[TowerTalk] ferrite beads on guys

Ian White, G3SEK G3SEK@ifwtech.demon.co.uk
Sat, 14 Jun 1997 08:00:05 +0100

Darrel Van Buer wrote:
>I looked up some likely ferrite beads in the Amidon catalog, and it looks
>like mix 43 has the right properties (used in many snap on split beads for
>RF interference, as well as standard toroids which could be threaded on
>before the hardware.
>Any of FT-50B-43 beads (.312 inch hole), 2X-43-251 split cores with 1/4 inch 
>hole or 2X-43-151 splits with 1/2 inch holes all result in about 8 to 11 
>microhenries per foot of beads/cores (the 50B is 1/2 inch long, the others
>1.125 inch long).  At 25 MHz, Amidon says Z=56 ohms for one FT-50B-43, Z=171
>for one 2X-43-251 and Z=159 ohms for one 2X43-151.
>At the higher HF frequencies, there is clearly enough inductance to pretty
>effectively break up the segments with a foot of beads.  At lower frequencies
>the reactance starts to fall into the range of values suitable for loading
>coils for shortened antennas, which might lead to some surprise resonances on
>40m and down.
>With enough experimentation on placement and quantity of beads I suppose it
>might possible to do some interesting things with guys loaded to resonance
>on 80 or even 160 meters but which tune out on the higher frequencies, but
>it would be a real killer changing the number of beads half way up a guy.

You can get a pretty good idea of the impedance of a bead by measuring
it with an MFJ-259 or Autek RF-1. Just slip the bead onto a wire and
read the "resistance" meter. 

Actually both the 259 and the RF-1 measure magnitude of impedance |Z|,
NOT true resistance, so you don't know whether the impedance is
inductive or resistive. Conventional RFI-suppression ferrites probably
create lossy inductance at lower frequencies, but the chokes become
predominantly resistive as the frequency increases.

If you use enough beads to create a high |Z|, it doesn't matter whether
they're resistive or inductive: losses will be low in both cases. But
most types of beads produce only a few tens of ohms of |Z| apiece at
1.8-7MHz, so if you don't use enough beads the choke may be the worst of
both worlds - both ineffective and lossy. 

A foot of beads for each guy, at all the different locations needed for
multi-band suppression, adds up to a lot of beads. Is this really the
most technically effective and cost-effective solution? 

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)

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