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
On 1/21/2019 6:45 PM, Todd Goins wrote:
Hello,
Per many people's recommendations I added 800ft of radials today. That is 8
x 100ft each. It made a difference on the analyzer which I'll summarize
below. It was dark when I finished but here are a few data points. I think
it is better. The wide SWR curve still bothers me but the resistance is
coming up.
This is also using the new K9YC cookbook choke with 18 turns of RG400
around a 2.4" type 31 toroid.
The values are Freq, SWR, R, X, Z
1810 1.43 42.0 -14.5 44.5
1820 1.31 43.2 -10.7 44.5
1830 1.21 44.1 -6.7 44.7
1840 1.13 45.0 -2.5 45.1
1850 1.10 45.9 1.7 45.9
1860 1.14 47.0 5.7 47.3
1870 1.23 48.0 10.1 49.1
1880 1.34 49.1 14.7 51.2
Any thoughts? The 160 CW contest is only 4 days away, I don't have a lot
more time to make changes but I could run a "few" more radials...
73,
Todd - NR7RR
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