On 4/5/2011 8:50 AM, Sam Carpenter wrote:
> I have always (almost?) run either balanced antennas with Baluns
> (yagis-dipoles). My problems happened on 75 mostly with phased sloper
> arrays, corner fed delta loops, and flat top 75m dipoles at 80' directly
> overhead. 3 ground rods in good soil and common point shack ground and
> braded ground wire fixed it. I cannot be certain that a lot of it was not on
> braids of feedline coming in, but it should not have been on center fed
> resonant dipoles with baluns, nor should the delta loop. It can be argued
> that a sloper could use the vertical support (tower) as the primary radiator
> and the sloper as the counterpoise. That could render the ground radials
> below the tower useless. These were not balun fed. I had always assumed that
> I was in the path of a good level of radiation and not that I had feedline
> radiating. I have learned something.
Sam,
There are at least a half dozen interesting antenna concepts in your
post. The "RF in the shack" part of the problem can be solved by more
effectively de-coupling your feedlines with very good common mode
chokes. See http://audiosystemsgroup.com/CoaxChokesPPT.pdf which are
the Power Point slides (as a pdf file) for a presentation I've done at
conventions and ham clubs on the subject. Just because an antenna
appears to be balanced and fed with parallel wire feeders does NOT mean
that it is ideally balanced, or that there is no feedline current.
Antennas can be unbalanced a bit by their surroundings, which puts
common mode current on the feedline, and in the shack.
See http://audiosystemsgroup.com/RFI-Ham.pdf for detailed advice on
winding very good chokes (much better than you can buy, and for a lot
less money).
As to your sloper -- radials at the tower ARE part of the antenna
system, but they may not be ALL that you need, depending on how you're
feeding that sloper. If you're not already using the NEC antenna
modeling software to study antennas, you can learn a LOT by spending
some time with it.
There are several ways to do slopers. I have two sloping wires for 160
coming off my 120 ft tower, one to the east and one to the west. They
are insulated from the tower and fed from the bottom (two feeds, one
for each antenna). The tower is grounded and acts as a passive
reflector, giving me a bit of directivity in the direction of the slope.
I also have radials under each of the two feedpoints. They are all
important -- when you study the NEC model, it's clear that there is a
lot of current at the base of the tower, just as there is a current peak
at the feedpoint (the bottom of the sloping wire).
73, Jim K9YC
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