Vic,
The simple answer is that the common mode chokes available to hams
are of the unbalanced type. That means they are designed to be
unbalanced in to unbalanced out. Some companies make baluns that are
both current baluns that act for choking common mode currents and
transform a balanced load to an unbalanced load. If you place a common
mode choke at the antenna feed point, the system will be unbalanced,
usually.
Next, most tuners that have built in baluns use voltage type baluns
and can be less than good in a high SWR situation. However, using a good
current balun at or neat the tuner usually prevents common mode issues.
However nothing is 100% when dealing with RF.
Putting reactance in the line to counter what is already there is
good when dealing with the impedance, but you are correct about
complexity. The very simplest configuration that I know in a multiband
situation is to erect a dipole for the lowest frequency to be used and
feeding it with ladder line, or equal. Where you enter the house mount a
4:1 current balun and connect coax from there to the tuner. As long as
your tuner can handle the SWR and the coax run is short, you need not
worry about losses due to SWR; I am speaking to the 1 or so db losses
over just the transmission line loss. If you use the tuner in your
Elecraft gear, I just wouldn't worry, period. Elecraft tuners appear to
be able to handle anything I've put in my backyard so far.
The Johnson matchbox line of tuners were excellent. I no longer have
mine; selling mine was a less than brilliant move on my part, in
retrospect. However, I don't usually recommend this approach. Getting
through some of the walls/windows in the houses around here with
balanced line can be problematic. So, I usually recommend the approach
above for overall simplicity and assurance of maintaining feed balance.
73,
Barry
K3NDM
------ Original Message ------
From: "Vic Rosenthal" <[email protected]>
To: "Al Lorona" <[email protected]>
Cc: [email protected]
Sent: 1/14/2019 1:12:37 AM
Subject: Re: [Elecraft] Slightly Unbalanced dipole?
Why would a common mode choke at the antenna feedpoint present more problems
than the common practice of feeding a balanced line through a single-ended
tuner and balun?
I’ve done a lot of experimenting with the latter and have learned that a) the
best configuration is with a 1:1 balun, and b) the balun will become lossy and
heat up if the impedance it sees is highly reactive. The solution to b) is to
cancel the reactance by putting the appropriate opposite reactance in both
sides of the line between the balun and the antenna. Of course this means added
complexity.
I would love to figure out how to make a choke that would work with balanced
line at the antenna feedpoint, on multiple bands without any additional
components.
FWIW I’ve switched to a real balanced tuner (Johnson Matchbox).
Victor 4X6GP
On 14 Jan 2019, at 1:42, Al Lorona <[email protected]> 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.
Al W6LX
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