> You should have a choke balun however. Not sure the simple coin of coax
> will work at 432. Might be a problem.
> 73,
Tom K6EU
>
Hi All: Here's my 2 cents on vhf/uhf choke baluns. What provided the best
choking action at 50MHz for me was a a set of 10~12 type 31 cable beads that
measured 34 db of (mismatch) attenuation in a 50 ohm pi circuit. Second
place and cheaper was a set of six .4 inch ID type 43 cable beads configured
as three binocular pairs and two turns of RG400 teflon cable through each
binocular core set. That came in around 30db in a 50 ohm pi circuit and
similar to using 10~12 type 43 cable beads in series on RG-213 type coax.
(You can convert the mismatch attenuation to j-ohms using a mismatch return
loss calculator like the one at
http://chemandy.com/calculators/return-loss-and-mismatch-calculator.htm
For example, set source and load impedance to 50 ohms and j-ohms to 10,000
and the mismatch attenuation should calculate to be about 40 db )
>From bench tinkering with my sprectrum analyzer, binocular chokes (and
coiled coax) had too much capacitive coupling above 100MHz or so in the 50
ohm pi circuit test (choke in series from signal generator to sprectrum
analyzer input starts to be too large a fraction of a wavelength to make
reliable measurements.) But, I did place single 1 inch long type 43 cable
beads on my 432 beacon antenna feed lines which is a halo 4 stack fed in
phase and bead near each feedpoint. I can barely key up the club repeater
with it 10 miles away (vertical antenna) with this horizontal omni antenna,
and yet WA9KRT hears the 432 beacon regularly over 250 miles away with his
horizontal array---so the type 43 cable beads definitely choke transmission
line radiation well enough through 432. In contrast, my Par loop horizontal
stack on 2 meters has no bead chokes and the local club repeater is full
scale on that band with that horizontal omni, (so the phasing lines are
definitely radiating.) I did not document the 2M and 432 bead testing on my
spectrum analyzer but if there is interest I will make another test run.
I'm inclined to think 1 or 2 type 43 beads would be adequate at 432 and 4~6
type 43 beads adequate on 144MHz---but that needs to be tested and verified.
You can check the choking impedance up to 650Z with an MFJ-259 to 175MHz and
something like 1500 or 2000Z with the MFJ analyzer version that covers
440MHz. Just use a short piece of coax with the beads in place and measure
it like any other inductor. Coils of coax (as a common mode choke) have too
much capacitive coupling to be of much use at 50MHz and up.
I'm also a fan of using ferrite chokes on both ends of any coax cable to
limit common mode noise egress, especially with all the PC routers that are
rather aggressive wideband noise generators these days. I'd be very
interested to hear from anyone who adds effective bead chokes to BOTH ends
of their 6M transmission lines and notes a worthwhile receiving capability
improvement. Recall that power line noise is predominantly vertically
polarized and that every extra db of choking attenuation for vertical
polarization signal pickup (cable running vertical up the tower or mast)
will make a difference as far as transmission line outer jacket decoupling
is concerned.
Here is a link to some cable bead choke tests.
http://www.w3kwh.com/newsletters/SCARC-KH-05-2009.pdf
Pages 8 & 9
One of these days I want to test some of the type 61 and 64 cable beads, but
I don't have any on hand.
Thanks for reading/listening....let the ham roast begin!
73 Mike wa3tts
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