Ferrite beads, torroids used for rfi suppression and Baluns, etc.

W8JITom at aol.com W8JITom at aol.com
Wed Sep 25 01:09:57 EDT 1996

First baluns:

I think the original question was what to use for a choke in an application
where dissipation is not critical... decoupling control lines. I'd use the
highest IMPEDANCE (a scalar quantity) bead I could find, which for HF would
be a lossy mix over 2000 in permeability. I'd never use a 43 mix at HF for
suppression because the impedance is too low, even if the Q is better. 

I'd use the same 2000 Ui up cores if I ran Dog-gnat power levels in a choke
balun. Now if I ran a manly contest type PA I'd use an air core balun,
because a string of high Q (low dissipation) beads would need to be five feet
long if the common mode voltage was very high. Remember an air wound coil
increases inductance by the square of the turns (cable length in the coil) if
the length is less than twice the diameter, but beads only increase at a
linear rate. If you need much impedance, beads get too expensive.


In a message dated 96-09-24 20:04:02 EDT, you write:

>My statement really was meant for the guys like me doing raised radials.
>two frequencies.  At least he did over at my QTH.  The results were that
>1 radial at each vertical was working.  Even though I had measured and
pruned them >last year with an Autek rf anayzer to be resonant.  

With a single tower array this seems like a valid estimate of high angle
radiation, and it is an understandable goal to want to minimize high angle
radiation. It is important to realize this does not mean field strength
improved. I'd have to think about what it means in a phased array. First
instinct tells me it may not mean as much at wave angle other than 90 degree
elevation, and may even make some unwanted radial radiation at lower angles
worse, but that is a guess right now. 

The radiation you worry about is one reason why four radials bothers me. The
current in each radial (assuming proper feedline isolation) is 1/4 the
current in the radiator, that can really murder receiving if you have higher
angle QRN or QRM, even in the verticals pattern null. 

>The reason this works is that the change in impeadance of each radial with
>frequency is a much smaller percentage when your dealing with 1/8 wave than
1/4 >wave.

I need to think about that, it initially strikes me as a contradiction... a
shorter loaded element having broader response. This isn't contest related,
so I'll e-mail.

Anyway, thanks for clearing this up. It makes more sense now. At first the
response you posted 

BTW K5IU has an excellent article comming in Communications Quarterly which
that 1/4 wave radials are absolutely the WRONG length for any vertical

left me thinking every measurement in history was wrong and we would soon be
hearing how 1/4 wl radials need to ripped out and thrown away.

73 Tom

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