[Antennas] Gain with wire ...

2002年4月02日 20:30:06 -0500

At 07:12 PM 4/2/2002 -0600, George, W5YR wrote:
George, Tim,
Varney says that if you want to use a balun, use a coiled transmission 
line. This is good advice, because it will keep the current off the 
outside of the coax, and the loss is no more than that of a similar length 
of transmission line. I use a large Radio Works 1:1 balun and it works fine.
I did some calculations the other day to see what ratio balun would be the 
best for a G5RV. Sevick says a 2:1, (50 to 100 ohms) but that is hard to 
buy or build. The impedance at the end of the open wire transmission line 
varies so much that a 1:1 is about as good as you are going to get.
GL Chas W1CG
>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.