[Amps] *** SPAM *** Re: Alpha 87a

[Peter Chadwick]

Ian said:
>That can only happen if either the cable, the SWR meter, the 
transmitter, the ATU or a dummy load has a substantial break in its 
shielding. Without any of those, the indicated SWR is not affected by 
common-mode currents on the outside surfaces.<
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. 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.
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.
One thing about MF that always amazes me is how well an unloaded 16 foot whip works at 2MHz. Just shows the importance of a good ground plane!! Some of the 'patent earths' used can be shown to have an effect - a bronze 'sponge' about 12 inches by 4 inches being about as good as 4 foot square sheet of copper when immersed in sea water. Something I didn't believe until I saw it....
Shakespeare got it about right:-
There are more things in heaven and earth, Horatio, than are dream't of in your philosophy....
73
Peter G3RZP