WA4PUB
Dave Budner
Marietta, GA
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160 meter vertical antenna's base loading and matching unit
The following pictures show what my 160M vertical antenna base loading and matching coil assembly looks like. Yes, I managed to shoot a line up over one of my HUGE (about 135 feet tall) Georgia Pine trees. Then I pulled up about 125 feet of wire for the antenna itself. I feed it at the bottom with a combination loading and matching unit. I operate the system as a quarter wave vertical.
The unit's coil form was obtained from military surplus (MARS Program). It is a ceramic unit. Its diameter is 3-1/2 inches by 10-3/4 inches in length.
There are actually two coils, connected in series. The upper part of the winding(s) consists of 37-3/4 turns of (silver plated) #10 AWG wire (spaced approx. 3/16" on center). It is silver soldered to a concentrically wound second lower coil, consisting of 4-3/4 turns of (1/4 inch diameter silver plated) tubing. This portion of the coil is wound on 1/2 inch centers.
The very lowest terminal on the "coax connection bar," about 2-1/2 inches below the coax cable connector, is where the driven ground rod and/or ground radial system attaches. The ground connection is made with the shortest but largest convenient AWG wire size connection. In my case, I used a sheet metal strap cut from 20 gauge aluminum, 1 inch wide and 6 inches long for connection to a driven 10 foot long, 1/2 inch diameter, copper clad steel ground rod. At this time, I don't have any ground radials and rely on the driven ground rod. We've had LOTS of rain these past several days.
For tune up, I connected an SWR bridge to the SO-239 coax cable connector. By adjusting the upper coil tap (this is where the antenna wire connects to the coil), I can change the antenna's resonant frequency. The center conductor from the coax connector is tapped up (from the bottom of the coil) for lowest SWR. Both taps interact somewhat, so it's usually necessary to alternately readjust them for unity (best) SWR.
In my case, the antenna connection is tapped 16-1/2 turns down from the top of the 10 gauge coil. The matching tap is connected 35-1/4 turns down from the top of the coil. Right now, I have 1.2 to 1 SWR. I didn't have enough time to make the adjustment. It is possible that I might need to place a series connected variable capacitor in the tap to tune out any inductive reactance in the matching portion. I'll worry about that later.
It really worked great. Even had a QSO with a guy in Missoula, Montana. I used 100 watts from my Kenwood TS-440SAT transceiver, and got a Q5 by S6 report. As expected with the vertical, I had lots of receiver noise. There were probably many signals buried in that stuff. I'll have to look at that problem later.
Dave, WA4PUB
12/6/04
