On Fri, 10 Sep 2004 14:25:15 GMT, "Jimmie"
wrote:


"Richard Clark" wrote in message
.. .
On Thu, 09 Sep 2004 19:42:41 GMT, "Jimmie"
wrote:

This equates to about 40 ohms
impedance seen at the antenna base. This also agrees with my antenna
analyzer. Antenna is a full length 1/4 wl antenna on 20m. I was figuring
37
or soo for this.


12 12f t wires, Center ground stake 10 ft long, far end of each wire has a
stake about 3 ft long. These are used more to just hold the wires in place.

Hi Jimmie,

My last post responding to this included a mistaken analysis. However,
it has been now thrice plagued. Originally I had picked up the sample
vertical and did not notice it was using MININEC ground, which is
unsuitable for this question. Then in doing the suite of grounds, I
had truncated the radials by half in a frequency translation. Then I
fumbled the wrong key -sigh-

On using the particulars you supply above, the results offer the low
end of ground resistance by your estimate. However this depends on
several things (not surprisingly - ground). Does your analyzer read
real resistance, or simply SWR? What does it read with a 50 Ohm
resistor (taking the average of several, all with close trimmed
leads)?

Anyway, by your SWR reading it appears your situation corresponds to
an "extremely poor ground." Curious thing about the nature of
"extremely poor," is that what does this mean? There is every
indication that it offers the least loss, and yet it also offers the
poorest signal out (at 10° as an indicator of DX).

With extremely poor ground (0.001/3 Zground = 209 Ohms):
Impedance = 39.46 + J 0.234 Ohms
SWR (50 ohm system) = 1.267
-6.01 dBi @ 10°

With poor ground (0.002/13 Zground = 104 Ohms):
Impedance = 42.34 + J 0.1014 Ohms
SWR (50 ohm system) = 1.181
-3.25 dBi @ 10°

With the default ground (0.005/13 Zground = 99 Ohms):
Impedance = 43.64 + J 0.03538 Ohms
SWR (50 ohm system) = 1.146
-3.36 dBi @ 10°

With "Pastoral" ground (0.01/14 Zground = 87 Ohms)
Impedance = 44.93 + J 0.515 Ohms
SWR (50 ohm system) = 1.113
-3.00 dBi @ 10°

With very good ground (0.0303/20 Zground = 57 Ohms):
Impedance = 44.5 + J 0.3779 Ohms
SWR (50 ohm system) = 1.124
-1.11 dBi @ 10°

For each change of ground I drove out reactance to less than one Ohm
by trimming the radiator to what you describe as a full size
quarterwave antenna.

Going back to the nature of ground, we return to the descriptions of
conductivity and how a "very good" conducting soil is NOT what you
want nearby in terms of matching. A "very good" conducting soil is
injecting several ohms more resistance into your feed than an
"extremely poor" quality soil. However, when we consider the Z of
ground, we observe that it is the ratio of this impedance to the æther
that is the better indicator of achieving good low angle launch
characteristics. Consistent here are the SWR of 6.6 for "very good"
soil compared to the SWR of 1.8 for "extremely poor" soil.

There is less conductive loss (but more capture of power) in the
"extremely poor" soil and there is more conductive loss (but less
capture of power) in the "very good" soil. Clearly it is the
REFLECTIVE attributes of the media interface that is the more
compelling characteristic than with the so-called boon of
conductivity.

Two more points of practical consideration he
1. With 12 radials laid out as you have, there is very little
variation over "average" grounds (no point in trying harder);
2. The DX angle is driven by ground characteristics many wavelengths
out (no point in worrying about ground either way).

73's
Richard Clark, KB7QHC