Oops! posted to the wrong list. This was supposed to go to the Topband list.
But some readers here still might find it interesting and informative.
Don k4kyv
> Thu Nov 20 W8JI wrote:
>>A 40 ft long wire laid on earth to the radials, even if Joe had 50 x 100 ft
>>radials, would almost certainly make the ground path impedance hundreds of
>>ohms.
>
> My 160m vertical tee has an extensive buried radial system that's probably at
> least 2× overkill. The ATU is located in a small wooden 'dog house' about 7
> feet away from the base of the series-fed tower. The grounded side of the ATU
> connects to the radial system via a 2" wide copper strap buried just below
> the surface of the ground, and the insulated tower base via apiece of #4 AWG
> solid copper wire elevated about 2 1/2 ft above ground. Sag and bends add
> about another foot to the total lengths of the ground strap and feed wire. I
> was amazed at how much that 8' run changes the measured impedance of the
> antenna.
>
> At 1800 kHz, right at the tower base, the impedance reads 117 ohms + j233. At
> the point where the feed wire is connected to the ATU, the reading is 140
> ohms + j269.
>
> With an earlier configuration, using the same tower and radial system but
> with a different top hat, the readings were:
> At the base of the tower, 28.8 − j10.5 At the ATU feed point, 36.5 − j16.1
>
> Even the few inches of wire running between the antenna disconnect switch
> inside the dog-house and the connection to the ATU coil made a measurable
> difference in the readings. It's not hard to imagine what a drastic effect
> the 40 ft run to the offset radial system would have on tuning and
> performance.
>
> I suspect the physical offset between the antenna base and the common point
> connection to the radials, and ground losses from the 40 ft run of wire lying
> on earth, affect the antenna performance more than the mere impedance of the
> ground path. If the extra impedance added in series with the inverted L were
> the only factor, that could easily be compensated for in the matching
> network, with negligible loss.
>
> I have long wondered how much loss my buried ground strap introduces
> into the system, and have considered replacing it with a second piece of
> wire closely parallel to the existing feed wire, but have never gone to
> the trouble to try this and take any measurements, plus the strap
> probably offers better lightning protection.
>
>
> Don k4kyv
>
>
>
>
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