View Single Post
  #7   Report Post  
Old May 12th 06, 10:28 PM posted to rec.radio.amateur.antenna
 
Posts: n/a
Default 2 Meter Choke Baluns

43 material ferrite beads are quite effective at VHF. I would (and do)
take this route at VHF because they're more compact than a coax choke,
and you don't need too many of them to get substantial choking
impedances.

I was pointed to this PDF (big) for impedance vs. frequency curves.

http://www.fair-rite.com/newfair/pdf...log.pdf#page=1

A 2643012702 bead of the sort that fits (most of the time!) over RG-58
has a typical 1 turn impedance of 38 ohms at 100MHz according to the
catalog. You can decide on an appropriate choking impedance and stack
beads in the number you need.

Don't worry about chokes further down the coax unless you're really
having feedline radiation problems. A choke at the feedpoint will take
care of RF current conducted onto the feedline. If you're getting
induced currents on the feedline, get it away from the antenna! It's
VHF! :-)

I'm using bead baluns on my 6m moxon, my 2m and 432m yagis... I might
not notice the difference without them, but I had a box of, I dunno,
800 of those 43 material beads lying around, so I'm using bead baluns
for everything.

I've hung around r.r.a.a. and read many posts about cracking at high
power if the choking impedance is low enough that more than a few watts
are being dissapated in the choke.

I've also read quite a bit about conducted RF from all the junk
electronics in everyone's house being kept OFF THE ANTENNA by an
effective common mode choke, and this is why I'm going to keep using
them whether I feel that I need a balun or not for antenna pattern.

Back when my very understanding parents let me bury all my feedlines
for 40 feet under the lawn to build my high-school antenna farm, I
didn't worry about choking off shield currents, but now that I'm in an
apartment, everything I can do to keep antenna and noise separated is
going to get done.

73,
Dan
N3OX
www.n3ox.net