[TowerTalk] Topband: Shunt Fed Question

[Peter Voelpel]

-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Peter Forbes
Sent: Montag, 8. Dezember 2008 04:05
To: Peter Voelpel
Cc: towertalk at contesting.com
Subject: Re: [TowerTalk] Topband: Shunt Fed Question

Peter,

The late John Lewis, VK3HW had a setup similar to what you describe.

He had a steel electricity transmission tower with a full size 3 el 40mx
yagi on top around 120 feet.

Initially he tried gammas up to the landing on the tower @ about 110 feet
with no success.

EVENTUALLY, he tried a feed point about 50 feet up the tower, but had to use
an omega match at the base to get a suitable impedance match.

Trying to use a gamma match and adjusting the spacing from the tower would
be a time consuming exercise.

If you intend running any sort of high power, then you really need to have 2
large capacitors, preferably vacuum variables.

Have a look at any of the Kraus textbooks or read some of the writings of
Paul Lee N6PL, W3JM, on matching vertical towers in backcopies CQ magazine

Good luck

Peter  VK3QI



Hi Peter,

I made the radials about 174 ft long initially. I connected only one at the
time through a variable capacitor to my MFJ-259 and tuned for SWR min on
1850kHz between the cap and the tower connection.
Then I tuned the second radial by pruning its length and height above ground
at the far end to the same frequency without changing the cap.
Finally I bonded both radials together and tuned them through a 500pf vacuum
variable to 1870kHz. SWR min. is 1.3, SWR2 bandwith is 90kHz. QRO is no
problem.

73
Peter