Topband: Electrical height of short shunt fed tower?

[James Hall]

This has been a very interesting thread and one that I took notes on. I will
have a new system going up that will be 120 feet of Rohn 45 tower with large
yagis at 60 and 120 feet - 48 ft. booms. I do not know how to model antennas
yet. Being so near 1/4 wavelength tall (and electrically taller), how do I
decide where to 'tap' the tower and how far off the tower the shunt wire
should be ?

73, Jamie
WB4YDL

-----Original Message-----
From: topband-bounces at contesting.com [mailto:topband-bounces at contesting.com]
On Behalf Of Guy Olinger K2AV
Sent: Tuesday, November 10, 2009 4:20 PM
To: Julius Fazekas
Cc: Mike & Coreen Smith; topband at contesting.com
Subject: Re: Topband: Electrical height of short shunt fed tower?

On Tue, Nov 10, 2009 at 7:39 AM, Julius Fazekas <phriendly1 at yahoo.com>wrote:

All the reading and discussions I have over the preceding 6 years suggest
> that anything above the first capacitance hat/wire provides little or now
> benefit. I think there have been articles through the years in magazines
> like Ham Radio, QST and QEX that have explored this topic.
>

In going up the tower to various conductors, the current tends to pick the
LONGEST conductor path "near" the top for the greatest current. Particularly
on 160 where a 1/4 wave is 125 feet, the typical longest path from a given
junction has the lowest Z and will therefore accept the majority of the
current out of the junction.

How this plays out with respect to "first" hat/wire is a purely case-by-case
basis.

.5 times longer may be as good a "swag" rule as any.

73, Guy.
_______________________________________________
160 meters is a serious band, it should be treated with respect. - TF4M




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