may help visualize how the current varies in a dipole. Visualizing the voltage is not quite so easy but bare with me. Think of someone holding two long broom handles. Arms out stretched. The person raises the right arm pointing that broom handle up. With the left arm he (or she) points the broom stick down and touches the ground with its tip end. Then he lowers the right one while simultaneously raising the left one and repeats this exercise several million times a second. What you have is the end points of the broom sticks are “oscillating”up and down between a maximum height of probably 8 volts…..I mean 8 feet and the ground or zero feet. The left side is up when the right side is down. Half way out on the broom sticks that point is “oscillating” up and down between a height if maybe 3 and 5 volts….I mean 3 and 5 feet. This is a good representation of the way the voltage standing wave varies on a half wave resonant dipole.
More tomorrow.
This leads us to another interesting thing about antennas. The voltage at the end will always be a maximum. The highest voltage that can exist on an antenna will be at its end. Do not ever touch the end of an antenna while it is being used for transmitting. I had a friend who had on a pair of gloves and when he touched the end of the antenna while it was transmitting on 160 meters he got bit right through the gloves.
At the center of the antenna (remember we are only talking about a half wave resonant dipole at this point. Once we understand that we will increase its length so that it is not resonant) the voltages will be out of phase and add to zero (well almost zero). The voltage at the center of a half wave resonant dipole will always be a minimum.
Lets review what we know for sure about a half wave resonant dipole. The current at both ends will always (by law) be zero. The voltage at the ends will always be a maximum. The current at the center will be a maximum, while the voltage will be a minimum. These currents and voltages are a result of the reflections (of both current and voltage) from the ends. There is a name for this kind of wave. It is a standing wave. You might say the voltage stands at a maximum at the end. The current stands at a maximum at the center. The current stands at a minimum at the end. The voltage stands at a minimum at the center. In reality these current and voltage waveforms do not actually stand still, they sort of “march in place”. At the end where the voltage is maximum, it really varies from a maximum positive voltage to a maximum negative voltage and back again. It does this at the same rate as the frequency of the radio signal. The current does the same, at the center it varies from a maximum positive value to a maximum negative value. At all points on the antenna the voltage or current will “oscillate” between a fixed maximum and minimum value. Think of what it looks like to watch some one jumping rope. The rope in fixed at both ends but the center goes up to a maximum height over the persons head then down to a lowest point almost on the ground and repeats over and over until the person jumping the rope stops transmitting…I mean stops jumping. This may help visualize how the current varies in a dipole. Visualizing the voltage is not quite so easy but bare with me. Think of someone holding two long broom handles. Arms out stretched. The person raises the right arm pointing that broom handle up. With the left arm he (or she) points the broom stick down and touches the ground with its tip end. Then he lowers the right one while simultaneously raising the left one and repeats this exercise several million times a second. What you have is the end points of the broom sticks are “oscillating”up and down between a maximum height of probably 8 volts…..I mean 8 feet and the ground or zero feet. The left side is up when the right side is down. Half way out on the broom sticks that point is “oscillating” up and down between a height if maybe 3 and 5 volts….I mean 3 and 5 feet. This is a good representation of the way the voltage standing wave varies on a half wave resonant dipole.
More tomorrow.
Tuesday, February 2, 2010
Current on a wire obeys the law
I hope the last post gave a good idea of why an antenna can act like a capacitor when short and like an inductor (coil) when long. It is all about the voltage waveform and the current waveforms either being in phase (increasing to a maximum, decreasing to zero and to a minimum all at the same time) or being somewhat out of phase (having one or the other reach a maximum, zero or minimum before the other).
There are various reasons for this to happen. I think I explained why it happens in a capacitor or inductor. You may wonder why it happens in an antenna or wire when that wire may not look very much like a capacitor or coil. (Note I will use the terms inductor and coil interchangeably. A coil is an inductor and an inductor is usually a coil.)
There are certain things that just happen in this world. You just need to recognize it and believe it. We call these things natural laws. You throw a ball up in the air and it will come down due to the Law of Gravity. There are also electrical laws just like that. They all have names but I will not try and name them just talk about them as the occasion arises.
The total current at the end of a wire is zero. The current at the end of any antenna wire (as long as that end is not connected to a feed line or some other conductor) is zero. That is a law. Anytime I see the end of a wire, I know one thing for sure. The current at the end is zero. The reason is simple. Suppose current is flowing down the wire toward the end. (It does that in an antenna you know) Lets suppose that you are looking at this wire and it is running from left to right in front of you. Lets send 5 amps of current down the wire from left to right. The end on the right is not connected to anything. (Like the end of a dipole antenna) When the 5 amps of current gets to the end of the wire what happens? Does it just stop and remain at 5 amps? No. That cannot happen or it would violate one of our electrical laws. (Maybe you know the name of the law, if not maybe you can look it up) Remember at this time we have 5 amps flowing from left to right. At the open end of the wire, those 5 amps is reflected or bounced back to the left. Now at this instant we have current of 5 amps flowing left to right and 5 amps flowing from right to left. We have equal and opposite currents existing right at the end of the antenna. Any time you have equal and opposite anything they tend to cancel. The total current at the end of the wire is +5 amps -5 amps = zero amps. This will be the case for all open-ended conductors. Now what happens to that current that is reflected back from the end of the wire? It will travel all the way to the other end of the wire or until it reaches some major change in the wire like a feed line another conductor attached or some other physical thing that will disrupt it for some reason. In the case of a center fed dipole antenna, it will go all the way to the other end and be reflected again. Every time it is reflected the total current will add to zero. So at both ends of any dipole of any length, the current will be zero. Visualize a center fed dipole (you have to visualize because I can not draw on a blackboard like I would like to here).
Current enters the dipole at the center and travels out to the ends where it is reflected and the value of current is always zero at the ends. If the dipole just happens to be a half wavelength long, several neat things happen. We will have zero current at the ends because the currents are always equal and opposite. When the currents are opposite we call that “out of phase” specifically we call it 180 degrees out of phase. You cannot get any more out of phase than 180 degrees.
At the center of the antenna the currents will always be totally in phase. We have what is termed a current maximum right at the center. So if you were to plot values of current all along the antenna you would start with zero current at say the left end, gradually build up to a maximum in the center and down again to zero at the right end. Can you visualize that? Sure. You could actually measure this effect by taking the temperature of the wire when it is being used for a transmitting antenna. The center of the antenna would be warmer than the ends. Current flowing through a wire of conductor generates heat. For example if the resistance was 1 ohm and 5 amps flowed, the heat generated would be 25 watts. The heat generated at the end of a wire will always be zero. So the wire would be much cooler at the end than it would be at the center. I have exaggerated this a bit. The resistance of a wire antenna is extremely small and the amount of current flowing from a high-powered amateur transmitter may be only 5 or 10 amps. So you probably could never detect it. You can however run your hand on a piece of coax used for transmitting and feel the warm and cool spots. A warm spot will be felt where there is a current maximum. These warm spots will occur every half wavelength along the line.
I found this out the hard way many years ago. I was running high power (about 300 watts) on AM and actually melted a piece of coax at one of these “warm”spots!
Next I will talk about how the voltage acts on a dipole. It is a little different than the current, however it always obeys the laws also.
There are various reasons for this to happen. I think I explained why it happens in a capacitor or inductor. You may wonder why it happens in an antenna or wire when that wire may not look very much like a capacitor or coil. (Note I will use the terms inductor and coil interchangeably. A coil is an inductor and an inductor is usually a coil.)
There are certain things that just happen in this world. You just need to recognize it and believe it. We call these things natural laws. You throw a ball up in the air and it will come down due to the Law of Gravity. There are also electrical laws just like that. They all have names but I will not try and name them just talk about them as the occasion arises.
The total current at the end of a wire is zero. The current at the end of any antenna wire (as long as that end is not connected to a feed line or some other conductor) is zero. That is a law. Anytime I see the end of a wire, I know one thing for sure. The current at the end is zero. The reason is simple. Suppose current is flowing down the wire toward the end. (It does that in an antenna you know) Lets suppose that you are looking at this wire and it is running from left to right in front of you. Lets send 5 amps of current down the wire from left to right. The end on the right is not connected to anything. (Like the end of a dipole antenna) When the 5 amps of current gets to the end of the wire what happens? Does it just stop and remain at 5 amps? No. That cannot happen or it would violate one of our electrical laws. (Maybe you know the name of the law, if not maybe you can look it up) Remember at this time we have 5 amps flowing from left to right. At the open end of the wire, those 5 amps is reflected or bounced back to the left. Now at this instant we have current of 5 amps flowing left to right and 5 amps flowing from right to left. We have equal and opposite currents existing right at the end of the antenna. Any time you have equal and opposite anything they tend to cancel. The total current at the end of the wire is +5 amps -5 amps = zero amps. This will be the case for all open-ended conductors. Now what happens to that current that is reflected back from the end of the wire? It will travel all the way to the other end of the wire or until it reaches some major change in the wire like a feed line another conductor attached or some other physical thing that will disrupt it for some reason. In the case of a center fed dipole antenna, it will go all the way to the other end and be reflected again. Every time it is reflected the total current will add to zero. So at both ends of any dipole of any length, the current will be zero. Visualize a center fed dipole (you have to visualize because I can not draw on a blackboard like I would like to here).
Current enters the dipole at the center and travels out to the ends where it is reflected and the value of current is always zero at the ends. If the dipole just happens to be a half wavelength long, several neat things happen. We will have zero current at the ends because the currents are always equal and opposite. When the currents are opposite we call that “out of phase” specifically we call it 180 degrees out of phase. You cannot get any more out of phase than 180 degrees.
At the center of the antenna the currents will always be totally in phase. We have what is termed a current maximum right at the center. So if you were to plot values of current all along the antenna you would start with zero current at say the left end, gradually build up to a maximum in the center and down again to zero at the right end. Can you visualize that? Sure. You could actually measure this effect by taking the temperature of the wire when it is being used for a transmitting antenna. The center of the antenna would be warmer than the ends. Current flowing through a wire of conductor generates heat. For example if the resistance was 1 ohm and 5 amps flowed, the heat generated would be 25 watts. The heat generated at the end of a wire will always be zero. So the wire would be much cooler at the end than it would be at the center. I have exaggerated this a bit. The resistance of a wire antenna is extremely small and the amount of current flowing from a high-powered amateur transmitter may be only 5 or 10 amps. So you probably could never detect it. You can however run your hand on a piece of coax used for transmitting and feel the warm and cool spots. A warm spot will be felt where there is a current maximum. These warm spots will occur every half wavelength along the line.
I found this out the hard way many years ago. I was running high power (about 300 watts) on AM and actually melted a piece of coax at one of these “warm”spots!
Next I will talk about how the voltage acts on a dipole. It is a little different than the current, however it always obeys the laws also.
Why an antenna is inductive or capacitive
Now we know how to calculate wavelength and half wavelength on free space using the formulas:
wavelength in meters = 300/MHz or
wavelength in feet = 982/MHz
It is of course more useful to get a half wavelength in feet from:
Half wavelength in feet = 491/MHz
As an example if we want to know what a half wave is for the middle of the ten meter band we would find it by dividing 491 by 28.5 and the answer is 17.22 feet.
This is the free space half wavelength in feet. It is based on the speed of the wave in space. If we are going to use a wire or some other conductor to carry this signal, then the speed will be less by several percent. Lets assume that we pick a wire with a certain type of insulation, such that the speed is reduced to 95% of the speed in free space. We then need to multiply the 17.22 feet by .95. We get 16.36 feet.
This tells us that a half wavelength for our radio signal is only 16.36 feet. If we were going to make a half wave long antenna it would be about 16.36 feet. (I say about because there are other usually minor factors that could affect the length)
If our wire were only 14 feet instead of 16.36 feet at our chosen frequency of 28.5 MHz it would be “short”. Remember that a “short” antenna acts like a capacitor. If the wire was 20 feet it would be “long” and act like an inductor or coil.
At this point you may be asking the question “what does it mean to be acting like a capacitor or acting like an inductor?”
The short answer is that the relationship between voltage and current is different.
In a pure resistance the current is proportional to the voltage. No voltage, no current. Maximum voltage, maximum current. Minimum voltage, minimum current. The current is said to be in phase with the voltage.
In a capacitor, when we first apply voltage the capacitor acts like a short circuit. Maximum current flows but the voltage is low. Then the capacitor charges up and the current flow stops. The voltage then is at a maximum but the capacitor is fully charged and the current is at a minimum or zero. We say that the current and voltage are out of phase. If we look at graphs of the voltage and current (voltage across the capacitor and current into or out of the capacitor) we will see that the current wave peaks before the voltage wave. The peaks are about ¼ wavelength apart. We like to talk about this type of wavelength lengths by using the term degrees. We take a wavelength and divide it into 360 equal parts. We say a wavelength is 360 degrees. So a quarter wave is 90 degrees. If the current is ¼ wave ahead of the voltage it is 90 degrees ahead. Another way to say the same thing is that the current leads the voltage by 90 degrees in a capacitor.
If we have a short antenna we find that the current peaks before the voltage peaks.
In a resonant antenna we find that the current and voltage are in phase.
In the case of the long antenna, it acts like an inductor. In an a inductor, when you apply the voltage, the current starts slowly then builds up. The voltage peak comes before the current peak. The common way of expressing this situation is to say that the current lags the voltage in an inductor.
If we have a long antenna, the current will peak after the voltage.
What does all this mean? Well If I have an antenna that is long or inductive I can sometimes use just a capacitor to tune it to resonance. If the antenna is short I may be able to only use a coil or inductor to tune it.
A practical example is the mobile whip antenna. Other than on ten meters the HF mobile antenna will be short. A short antenna is capacitive. A coil placed at the base of a mobile whip is frequently all that is needed to tune that antenna. Depending on the frequency, the capacitance of the antenna will vary and a coil will have to be chosen to suit.
wavelength in meters = 300/MHz or
wavelength in feet = 982/MHz
It is of course more useful to get a half wavelength in feet from:
Half wavelength in feet = 491/MHz
As an example if we want to know what a half wave is for the middle of the ten meter band we would find it by dividing 491 by 28.5 and the answer is 17.22 feet.
This is the free space half wavelength in feet. It is based on the speed of the wave in space. If we are going to use a wire or some other conductor to carry this signal, then the speed will be less by several percent. Lets assume that we pick a wire with a certain type of insulation, such that the speed is reduced to 95% of the speed in free space. We then need to multiply the 17.22 feet by .95. We get 16.36 feet.
This tells us that a half wavelength for our radio signal is only 16.36 feet. If we were going to make a half wave long antenna it would be about 16.36 feet. (I say about because there are other usually minor factors that could affect the length)
If our wire were only 14 feet instead of 16.36 feet at our chosen frequency of 28.5 MHz it would be “short”. Remember that a “short” antenna acts like a capacitor. If the wire was 20 feet it would be “long” and act like an inductor or coil.
At this point you may be asking the question “what does it mean to be acting like a capacitor or acting like an inductor?”
The short answer is that the relationship between voltage and current is different.
In a pure resistance the current is proportional to the voltage. No voltage, no current. Maximum voltage, maximum current. Minimum voltage, minimum current. The current is said to be in phase with the voltage.
In a capacitor, when we first apply voltage the capacitor acts like a short circuit. Maximum current flows but the voltage is low. Then the capacitor charges up and the current flow stops. The voltage then is at a maximum but the capacitor is fully charged and the current is at a minimum or zero. We say that the current and voltage are out of phase. If we look at graphs of the voltage and current (voltage across the capacitor and current into or out of the capacitor) we will see that the current wave peaks before the voltage wave. The peaks are about ¼ wavelength apart. We like to talk about this type of wavelength lengths by using the term degrees. We take a wavelength and divide it into 360 equal parts. We say a wavelength is 360 degrees. So a quarter wave is 90 degrees. If the current is ¼ wave ahead of the voltage it is 90 degrees ahead. Another way to say the same thing is that the current leads the voltage by 90 degrees in a capacitor.
If we have a short antenna we find that the current peaks before the voltage peaks.
In a resonant antenna we find that the current and voltage are in phase.
In the case of the long antenna, it acts like an inductor. In an a inductor, when you apply the voltage, the current starts slowly then builds up. The voltage peak comes before the current peak. The common way of expressing this situation is to say that the current lags the voltage in an inductor.
If we have a long antenna, the current will peak after the voltage.
What does all this mean? Well If I have an antenna that is long or inductive I can sometimes use just a capacitor to tune it to resonance. If the antenna is short I may be able to only use a coil or inductor to tune it.
A practical example is the mobile whip antenna. Other than on ten meters the HF mobile antenna will be short. A short antenna is capacitive. A coil placed at the base of a mobile whip is frequently all that is needed to tune that antenna. Depending on the frequency, the capacitance of the antenna will vary and a coil will have to be chosen to suit.
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