On 10/1/2025 8:22 PM, Jack Brindle via TowerTalk wrote:
One of the problems we see in SO2R and Multi-transmitter stations is that over time
connectors may loosen and cause leakage. I periodically check all my connections and
make sure they are tight. Even when PL-259s are tightened with a wrench, they can
loosen over time. It’s not a good experience to discover this has happened in
the midst of a contest!
I've never heard any ham express concern about the connector heating
that takes place with Uber Power. However, it is a fact that connectors
of the 1/2"-size (which includes UHF, type N, and even type C with their
extra-thick center pin) do heat up when a kilowatt or more is passed
through them, particularly at V/UHF. Of particular note should be the
fact that during operation, those connectors will go through thermal
expansion as they heat up with high power then contract as they cool
again, gradually causing them to loosen. The same happens, of course,
with connectors that are exposed to the hot sun.
Decades ago, I designed and helped produce a rack-mounted
1-kilowatt-minimum-output solid state transmitter for a government
agency operating from 400 - 450 MHz. Due to size limitations due to the
rack depth, I was limited to using type N connectors on the outputs of
the power combiners instead of more-appropriate type HN or even DIN
7/16" (my preference, but they were unavailable at the time to the power
combiner manufacturer).
During acceptance tests, while looking for hot spots throughout the
transmitter during key-down time periods as long as six continous hours
of 1-kilowatt output power using a VERY expensive thermal IR camera and
other thermocouple probes, I found that among the hottest spots were the
type N plugs screwed into the female chassis connectors of the power
combiners; they would approach 300-degrees F after just ten minutes and
slowly climb higher as the rack's internal ambient temperature also
climbed. After time runs, while disconnecting the dummy loads, those
type N connectors were sometimes found only finger-tight whereas they
had been torqued tightly before the time runs! That experience taught me
to watch and test the tightness of connectors both before and AFTER
being used at very high powers, regardless of the frequency.
One of the precautionary operating notes we included on the face of
those transmitters was to ALWAYS check the torque of those coax
connectors on the output spigots before every operation. During training
classes, I later had technicians who had maintained the agency's old
vacuum tube kilowatt-level transmitters tell me that they, too, had
encountered that on their original transmitters, so were familiar with
the issue. I was shown several such connectors that had failed
catastrophically during runs; the silver plating was invariably burned
off the connector nuts and the chassis connectors, especially center
pins, might even be melted.
Steve, K0XP
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