The last post on the 15 meter antenna brought up the radiation angle and the height of the ionosphere. I thought I would switch gears here and discuss some radio wave propagation. To make effective use of any antenna or to choose the right antenna it helps to know something about propagation or how a radio wave gets from here to there.
Most of our high frequency radio communication depends on the ionosphere bending the radio signal so it will return to earth. The ionosphere is an area of the upper atmosphere where the air is much less dense than here on the surface of the earth. It is also gets the full force of the radiation from the sun. It is the radiation from the sun that causes the air molecules to form ions. Positive and negative charges are created by the radiation from the sun. The strength of the radiation from the sun varies considerably. To put it simply, the radiation varies somewhat predictably with the number of sunspots following an eleven-year cycle. In other words there is usually a peak in the number of sunspots every eleven years. This peak in sun activity coincides with extremely good radio conditions on the higher frequency amateur bands such as 20, 15 and 10 meters.
In addition to the effect of the sun spot cycle, there is a seasonal and daily variation. The daily variation is mostly due to the fact that as the earth rotates, areas of the atmosphere or ionosphere are in view of the sun or being shielded by the sun. We find that during the day the ionosphere is “stronger” than at night. This is really a two edged sword. The higher daytime ionization helps return the higher frequencies such as 14, 21 and 28 MHz back to earth. It has a detrimental effect, however, on the lower frequencies (longer wavelengths). During the day a dense lower ionosphere layer forms. It is called the “D” layer. This strong daytime or “D” layer of the ionosphere absorbs the longer wavelengths. For this reason there is practically no sky wave propagation on 160 or 80 meters during the day. The 40 meter band is right in the middle. There is not much absorption on 40 meter signals so there is some sky wave propagation on 40 during the day. During the evening the “D” layer disappears and there is good sky wave propagation on the 160 and 80 meter bands and signals even get stronger on 40 meters.
Very high sun activity causes so much ionization that the 20, 15 and 10 meter bands can stay open for sky wave propagation 24 hours a day. Except for these periods of high sunspot activity, these bands are mostly considered “daytime” bands.
In general the higher the frequency, the more ionization necessary for sky wave propagation. This means that in the evening 10 meters will close first, then 15 meters will close and the last high band to close will be 20 meters. In the morning the reverse is true. 20 meters opens first, then 15 and then 10. The times all depend to how much radiation there is from the sun at any given time.
Saturday, March 6, 2010
Tuesday, March 2, 2010
The 15 Meter Dipole and elevation angles
The higher bands (15 and 10 meters) are now starting to open for DX.
If you do not have an antenna for these bands yet, I suggest building a simple dipole fed with coax. For lengths of up to 100 feet, RG 8X is not a bad choice. The loss for 100 feet of RG8 X should be less than 2 dB at 15 meters. Less loss will require larger and heavier coax. I feel it is most important to get the dipole outdoors if possible. It does not have to be real high as long as it is away from any nearby objects, especially metalic objects. On 15 meters I consider 20 to 25 feet high. That height puts the antenna a halfwave above the ground. Any antenna that is a halfwave above the ground is frequently a better general purpose antenna than one twice as high. I will go into more detail about antenna patterns at a later date. For now let's just say that I think the magic number for the height of a horizontal antenna (not a vertical antenna) is one half wavelength!
That's 16 feet for 10meters, 22 feet for 15 meters and 33 feet for 20 meters. Antennas that are in the clear, away from any nearby objects work very well at these heights. One problem may be 16 feet on 10 meters. It may be hard to get a 10 meter antenna in the clear at only 16 feet. In that case the best solution is to double the height to 32 feet. The same goes for the other bands. Doubling the height to one full wavelength will give added performance at long distances but will produce a reduction in performance at an intermediate distance. I am most interested in DX on 15 meters, so I chose to put my 15 meter dipole at about 44 feet. That height is acheivable because I have some nice tall trees I can use for support. I chose to do the same for my 10 meter dipole. If I ever put up a tower again I expect to only go about 35 feet. I have had my 20 meter beam at 70 feet and if I put it back up I think it will do almost as well at a halfwave as at one full wave. I also will not have the null at a vertical angle of 30 degrees.
All horizontal antennas will exhibit a decrease in performance or null at an elevation angle of 30 degrees. This elevation angle translates to loss of performance at distances of 350 and 700 miles (1 or 2 hop E skip) and distances of 550 and 1100 miles for F layer skip. These numbers vary quite a bit because they depend on the height of the ionospheric layer layers. These heights vary with the time of day and sun activity. Variations in the F layer height could cause this distance of reduced performance for a horizontal antenna one full wavelength above the ground to vary form about 400 to 800 miles.
If you are most interested in these distances be sure your antenna is not a full wavelength above the ground. A horizontal antenna half as high (one-half wavelength) will have its maximum performance at these distances and (all other things being equal) could actually be 10 dB stronger than the higher antenna!
If you do not have an antenna for these bands yet, I suggest building a simple dipole fed with coax. For lengths of up to 100 feet, RG 8X is not a bad choice. The loss for 100 feet of RG8 X should be less than 2 dB at 15 meters. Less loss will require larger and heavier coax. I feel it is most important to get the dipole outdoors if possible. It does not have to be real high as long as it is away from any nearby objects, especially metalic objects. On 15 meters I consider 20 to 25 feet high. That height puts the antenna a halfwave above the ground. Any antenna that is a halfwave above the ground is frequently a better general purpose antenna than one twice as high. I will go into more detail about antenna patterns at a later date. For now let's just say that I think the magic number for the height of a horizontal antenna (not a vertical antenna) is one half wavelength!
That's 16 feet for 10meters, 22 feet for 15 meters and 33 feet for 20 meters. Antennas that are in the clear, away from any nearby objects work very well at these heights. One problem may be 16 feet on 10 meters. It may be hard to get a 10 meter antenna in the clear at only 16 feet. In that case the best solution is to double the height to 32 feet. The same goes for the other bands. Doubling the height to one full wavelength will give added performance at long distances but will produce a reduction in performance at an intermediate distance. I am most interested in DX on 15 meters, so I chose to put my 15 meter dipole at about 44 feet. That height is acheivable because I have some nice tall trees I can use for support. I chose to do the same for my 10 meter dipole. If I ever put up a tower again I expect to only go about 35 feet. I have had my 20 meter beam at 70 feet and if I put it back up I think it will do almost as well at a halfwave as at one full wave. I also will not have the null at a vertical angle of 30 degrees.
All horizontal antennas will exhibit a decrease in performance or null at an elevation angle of 30 degrees. This elevation angle translates to loss of performance at distances of 350 and 700 miles (1 or 2 hop E skip) and distances of 550 and 1100 miles for F layer skip. These numbers vary quite a bit because they depend on the height of the ionospheric layer layers. These heights vary with the time of day and sun activity. Variations in the F layer height could cause this distance of reduced performance for a horizontal antenna one full wavelength above the ground to vary form about 400 to 800 miles.
If you are most interested in these distances be sure your antenna is not a full wavelength above the ground. A horizontal antenna half as high (one-half wavelength) will have its maximum performance at these distances and (all other things being equal) could actually be 10 dB stronger than the higher antenna!
Verticals if you can not get horizontal antennas "high"
I have recommended that horizontal antennas be a half wave above ground for best performance. For the lower bands, this states to be a problem for most of us. While a half wave on 20 meters is only 33 feet, on 40 this becomes 66 feet and on 80 meters this is now 120 or 130 feet! Most of us can only have "low" dipoles on 80 or 160 meters. In order to have a good performing antenna on these bands, especially for DX, we have to either put up a very high tower or use some form of vertical. It is amazing how much improvement a simple bent vertical or inverted L antenna can be over a dipole at the same "low" heigth.
The low dipole does a great job for distances up to maybe 500 miles on 160 or 80 meters. If you want a reasonably strong signal at greater distances, some form of vertical is porbably your answer.
I was able to hang a half square antenna for 80 meters between two pine trees. I was not able to get the vertical elements the desired 66 feet high so I bent the bottom ends. This does not affect the performance much at all because, in this antenna, the two ends at the ground are open. (Not connected to anything). What do we know about the open ends of an antenna? The current is zero at the ends. So the last 5 or 10 feet on a 80 meter antanna wire dies not cary much current. If there is not much current we find that there is not much radiation. Most of the radiation from an antenna is from the high or higher current areas on the antenna. The maximum height of my half square antenna is only about 50 feet. For working DX, say Europe for examply, I would need an 80 meter dipole about 130 feet high to equal the 50 foot high half square! The antenna mocking programs such as EZNEC predict this, and I can say that my experience with this antenna has verified that prediction time and again.
The low dipole does a great job for distances up to maybe 500 miles on 160 or 80 meters. If you want a reasonably strong signal at greater distances, some form of vertical is porbably your answer.
I was able to hang a half square antenna for 80 meters between two pine trees. I was not able to get the vertical elements the desired 66 feet high so I bent the bottom ends. This does not affect the performance much at all because, in this antenna, the two ends at the ground are open. (Not connected to anything). What do we know about the open ends of an antenna? The current is zero at the ends. So the last 5 or 10 feet on a 80 meter antanna wire dies not cary much current. If there is not much current we find that there is not much radiation. Most of the radiation from an antenna is from the high or higher current areas on the antenna. The maximum height of my half square antenna is only about 50 feet. For working DX, say Europe for examply, I would need an 80 meter dipole about 130 feet high to equal the 50 foot high half square! The antenna mocking programs such as EZNEC predict this, and I can say that my experience with this antenna has verified that prediction time and again.
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