[Antennas] coaxial balun

[Charles Greene]

At 09:31 PM 1/2/2002 -0600, George, W5YR wrote:

Hi George and Paul,

George is correct generally speaking.  I once made a coax balun consisting 
of a 1/4 wave length of RG59 (still have it), 75 ohm coax to do a match to 
50 ohms and also convert between balance and unbalanced for a 15 meter lazy 
H antenna which was on the order of 100 ohms.  It is of course a 1/4 
wavelength on just 15 meters, but the antenna was for just 15 meters 
too.  Unless you have a specialized situation, just feed your dipole with 
RG-8 or RG8X type coax, there's no need for the LMR-400 in the HF band, or 
use a 450 or 300 ohm ladder line and an antenna tuner and get all band 
coverage.  If the coax does not comes off the dipole at right angles or 
nearly so, you can use a 1:1 balun at the antenna to reduce unwanted feed 
line radiation.  The information for construction a coax balun is in the 
ARRL handbook.  They say don't use RG8X coax for a coax balun as the center 
wire creep toward the braid, but RG8 or RG58 do a reasonable job.

>Paul, the very practical problem here is that it is virtually impossible to
>do much more than estimate what the driving point impedance of any antenna
>will be. It depends upon every scrap of detail concerned with construction,
>materials, location, height, ground/soil conditions, other objects within
>several wavelengths, etc. etc.
>The list of affecting agents is almost endless.
>
>So, in real life, we put up antennas and by making measurements or just
>plain guessing, we find what feed system works best according to whatever
>criteria are important to us.
>
>It is probably an overstatement of fact, but I think that regarding a balun
>as a transformer is somewhat confusing and misleading. A balun serves to
>connect a balanced source/load to an unbalanced load/source. Any impedance
>matching aspects to that connection should best be done elsewhere.
>
>I am unaware of any strictly coax balun that could be constructed that
>would assure a match between an arbitrary dipole installation and a coax
>feedline. For many applications, it is far simpler to accept small losses
>in the feedline due to the antenna/line mismatch and manage the impedance
>matching implications in the shack with a "tuner."
>
>If that is unacceptable, then there are many techniques for matching the
>line to the antenna at the antenna, such as gamma, delta and T matches, but
>none come to mind that could be described as a "coax balun." In any event,
>such methods may be  narrow-banded in their operation, especially on the
>lower bands.
>
>Perhaps someone else can make a positive contribution to answering your
>question.
>
>72/73, George W5YR - the Yellow Rose of Texas   QRP-L 1373 NETXQRP 6
>Fairview, TX 30 mi NE of Dallas in Collin county EM13qe SOC 262 COG 8
>Amateur Radio W5YR, in the 56th year and it just keeps getting better!
>Icom IC-756PRO #02121  Kachina #91900556  IC-765 #02437
>
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>
>
>Paul McInnish - K4BET wrote:
> >
> > For a dipole (or flat top as some call 'em) that is fed in the center of
> > which the impedance is NOT a true 52 ohms... can some one advise the
> > construction details of a coaxial balun (a balun made of coaxial cable) to
> > get a good match at the feed point and with the entire antenna being fed
> > with 52 ohm coax (LMR-400)?
>
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73, Chas, W1CG