Topband: FCP model
weeksmgr at hotmail.com
Wed Aug 1 12:26:58 PDT 2012
On 1 Aug 2012 09:49:13 Jim at audiosystemsgroup.com wrote:
> You can get at least a first approximation by modeling a dipole at the
> height where you plan to put radials. I did that this spring in
> preparation for this elevated radial project. I did a series of
> computations, each for a different height, varying the length for each
> height until the "dipole" was resonant. What we are seeing is the
> change in the velocity of propagation along the antenna caused by
> proximity to the earth.
I can model the inverted L and get good results, but the wire length for the radials always ends up in the model being 10' to 15' longer than in the real world.
I started out making the radials 130' long and trimming from there, an equal amount on each radial. After building several on my site, it became obvious these were ending up ~120' long or slightly less, so when building one here 120' is now the starting length. They are usually pretty close if the wire is ~6' above ground.
I have noticed that radials installed on other local sites may require a different length from my site, likely due to different ground properties.
Here at my place, the measured velocity factor of a DOG (on ground dipole) is lower than what you measured. I also find that changing the height within the 0 to 6' range changes the resonant frequency through the range. Not sure at what height the resonant frequency would stabilize here.
If I could put down a dense radial field, that would be the way to go. Unfortunately, that is not in the cards here. Operationally, I am happy with the way these antennas perform, but it could always be made more efficient. Currently, my 160 success seems to be receive antenna limited, and we are working on improvements.
73 Charlie N8RR
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