"Mike Newman" wrote in message
...
On Tue, 07 Mar 2006 15:18:33 GMT, "Jerry Martes"
wrote:


"Mike Newman" wrote in message
. ..
Any advice on which grade of ferrite would be most useful for a
feeedline balun choke effective at 70 cm?
Thanks de miken zl1bnb

Hi Miken

I've been using the tubular, high permeability ferrites designed for RFI
suppression slipped over the RG-223 coax for a balun for vhf antennas. I
am
aware of the high loss with those ferrite tubes. I have been hoping the
high permeability would present a high reactance to currents along the
outside of the coax. So far, the tubes are working OK for my needs.
Your mileage may vary.

Jerry

That's exactly what I intended doing, feeding a moxon rectangle in
this case, my win4nec model gives 50+0.1j.

Given the correct grade of ferrite, it should probably be ok on
UHF.

But how to to determine the effectiveness? Maybe a small bead with a
couple of turns to pick up voltage nodes on the coax feedline, with
and without the choke balun beads.

MikeN

Hi Mike

Since you are acknowledging the existence of 1/10th of an ohm of inductive
reactance, at UHF, you are way beyond my level of accuracy in any
measurement of effectiveness.
But, I once measured the inductance across the ferrite tube by connecting
the impedance "meter" to the exposed ends of the coax braid that was
threaded thru the center of the ferrite. Then I removed the ferrite and
repeated the test with the ferrite removed.

Roy Lewallen has an excellant paper written to show how to properly
evaluate baluns. I have the paper saved and will be happy to share it with
you. It is almost a *must read* since it is so thouough and informative.

Jerry