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Topband: Shunt feed tap point and resulting radiation pattern

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Subject: Topband: Shunt feed tap point and resulting radiation pattern
From: (John Devoldere)
Date: Mon, 31 Dec 2001 10:26:13 -0000

The radiation pattern is determined by the antenna (including ground,
radiale etc which are really part of the antenna), but is independent of the
matching (feed) system, provided we are talking feed systems that produce
the same current distribution on the antenna.

Whether you use a Gamma or an Omega match to shunt feed the tower makes no
difference in radiation pattern. The only thing that could be differenet is
the power loss in the feed/matching system, and also its bandwidth.

The best matching system is the system wiith the least number of components,
so a gamma is to be prefered over an Omega. That is theory. Whether the
extra loss in the extra component is meaningful is a different thing.
Probably NOT. But it is certain that a Gamma match, achieving a direct 50
ohm feed impedance, will, with the same tower, achieve a greater bandwidth
than an Omega match.

The antenna can "work quite well" (as you quote it) with a "not at all
lossless Omega". Who can ever tell the difference? We are probably talking
fractions of a dB. But of course, a top notch station is a station where no
compromises are made, because a fraction of a dB here added to a fraction of
a dB there etc etc adds up to many lost QSO's... It is good engineering
practice to opt for the best solution, if the price is right. And the Gamma
solution should not cost more (less!, there is one less component!) than the
Omega solution.

Happy New Year to all, and let's hope for a revival of Top Band conditions.


John, ON4UN

----- Original Message -----
From: "Pete Smith" <>
To: <>
Sent: Sunday, December 30, 2001 1940
Subject: Topband: Shunt feed tap point and resulting radiation pattern

> For a given shunt fed tower, mast, antennas, radial field, etc., is it a
> good generalization to assume that the only free variable in determining
> the radiation pattern shape and strength is the height of the tap?  Put
> another way, if the antenna seems to be working well, but the tap
> is quite low (matchable to 50 ohms with an omega match), is there any
> reason not to leave the tap where it is, rather than searching for a
> point?
> 73, Pete N4ZR
> The revised World Contest
> Station Database
> is online at
> _______________________________________________
> Topband mailing list

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