Jeff,
I found an FT-240-77 wrapped around the power cord of my
R4C (long story). In any case, I took it off and measured it with
a Network Analyzer and then dumped the data to M-S Excel:
http://www.dellroy.com/W4EF's-Ham-Radio-Page/Ferrite_Data/Ferrite.htm
Please excuse the quality of the plot image (I need to figure out
a better way to display these Excel plots in Frontpage). In any
case, it's pretty clear that this core provides maximum choking
impedance around 1.2 MHz. Beyond that the permeability starts
to fall off rather significantly (you can see this in the Fair-Rite
products mu-s data for type 77 material). At 0.8 MHz, the phase
angle of the impedance is about 45 deg (|R| and |X| components
about equal). In the lower HF region (including 160 meters) the
impedance becomes mainly resistive (don't use this material for
transmitting applications). I only measured 1 to 6 turns, but
extrapolating from the data which seems to follow the Z = Al * N^2
relationship, you could probably get > 800 ohms of choking |Z| from
the material on 160 meters by increasing from the number of turns
from 6 to 8.
I have data out to 50 MHz if anyone is interested.
73 de Mike, W4EF...........................................................
----- Original Message -----
From: "Jeff Maass" <jmaass@columbus.rr.com>
> This has been a very interesting, and somewhat frustrating,
> discussion!
>
> My applications are:
>
> 1) For keeping RF out of audio lines in a high RF multi-
> transmitter station. I think I understand this one,
> although I'm interested in seeing K9YC's article to
> learn to better select ferrite materials.
>
> 2) Reducing the effect of the feedline on the effectiveness
> of a Beverage or pennant/flag antenna. From Tom's
> description above, ferrite is pretty much useless in
> the case of my pennant/flag (long feedline, poor ground
> at both ends), but we are told in the various pennant/flag
> articles that a choke at the feedpoint is pretty much
> essential to proper operation.
>
> So, who has *practical* advice of how to apply a choke
> for flag/pennant antennas? I've been winding 15-20 turns
> of RG8X in an FT240-77 core.
>
> Next week I'll be flying down to Curacao for a few weeks, and
> I'll have some time to try out some more receiving antennas.
>
> 73,
>
> Jeff Maass K8ND
>
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