Well, I haven't used 31 mix material for common-mode chokes or current
baluns, because I didn't have any - but it would be my choice. I used higher
frequency material because I had plenty of it - some in substantial sizes
that would accommodate one or more passes through with RG-213. Generally I
have installed several "beads" or "tubes" of ferrite material on coax - both
to build up the choke impedance and to distribute the heat due to the
resistive loss over a larger volume and surface area. On occasion I have
passed multiple turns of RG-213 through larger cores.
I've had fairly good results, but I don't remember doing much in the way of
detailed measurements of the resulting "baluns" or common-mode chokes - and
yes, I'm quite comfortable "walking around" a Smith Chart from inductive to
capacitive line lengths. About the only "baluns" that I've done detailed
measurement on were at VHF, 900 MHz or 2.4 GHz. - Certainly not topband or
80m stuff!! I have sometimes had very good results winding transformers for
receiving loops or amplifier input matching transformers on more modest
cores - usually binocular - and sometimes made up "binocular" cores by
taping two tubes side-by-side. Not elegant - but seemed to work well.
Regards,
Charlie, K4OTV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Tom W8JI
Sent: Tuesday, May 21, 2013 12:42 PM
To: topband@contesting.com
Subject: Re: Topband: Thanks!
> These are not "excellent" 1:1 current baluns below about 75 MHz. Below
> that range, they are a lousy common mode choke, because they are
> inductive. Inductive chokes resonate with a line that is capacitive, so
> they actually increase the current. A line shorter than a quarter wave is
> capacitive; so is a line between a half wave and 3/4 wave. And so on.
> That's why we want a choke that is highly resistive -- the resistive
> component always reduces common mode current, which is the objective of a
> common mode choke (the so-called "current balun.")
You may be confused, or I am. He asked about 31 mix beads:
<<<<
Hi Folks;
Thanks a bunch, to all those who responded with source possibilities for 31
mix toroids ! I will add a note here, after I actually make a purchase to
designate the lucky victim of my small purchase.
73 Dean W5PJR
>>>>>>>>>>
31 materials has a Q of about 1 on 80 meters. This means a good part of the
impedance is resistive.
While I don't beads are a good choice with large coax, only one pass can be
made through the window with normal types of high power coax, I don't think
the material itself is all that bad.
Also, attenuation has as much to do with grounding and wise engineering of
common mode shunt impedances as it does choke impedance. Choke material is
almost always a compromise between heat and Q at high power. For receiving,
lossy common mode choke materials always work. Not so for transmitting. One
size will never come close to fitting all.
73 Tom
All good topband ops know how to put up a beverage at night.
_________________
Topband Reflector
All good topband ops know how to put up a beverage at night.
_________________
Topband Reflector
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