[TowerTalk] Limited space for 80 and 50
k0dan at comcast.net
Wed Nov 20 13:46:22 EST 2013
I agree that the dipole/Zepp is worth rescue. I would consider trying 450 or
600 ohm line.
Ideally balanced antennas should be perfectly balanced, but in my experience
you can tolerate a lot of slop and still have an excellent antenna. Which is
to say you don't need to achieve "perfect" balance. The more balanced the
better, but there is a lot of tolerance in the system.
With the feedline, close proximity to metal objects should be avoided (by at
least a foot...more distance is better), sharp bends in the direction of the
feedline should be avoided. Twist of the feedline is OK, provided it doesn't
fold back on itself. If a small portion of the antenna or feedline is close
to large metal objects (metal roof, window sash, etc.) it is probably OK.
Where large sections of the antenna or feedline are in close proximity to
metal objects, this needs to be changed.
Antenna tuners which claim to handle "balanced lines" should be looked at
with some skepticism. Most commercial tuners are not truly balanced (both
sides should be above ground potential), and if your antenna network has
impedance issues, a T or L tuner may not help things. There are a number of
true balanced tuners: Rich Measures had an article for a homebrew one, and
the EFJ "Matchbox" tuners are link coupled balanced devices. I think there a
few manufacturers who make true balanced tuners today.
Sometimes a slight change in the length of the open wire feedline will help
"tune" the antenna network and achieve better balance (see Cebik articles).
Sometimes a balun or coax choke does not accomplish what is expected of it.
If you are having feedline radiation which is getting into consumer devices,
it is a sign that the antenna NETWORK is out of balance.
While a vertical is a good addition to any antenna farm, so is your
dipole...I'd take a look at what is causing the problem before tearing it
GL es 73
From: ve4xt at mymts.net
Sent: November 20, 2013 08:20
To: Jim in Waco wb5oxq
Cc: TowerTalk at contesting.com
Subject: Re: [TowerTalk] Limited space for 80 and 50
It would be a shame to not be able to use this dipole, since your vertical
antenna won't come close to it without a lot of radials.
I suspect the problem isn't the antenna but rather the feedline.
I suspect the interference is a combination of feedline radiation and the
Pin 1 problem on your entertainment gear. Even some so-called high-end
consumer electronics suffer from poor engineering.
Have you read any of K9YC's information on chokes and RFI (incl. Pin 1)?
Sent from my iPhone
> On Nov 19, 2013, at 21:57, "Jim in Waco wb5oxq" <wb5oxq_1 at grandecom.net>
> I have a full size 80 meter dipole at 60' in the center fed with 300 ohm
> window line to a good tuner with the balanced input. I cannot use it on
> 80 or 40 meters even at levels of 25 watts or more because of
> interfearance to my entertainment center despite snap on chokes on all
> input leads and every speaker lead and a high end ac line filter.
> Therefore if I want to operate any frequency below 14mhz I feel I must try
> a vertical in a side yard located outside the shack. The area is about
> 25X40 feet. Radials can be put down in that area There is a 50' tower
> with a triband beam in one corner of that area. No other metal is in that
> area. The feedline length to the shack is about 20'. There is no height
> restriction in this area. I only need the antenna for 75 and 40. There
> is a 6' tall wood privacy fence around this area. I would hope that using
> a vertical in this area would fix the rfi problem since the antenna would
> be a good 40' away from the entertainmant center whe
> as the center of the dipole is directly above it. Suggestions please.
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