Alan,
The capacitance per foot of the RG 58 is higher than the foam larger coax.
This probably made the cables not resonant near 12M, and now you have
differing lengths, and differing capacitance represented by the cables, thus
your transmatch tunes the critical band at differing settings, and you have
the transmatch looking at higher swr point because of the longer cables have
shifted the SWR peak close to the transmatch. That would be my take on it.
What lengths are your new coax cables overall, all added together, compared
to all the old ones added together? Remember a quarter wave on 12 would be
only 10 ft. approx., and the end of a quarter wave is a high impedance, if
that is what you have constructed by changing cables.
Add another 2 feet in and see if the SWR goes back down.
73, Stuart K5KVH
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