Date: Sat, 17 Jul 2010 22:46:28 -0400
From: Dan Zimmerman N3OX <n3ox@n3ox.net>
Subject: Re: [TowerTalk] Beta match adjustment
> > The info that came with the beta match suggests experimenting with the
> match
and driven element length to tune it at ground level, but the info with it
is vague as to the process. And, I can?t uncover a correlation that would
permit me to pre-tune it at ground level that translates to being tuned when
raised on the tower at 65 feet.
You don't really need to experiment. The correct settings are quite
specific. You need to SHORTEN the driven element so it shows about 25 ohms
of capacitive reactance and adjust the beta match hairpin so its about 50
ohms of inductive reactance.
26-j25 ohms in parallel with +j50 ohms is about 50+j2 ohms.
### correct. It's just an L network. [LC] and the 'XC' comes from the DE,
being
shortened a tiny bit.
## another method would be a 1:2 xfmr... like what array solutions sells.
Presto,
flat swr. [in this case the DE is NOT shortened]. Nothing to shorten, and this
is the
quick and easy solution, plug and play, broad-banded, and you can't blow it up.
You also
DON'T require a 1:1 choke balun.
## since you already have the hair pin, you may as well use it. But you might
have to try 1-3 x
attempts till you get the DE shortened by the correct amount. Then you also
have to tweak the
hairpin for flat swr. You also require a 1:1 choke balun.
## Instead of RG-83 coax, you may well be able to just use parallel RG-11...
which would
be fairly close. 75/2= 37.5 ohms. You might also be able to wind it up, and
make a choke balun
from it?
## The 1:2 xfmr is the cleanest, simplest route.. and kills 4 x birds with one
stone... and just one main
50 ohm coax to plug into the balun.
Later... Jim VE7RF
L.B. Cebik's website has a good explanation of what's going on and formulas
to calculate the hairpin inductor and antenna capacitive reactance: the beta
match hairpin is the shunt inductor in a balanced L network. The two series
capacitors are formed by the capacitive reactance of the antenna. Just like
if you did this with an antenna tuner, all values are fairly critical. Once
you get it right, it will work fine, but juggling both and not actually
having the thing in the air could be a headache.
In your particular case, you might start with setting the beta match hairpin
to +j50 ohms using your analyzer and then try shortening the driven element
to get a good match. I think it will help nail down your variables a bit.
K3LR writes:
You can easily match 26 ohms (with no reactance) to 50 ohms with a one
> quarter wavelength piece of RG-83 coax (35 ohms) in series.
You can also do it with a twelfth wave transformer made of 25 ohm and 50 ohm
sections. The 25 ohm can be paralleled 50 ohm cable, so you don't need any
special coax. The technique is show at the bottom of this page:
http://www.tuc.nrao.edu/~demerson/twelfth/twelfth.htm
Then again, if I had a roll of 35 ohm coax I'd use it all the time :-)
Either way, series section matching is certainly a good option here. Should
be broader bandwidth than the beta. And there's nothing really to adjust...
just install and go.
73
Dan
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