[TowerTalk] Balun on a vertical

[Andrew Roos]

Hi Duncan

I had a look at the website, and the antennas appear to be groundplane
verticals, using two radials and fed with open wire line or similar. The
GP 80/10 is 9.8m tall, and specified for 80m - 10m. The GP 160/10 is
12.6m tall, specified from 160m to 10m. I'm guessing that the antennas
aren't loaded or fitted with baluns as the power rating is specified as
"depends on used tuner and feedline".

My concerns are:

1. If fed with balanced line as recommended, there will be unbalanced
currents in the feeder, causing it to radiate. This is likely to 

(a) reduce the efficiency of the antenna (because feedlines typically
run close to ground and other places that we wouldn't put an
intentionally radiating element), 
(b) upset the pattern (e.g. with HP radiation from feedline filling in
the overhead null), and 
(c) may cause "RF in the shack" problems. 

2. The antennas will suffer from elevation pattern breakup at the higher
frequencies, where the length exceeds about 5/8 wavelength, resulting in
a high elevation angle of maximum radiation and poor DX performance. For
the 160/10 this would be a problem on the 15, 12 and 10m bands. The
80/10 should be OK up to 15m. 

3. The 160/10 seems a bit short for efficiency on 160m. I make the
impedance of the driven element about 2.6 - j 764 at 1.8 MHz. Ground and
tuner losses will nail you badly.

My recommendation to make this antenna work would be to use a remote
auto ATU at the antenna base (which solves the radiating balanced line
problem), use the 160/10 but only from 80m to 20m (to solve the
efficiency and pattern breakup problems).

BTW I'm not sure whether the 160/10 has one or two ground radials. I
would definitely use two running in opposite directions. 

73
Andrew ZS6AAA



-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Duncan Lindsay
-MSC Valencia-
Sent: Tuesday, October 10, 2006 10:31 AM
To: towertalk at contesting.com
Subject: [TowerTalk] Balun on a vertical

All very interesting.

 

My original question, which I think got buried at the end of my message
was:

 

How should we expect the Titanex multiband verticals to work? (I'm
talking about the models GP 160/10, GP 80/10. etc). They are essentially
unbalanced verticals (a vertical element on one side, radials on the
other) which they say can be fed with balanced feeder up to an ATU.

 

I am still going round in circles to find a way to put up a cheap and
cheerful multiband vertical for using on the low bands at our new
contest site. I liked the look of the Titanex idea, nice and simple, but
in the back of my mind have a niggling worry that it's not a 100% good
idea... 

 

73 de Duncan EA5ON

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