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Re: [TowerTalk] Feedline (choke) question

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Feedline (choke) question
From: Steve Harrison <k0xp@k0xp.com>
Date: Thu, 2 Oct 2025 07:55:33 -0700
List-post: <mailto:towertalk@contesting.com>
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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