Fw: RE: [Antennas] questions

[windy10605@juno.com]

Ahhh, choke baluns again............

I made some choke balun measurements. The 5 or 6 turn coil is a very
good balun for covering 20m to 10m and that's why they tell you to add
them onto beams. The observed impedance for 5 turns of RG-8U (non
foam) on a 4-1/2" dia piece of PVC was 689 at 14Mhz, 1K+ at 18Mhz, 
1K+ at 21Mhz, 1K+ at 25Mhz, and 654 at 28Mhz. The things which are 
variable are the type coax (physical size and velocity factor), coil 
diameter, number of turns, and it has to be solenoid wound (one turn 
next to another ...that's why a PVC coupler as a form works great and
you can use cable ties to hold the turns in place).

Ferrite beads, which are highly dependent on the ferrite material, are
generally better at lower frequencies. The typical store bought ferrite 
bead chokes are generally very effective in the 80m to 20m range. At 
10m they tend to fall off some ...impedance of 400 ohms or so. You
can raise the frequency range with fewer beads or different material. 
The thing about ferrites that jumps out at you is that they are effective

chokes over a relatively broad frequency range (much less "Q" than 
any coil). But don't assume you can just put a few ferrites on the 
coax and have it work. I heard a discussion about buying "red"
ferrites because they work across the ham bands ....what you want 
are the ones that DON'T "work", but "choke" at those frequencies. 

One of the easiest ways of doing a quick check of choke baluns is to 
drive the balun with a signal generator over a ground plane (a piece 
of thin copper scrap in my case) tie the input to the generator and 
ground the shield to the plane. On the output side, tie a termination 
resistor from the center conductor to the copper plane and tie the 
same value termination resistor from the shield to the ground plane. 
At first glance it looks like that resistor is shorted ......but, not so 
because of the choke effect on the OUTSIDE of the braid. If the 
voltages you measure across the resistors are about the same, the 
choke works and you have a balanced output at that frequency.

The biggest problem is finding one which works from 160m to 10m
for an antenna tuner. The only one which I've built which does this 
is using a simple switch to short the shield ONLY on a multi turn
choke balun (don't see why it wouldn't work on a bead balun also)
giving me a hi/med/lo frequency range. Of course this choke balun 
is only good at the tuner end due to the switching.

Well, off to go mess with my "simple" vertical with multi band
trap radials.

73 Kees K5BCQ