So, now we're (apparently) recommending he cut back his already minimal
radial field..uhhh, really Wes?
=-Mike VE9AA
I started this message a day or so ago. Others have commented since with
some similar thoughts, nevertheless, here is my take.
Todd you're going the wrong direction. The feed point resistance should be
going down.
A 1/4 wave wire vertical should have a radiation resistance(Rr) of around 35
ohm. Ignoring conductor loss, over a perfect (zero ohm) ground, the feed
point resistance is also ~35 ohm. The Rr of a shortened, top loaded
vertical (inverted-L) will be lower than that, the shorter, the lower.
Ground loss resistance (Rg) that appears in series with the Rr increases the
feed point resistance to Rr + Rg, assuming resonance. Rg also lowers Q.
Since your total feed point resistance is increasing, as is BW, you must
have increased Rg. Although this seems counter-intuitive, experiments have
shown that, on or in, the ground radials can be too long. See: "Vertical
antenna ground system experiment No. 4" by N6LF.
For my inverted L, I cut my insulated-on-the-ground radials to 55' because
1) that's the height of the vertical part of the L; 2) it gives me 9 radials
with no waste from a 500' roll of wire and 3) that's the longest length I
can use and still maintain symmetry. I recently measured the Z of one of
these against the rest of the ground system with a network analyzer and by
serendipity found it resonant at ~1.9 MHz.
At the moment I have only 12 radials, although I plan more (see my QRZ page
to see why I haven't rushed into this). Around resonance, a Smith chart
display of a model of the antenna feed point Z overlays nearly identically
the measured Z, if I add 13 ohm simulated ground resistance to the model.
That is the apparent ground loss.
Wes N7WS
Mike, Coreen & Corey
Keswick Ridge, NB
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