[TowerTalk] Line Isolators (was: common mode chokes)

[Michael Tope]

>
> So, even in your idealized circumstances, for 20 meters the power
> rating is 750 w; and for 10 meters the power rating is 375 w.
>

Hi Chuck,

There should really be two power ratings for
these isolator things. The first one should be
the power handling for the normal differential
transmission line mode. This test could be done
with a dummy load, a good PA, and some
stubs/transformers to create the required worst
case matching conditions.

The other test would involve stressing isolator
in the common-mode transmission configuration.
I had some HB chokes made from RG-142 and
Type 77 ferrite beads that ran cool when driving
a dummy load where there was no common mode
stress. When I attached one of these same chokes
to the shack-end of the feedline for my 80/40 meter
parallel dipole, it ran warm on 40 meters. Probably
no coincidence that I had lots of RF in the shack
on 40M. In this configuration, the lossy high-mu
beads were being stressed in the common-mode
by all of the RF on the outside of my feedline
whereas the choke could probably handle
10 KW in a normal transmission line mode.

A decent test for this common-mode power
handling performance would be to short the
center conductor and shield together on each
end of the choke and then place the resultant
2 terminal device in parallel with a dummy
load. In this configuration the isolator would
see the same RF voltage across its terminals
as the dummy load. If the test device really did
have 80K ohms of common-mode impedance,
it would stay cool in this configuration even at
high power levels.

>
> Now regarding common-mode choking impedance:
>
> The web page (URL given above) states that the "Isolation @ 14 MHz"
> is "80,000 ohms."  The catalog page (ditto) states "Winding Z @ 3.5
> MHz >33K" and "Winding Z @ 14 MHz >50K".
>

Check the TT archives to be sure, Chuck, but I think
someone else measured one of these Radio Works
isolators a while back and got numbers similar to
yours.

73 de Mike, W4EF.........................................