Don et al,
I sense this discussion is targeting the use of a dipole on a single
band. What is being missed is that many of us use a dipole cut for for
one band and use it for the bands above. What we see are high SWRs on
all of the higher frequency bands. We use balanced feeds, open wire or
ladder line, as far as we can before we transition to coax. That
transition is a good current balun. What this does is control losses due
to high SWR and keeps RF currents off the outside of the coax shield.
What has been said in previous notes might be true, but they will
prevent or make use of balanced feeders quite difficult.
An 80 meter dipole used on 40 or 20, for example, is a pretty good
antenna. However, it begs to be fed with open wire or equal. That means
no balun can be used at the antenna feed point. The exception is to use
very expensive coax. :-)
73,
Barry
K3NDM
------ Original Message ------
From: "Don Wilhelm" <donw...@embarqmail.com>
To: elecraft@mailman.qth.net
Sent: 1/13/2019 6:59:18 PM
Subject: Re: [Elecraft] Slightly Unbalanced dipole?
Al,
If not placed at the antenna feedpoint (up at the antenna), then the feedline
may radiate due to common mode current.
For feedlines using parallel conductors, the problem is not as bad if the
feedline exits at right angles from the radiator for at least a half
wavelength. In that case, the common mode choke can be placed at the junction
of the parallel line and coax.
In all cases of a radiator fed with coax, a good common mode choke at the
radiator feedpoint is necessary. Even though the RF on the inside of the coax
is balanced, when the coax reaches the radiator, the side of the antenna
connected to the coax shield sees two paths. One is the side of the radiator,
and the other is the outside of the coax braid - it will dutifully split to
follow both paths depending on the impedance of each one. A good common mode
choke will prevent the RF current from seeing the coax shield as another
conductor.
A similar situation exists at the transition between parallel feedline and coax.
73,
Don W3FPR
On 1/13/2019 6:42 PM, Al Lorona wrote:
By the way, whenever Jim says, "...a very good common mode choke at the feedpoint of
an antenna...," he means an antenna fed with coax. For the rest of us, of course,
that choke would (should) go at the output of the antenna tuner, whether in the shack or
close by.
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