A good ham friend of mine in former East Germany, together with a very
reputable German antenna specialist, many years ago made some measurements
of 432 MHz yagis fed with split dipoles and coax looking for pattern
distorsion without baluns. Plots were first made with baluns fitted, then
they were taken off and new plots were made.
The tests were pretty conclusive, there was not much pattern distorsion at
all without the balun. The measurements were made at the official German
PTT antenna range, so we can indeed trust the integrity of the plots. I
have seen copies and the pattern distorsion was indeed minimal.
But all in all, I suppose a balun won't hurt if it is the right type and
can take the power... but it is also an extra cost and a single point of
failure.
73/Peter SM2CEW
At 15:57 2004-07-13 , you wrote:
>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
>>
>> _______________________________________________
>>
>> See: http://www.mscomputer.com for "Self Supporting Towers",
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>>
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>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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