> I have found that the 'Monimatch' type used at HF with a
> wire under the coax braid is particularly susceptible,
> because the pick up loop has its return, by necessity, to
> the outside of the cable.
That makes sense if the monomatch is not in an RF tight box
or on a panel with the connectors grounded, and it is not
built in a loop with the shiled at a common point for
entrance and exit. But that type of indictaor along with bad
construction is anywhere remotely near a common case on HF.
A similar type is used on 2m in the IC251, and I can get
the internal SWR meter to change by adding ferrite on the
outside of the feeder - admittedly, not much, while the Bird
43 is unaffected.>>
Then they have very poor RF grounding of the output
connector Not an easy thing to do at HF even through design
or construction accidents. Or you could actually be changing
the SWR from poor antenna design. Bid meters are pretty poor
at reading small SWR changes when the same slug is used for
forward and reverse.
> More years ago than I care to remember (when both Ian and
> I were 'nobbut young lads', as they say in Yorkshire!), I
> was involved with a fitting on an oil rig. There we had a
> tuner and a 35 foot whip on 2 MHz, fed against the steel
> super structure, and had very weird problems with the SWR
> meter. The transmitter was also grounded to the steel
> superstructure: I think that the resistance of
> superstructure from the tuner output was comparable to the
> resistance of the coax outer, so that we got currents on
> the coax outer, and it certainly gave difficulties with a
> both a Bruene type SWR meter and a Bird. Fortunately, some
> dust iron toroids were available (spares for the PA tank
> coil) and those gave enough impedance when wound around
> the feeder to clear things up.>>
A Bird coupler is totally insensitive to common mode
currents. The Bruene type is too unless the particular
construction overlooks some very important details.
It is NOT normal and not common to have SWR change from the
addition of an "RF Isolator" between the PA and the radio,
and if I had that odd problem I'd want to know why it was
really happening. The last thing we should do is choke the
shields on coaxial cables connected between various pieces
of transmitting gear on the same desk. While it may change
something in cases where there is a severe common mode
problem, it would simply be masking the real problem. Better
to fix the REAL problem at the source of the problem and be
done with it.
73 Tom
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