At 05:03 PM 2005-06-11, Tim Kass wrote:
>If you slope the radials, that will lower the impedance a bit, for a full
>size 1/4 wave ground plane it will match 50 ohms...either way it shouldnt
>change more than about 20 ohms and should still present a decent match to 50
>ohm coax.
I'm sure Tim meant to say that the impedance will go up with sloped
radials. I modeled quarter-wavelength verticals on a 50 foot tower, with
four 45 degree sloping radials. The feed impedance at resonance varied
between 33 and 50 ohms, not bad. (Many people don't realize that a ground
plane with four horizontal radials in free space is about 22 ohms instead
of the 36 ohms plus ground losses seen with the radials on the dirt.) Of
course, with a trap antenna, things will likely be somewhat different.
>However another point, moving the vertical that high may help
>with receive signals, but a lot of your transmitted signals will go straight
>up or nearly so, fine if you want short DX.
There is some increase in high angle energy, but there will still be fairly
good signals at low angles. The worst case I saw in modeling was on 30m,
where a 1/4 wavelength vertical on a 50 foot tower had a lobe of +4.2 dBi
at a 46 degree takeoff angle and +0.8 dBi at 15 degrees. On 20, 15, and
10m, the higher angle lobes were roughly +/- 1 dB with respect to the low
angle lobes. On 40m, the high angle lobe was down -2 dB. Don't forget
that raising a vertical above average-conductivity ground increases the low
angle energy quite a bit. Not like sitting on salt water, but a nice
improvement. (I have the modeled impedances and gains vs. angles if you're
interested.)
Part of the situation, as W8JI often reminds us, is that a ground plane
antenna doesn't decouple the feedline (or in this case, the 50 foot tower)
very well, if at all. When the tower is near a multiple of 1/2 wavelength,
there can be lots of current flowing down the tower (or unchoked
feedline). This is the case on 30m, 15m, and 10m. The modeling I did
assumed the radials are connected to the top of the tower.
73, Terry N6RY
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