On Thu, Dec 27, 2012 at 8:45 AM, Robert Costa <[email protected]> wrote:

> I was on the phone with Bob from Balun Designs. He suggested a dual core
> 1115DU. The cost is $79.00 plus shipping. The item has great
> characteristics as far as SWR, line loss and choking impedance from 160-10
> meters.
>

SWR and line loss, while interesting, aren't that important because, after
all, it is just a straight piece of coax. If they used good coax then the
SWR and line loss numbers will be that of a good piece of coax.

The choking impedance from 160m-10m is the crucial information. The bigger
that number is at your operating frequency, the better it will choke off
currents on the outside of the line. One thing to consider is that, if the
impedance is too low, the choke will run hot and NOT do the job you want it
to do.

And one other thing. Where you place the choke in the line makes a
difference. RF current appears in standing waves just as RF voltage appears
in standing waves. There are points on the line that are high current (low
impedance) and points that are low current (high impedance). If you happen
to place your new choke at a high-impedance point on the line, the effect
will be minimal. If you place it at a low impedance point on the line, the
effect will be maximal, i.e. reducing current all along the line. If you
have a way of measuring RF current along the line you can find these high
current nodes.

And measuring current on the outside of the line is surprisingly easy. Put
a ferrite bead over the coax and wrap several turns of hook-up wire around
it too. You have just made a current transformer. Connect a meter that will
measure RF voltage to the free ends of the hook-up wire and now you can
find the current peaks and nulls on the coax. (It doesn't matter if it
accurately measures current, only that it will show you where the peaks and
nulls are.)



> My friend in a QSO last night suggested the MFJ-915. This item is only
> 29.99. It consists of a bunch of ferrite beads. As it is MFJ, I do not
> expect them to publish any specifications or sweeps from a spectrum
> analyzer.
>

Right. You have no idea what kind of ferrite beads were used so you have no
idea how much choke impedance there is. You are shooting in the dark. Of
course, you COULD measure the impedance but I am guessing that will cost
you a lot more than buying a known quantity (unless you already have the
kit).

-- 
Brian Lloyd, WB6RQN/J79BPL
3191 Western Dr.
Cameron Park, CA 95682
[email protected]
+1.767.617.1365 (Dominica)
+1.916.877.5067 (USA)
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