[TowerTalk] Broadband Inverted L

[Donald Chester]

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