Topband
[Top] [All Lists]

Topband: Detuning towers

To: <topband@contesting.com>
Subject: Topband: Detuning towers
From: jon.zaimes@dol.net (Jon Zaimes)
Date: Mon, 20 Jan 2003 16:43:26 -0500
Pete,

As I was planning my antenna farm here at the new QTH 5 years ago, I took 
great pains to lay out most of the Beverages on one side of the property 
(12 acres) and the towers on another. Until recently I only had one 
Beverage (switchable 342/162 degrees) that came close to a tower, and it 
was 130 feet away from the series-tuned 1/4 wave 160m radiator (100 ft 
tower plus top loading and small linear base loading).

All the other wires were at least 200 feet, most more, from any of the towers.

I did not find any increase in noise floor on the wire that was closest to 
the tower.

Recently, after reading some of W8JI's antenna comparisons, I decided to 
try improving my RDF (receive directivity factor) toward JA and 8N1. I 
decided on two 935-ft phased Beverages aimed at 320 degrees. This meant one 
wire would pass about 20-30 feet from the 160-meter tower, and another wire 
would pass about 10-15 feet from 100 feet of Rohn 45 that holds a 3-stack 
for 20m, 3-el 40m and 5-el 10m yagis.

I first installed the wire closest to the 160 meter tower, and used this 
for several days by itself until I put the second wire up. The single wire 
was a significant improvement over my old, shorter JA Beverage (450 feet) 
that is more than 200 feet away from any tower. I could hear JA signals 
last week on the 935-foot wire that were not there at all on the short 
wire, nor on an 880-foot wire at 342-degrees or other wires at 205 or 290 
degrees. To my surprise, I did not see any significant increase in noise on 
the new long Beverage near the tower. I had fully expected I might need to 
detune the tower during reception.

When I  installed the second half of the phased pair, I initially just 
hooked it up to a separate feedline so I could do some A/B comparisons on 
the two wires. The second wire -- which runs near the Rohn 45 tower -- also 
runs a few feet from the pole carrying the power line feed to my house, 
which has a transformer on it and a vertical service feed down the pole and 
then buried to the house.

The second wire was slightly, but noticably, more noisey than the first. 
This could be from coupling to the Rohn 45 or the power line feed, or both. 
Yet despite the slight noise increase, I found I could hear the weakest 
signals about the same on either wire; the noise on No. 2 wasn't enough to 
mask any signals that I could also hear on No. 1.

So I went ahead and installed the phasing lines on the pair on Saturday 
morning. A benchmark test against a BC signal a few hundred miles to the NW 
found the pair had several DB more gain than the single wires, as expected. 
These wires are spaced 177 feet. As Tom has pointed out, 350-400 feet would 
have been more ideal but I don't have the room to do that without going 
under the power line feed, and I had tried that previously with some 
Beverages and found they picked up a lot of noise from the power line.

Sunday we had zero signals from JA here, so no comparisons possible. Today 
we had a good opening from 1155-1240z (sunrise was 1218) and the phased 
pair was clearly the best antenna -- in many cases the only antenna I could 
hear the JAs on, and in all cases better than any of the other wires.

During this comparison period we have had fairly clear weather conditions 
-- only one snow day and the storm center had already passed to the NE of 
us by 1100z, and that day seemed as quiet as the rest. I expect I may see 
more noise pickup under more extreme weather conditions, as I found with a 
much shorter (185 ft) Beverage that ran 50-feet from my shunt-fed tower at 
another QTH back in the '80s. Coupling to the tower with that antenna 
actually increased signal pickup, but if a storm was nearby I had to detune 
the tower (by switching out the feed in the shack) and the noise and 
signals dropped.

73/Jon AA1K


<Prev in Thread] Current Thread [Next in Thread>