[Antennas] Gain with wire ...

2002年4月02日 19:12:10 -0600

Well, Tim, most of the "hoopla" comes from folks that have not done their
homework and really developed an understanding for what a balun is, what it
does, and where it should be used. Not trying to sound mean, but that has
been my observation. Many very good articles have been written on the
subject, probably the best of which is by Roy Lewallen in the ARRL Antenna
Compendium No. 1.
On the G5RV, the balun is intended to eliminate common-mode current on the
coax - nothing more - as on any balanced antenna fed with coax. On 20
meters which is the band which Varney designed it for and basically the
band where it operates best, despite all the claims and "hoopla," the
antenna is 1.5 wavelengths and fed at a current max or voltage minimum.
Thus, at the end of the 1/2 wave of ladderline, there is the same current
max and voltage minimum. A balun at that point acts to decouple the shield
as it is intended to do.
On other bands, though, the antenna is not resonant and thus the
driving-point impedance is not resistive. And the ladderline stub that was
1/2 wavelength on 20 is now some other length and it operates differently.
Result is that there is no longer a simple current max at the coax
transition point and a balun placed there will not see the impedance it is
designed for, etc. so it probably doesn't do nearly as good a job.
Even Varney recommended tuned ladderline all the way from the antenna to
the shack. The other approach was just a way to bring coax into the shack
and avoid some of the problems with balanced line. Now we resolve that
issue with a 1:1 bead balun and let the tuner handle the impedance
matching.
The bottom line on using a balun to feed any balanced antenna with coax is
simply that if the inevitable common-mode current does not present a
problem, then you do not "need" a balun. If it does, and it can in many
instances, then a balun may be the only solution.
73/72/oo, George W5YR - the Yellow Rose of Texas 
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe 
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
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Tim Billingsley wrote:
>> Thanks George.
>> So why all the hoopla about baluns on things like G5RV and other multiband
> wires?? I ran a G5RV for a couple of years and I was always asked why I
> didn't have a BalUn on it .... my answer was that I did not build one and
> that LB Cebik, W4RNL said it wasn't necessary.

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