Modifying coax connectors

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 Custom N-Adapter for the Amplifier Build



Yesterday during the amplifier build, I needed a coax adapter – an N male to N female – but a bit longer than usual, to connect the amplifier output from the directional coupler to the T/R board.

It turned out I hadn’t paid attention to the minimum bend radius of the coax, so I had to get creative.

At first, I planned to just order an adapter, but I couldn’t find exactly what I needed. Instead, I decided to modify some existing connectors.


Interestingly, the backshell thread on many clamp-type N connectors is the same as the retaining ring thread – 5/8"-24 TPI – meaning you can actually thread a female connector into the back of a male RG213 clamp connector. Unfortunately, that combination would have been too long.





I own an early 1940’s Blomqvist lathe, a close copy of the famous South Bend 9". It’s fully equipped with all the gears for cutting threads, so my solution was to set up for 24 TPI and cut the thread directly onto the connector body (after removing the mounting flange).





Once the thread was cut, I shortened the connector until the center pin was exposed, chamfered the thread, and adjusted the total length so the two connectors would screw together to the correct total length.

The clamp connector was bored out to increase isolation between the center conductor and the connector body. I then turned a PTFE insulating sleeve on the lathe. Its length was chosen so that, when the connectors are threaded together, the smaller diameter of the PTFE sleeve is forced snugly into the "isolating" bore.



For the center pin, I used two male pins placed back-to-back. Each was shortened by 2.5 mm to get the correct overall length. They were soldered together and then soldered to the exposed female pin of the female connector. The PTFE sleeve was slid over the pins, and the connectors were threaded together.




Finally, I soldered the two connector bodies together. I had noticed a small gap around the circumference at the female connector end – likely from manufacturing – and wasn’t sure if it was pressed or threaded in place. To be safe, I applied a continuous solder joint around the seam.




I’m happy with the end result! Now I can clamp the N connectors onto the cable and continue the amplifier build.


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