From: Roy Lewallen on Jul 13, 12:39 am


tim gorman wrote:
. . .
Unless you are using rg174 the 7 foot of extra cable should not make this
much difference unless the input impedance of the amplifer is not 50ohm
resistive. If it is not purely resistive then changing the cable length can
impact the SWR seen at the transmitter end significantly. . .


I agree with the suggestion that the OP measure the SWR and if possible
the power at both ends with both cables. Something else is going on,
like maybe a bad cable or connector.

I don't think the OP said what frequency this is happening at. That
might give some additional clues.

Good suggestions on simple checking of a cable AND connectors.

The six-decade-old design of the PL-259 is not the best on
"wiping" action of contacts on the sleeve (outer conductor
portion). It is a license-free standard design, standard
because it is relatively cheap. Cheap silver plating can
corrode fairly easily (rhodium flash over silver plating is
much better but costs more)

Yesterday my "ancient" HP-722 inkjet printer had no primary
power. Check of the AC cord, external supply, said that was
okay. Turns out that the coaxial connection to the back of the
printer had developed some kind of minor corrosion. Simple
unplugging and re-plugging that DC connector brought back
primary power.

Three decades ago I was involved in a seemingly "unsolveable"
problem in Navy-flown L-Band R&D system. Signals would just
cut out at altitude, said altitude varying depending on day
of flight test. System AND RF cables (to top and bottom
fuselage antennas) all checked out fine on the ground. Nothing
intermittent. Blade antennas were taken off and checked
okay, put back. All type N connectors, good ones. Trouble was
in an unlikely form of a "doubler plate."

"Doubler plates" are often used in retro-fitting antennas and
other things on aircraft, just a sheet of metal to re-enforce
strength of the metal skin. The doubler plate drawings had
clearance holes just too close to type N connector sleeve
outer diameter. Connectors mated, but NOT fully. As altitude
increases, temperature drops. The not-fully mated center
conductor pin just contracted until it lost contact at cold
temps. Enlarging the doubler plate clearance hole allowed
full mating, no shrinkage of contacts. Unlikely problem
solved at about quarter to 8 PM in a cold hangar. :-(

Sometimes the "unlikely" not-described-in-text things are to
blame.