Topband: Rationalizing my radial field through measurement

[Shoppa, Tim]

My 160 TX antenna is also my 130-foot-doublet. It's up about 80 feet and fed with ladder line. To use on 160, I tie the feedline together at the bottom and feed against ground as something like a "Marconi T" using a L-network match right at the base.

The rest of this post is about our favorite subject, "ground".

The base of the T is the corner of the house. At this corner... one quadrant is the house. Another quadrant is the garage. The third quadrant is the driveway. And the fourth quadrant is the back patio. Everything is covered by building, concrete, or asphalt except for like a 45 degree wedge towards the back patio.

So the radial system is at best haphazard. On contest weekends I make an effort to go out and unreel about 1500 feet of wire over the backyard, across the driveway to the yard, and across the garage floor and out the garage windows and over the lawn, and across the basement ceiling to the far side of the house. In all there ends up being about 24 radial wires of lengths ranging from 30 feet to 120 feet (the corner of the house is in no way, in the center of the yard, which is rectangular but also with some odd triangle corners too!). I might take a picture and post it just so you guys can laugh at it.

If I were to measure RF current in each radial when TX at the 100 watt level, with the antenna system impedance of 30+100j (from modeling and in good agreement with actual L match), I would expect that antenna RF current equals ground current equals about one amp. So if one amp is evenly spread amongst 24 radials, an average AC current of maybe 40mA.

There's probably some net RF current back through feedline too even though I've got a #31 ferrite core with coax round through it to serve as a choke there.

If I wanted to measure the radial and feedline AC current, not having any RF ammeters, I might just take a #47 lamp or other small pilot lamp and stick it in series with the radial and transmit into it. 40 mA will make a #47 lamp glow very dimly. Is this a good thing to check on? If I find that some radials don't carry enough current to light up the lamp... is that a sign that the low current radial is not needed? If other radials are carrying way more than average current... is this a sign I need more radials in that direction? Or is it the other way around? Or is this all academic and I just need another few thousand feet of copper wire? :-)

Tim N3QE