[TowerTalk] TA-33 Mosley Senior-Choke Balun needed?

[Bill VanAlstyne]

If you look at Mosley's website (http://www.mosley-electronics.com/faq.htm),
they clearly use their split-dipole direct-feed approach as a marketing
strategy -- simplicity, stability, etc. But any radiator fed in the center with
an unbalanced transmission line is going to produce common-mode currents on the
transmission line. That's agiven. The bottom-line question is whether this is a
"big deal" or "not a big deal".

In my experience, coax common-mode currents are a big deal if they cause actual
problems with loading, RF in the shack, pattern distortion, etc. Otherwise, for
practical purposes, probably not. In any case, however, putting a choke balun in
the transmission line won't produce any untoward side-effects (loss or
attenuation) and I would do it as a matter of course, as you did. There are also
other ways to choke off common-mode currents, all detailed in the aforementioned
Antenna Book, and you can take your choice. My philosophy is simple: We know
that common-mode currents on the coax CAN cause problems -- and since it's so
easy to prevent this, why not do it?

Bill / W5WVO


Bob Allen wrote:
> Hi,
> We were working on our 2 Field Day antennas and the question came up
> about choke baluns and
> whether they are needed on this antenna.
> The manufacturer say no but Cushcraft's instructions for one of their
> tri-banders of similar
> construction and frequency says to make a choke balun by wrapping
> coax in a 6" circle 8 turns.
> Given that and the assumption that the coax hitched direct could have
> problems with common mode
> currents as described in the ARRL Antenna Book 19th ed. sec.26 pg. 16
> we use choke baluns as
> described above.
> The question is: are we doing anything that might stand out as
> particularly wrong in the sense
> of signal loss or maybe SWR issues though these antennas tune fine
> like this (except when we get
> the dipoles too close)?
> We feed these with rg-8
>
> Bob
> KB1FRW
> 73
>
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