Counterpoise or radials?
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| Oct 8th 2011, 03:56 | |
KD5KJTotal Topics: 0 Total Posts: 0 |
I guess I am just dense but in the ARRL Simple and Fun Antennas Book (great BTW), a Cheap and Dirty Multiband antenna is described with a single 1/4 wave counterpoise. Of course, the author says it works great and it is horizontally polarized apparently. However, if the same antenna was to be installed vertically at near ground level, most "authorities" require a series of over 16 radials (counterpoises, if you will). I somewhat understand ground reflection and resistance losses but it logically seems that with added ground reflection this antenna installed vertically should require no more counterpoise than a horizontal antenna with a single "radial". For example, a ground plane antenna with only two radials performs almost as well as one with four or more radials. I have installed several "L" antennas (not inverted L) for HF with the bottom "counterpoise" and feed point at about 8 feet over ground and they seemed to work very well. As I said, I am a little slow but I have been a ham for over 40 years and this still mystifies me Anyone else have this type of experience? |
| Oct 8th 2011, 06:29 | |
KM3FTotal Topics: 0 Total Posts: 0 |
I may not understand what your question is fully by the way you present it. .. As I have understood for along time, radials and counterpoise is actually one and the same, just different names given to same part of the antenna system. Either one is the "other half" of the total antenna system no matter what the configuration is. Think of a tank circuit with the RF circulating between the inductor and the capacitor. An antenna system also is a circulating configuration. Just that the radials or counterpoise is one half of the total antenna system the antenna current circulates between. A difference is the radials present resistance/higher loss to the circulating current and have an effect on the pattern when there are only a few, two or one. The radiating element also has resistance and contributes to the losses. This means that more radials offer less loss to the circulating current much like resistors in parellel end up with a total value 'less than the least' of the lowest value in the total number. The lower these losses the more current is left to radiate. This is the efficiency of the system in total. In a tank circuit, how do you 'take power out? It is coupled out by a capacitor, or an inductive link of some sort. With an antenna, coupling to the surrounding meduim one way power is coupled out. As power is coupled out, the RF power source replaces what is lost by both radiation and losses in the hardware and it's configuration. And we have not even addressed system Q yet, in all this. . As to reflection, be a little carefull how the term is used. Right under the antenna, the system sees a mirrior reflection of it'self. This has direct effect on the radiation pattern. As a vertical with it's radial sytem is raise off the ground, the radiating element begins to see the raised radial as it's main ground instead of the earth. After all ,it is the 'other half' of the system Good luck. |
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