Intermittent Connectors

coax less connectorIntermittent comms from particular sites that are well within the fade margins can often be attributed to badly wired RF connectors. These, especially the more complex types such as 'N' connectors, are somewhat tricky to fit and have been the bane of many an installers life.

The accompanying photo shows the end of a coax that should have had a BNC on it. It was found pushed into the connector without the connector ever having been taken apart. The section "wiring up RF connectors" covers fitting such connectors in detail.

Although this was a simple fault to trace, finding a badly fitted connector or 'dry joint' can be a nightmare if there is more than one connector in the system and they are possibly crimped.

You would not be blamed for having a bad attitude towards have to trace a bad connection after everything has been sealed with self amalgamating tape and strapped to a mast. However, with two very simple devices not only is finding the offending culprit made easy but you also get to go home early.

The principle is simple. All bad connections exhibit the same property - scratch! The solution below allows one to trace the bad joint without having to 'down' the system while doing so.

Using an audio test amp in combination with a device known as a "Bias T-coupler", more commonly used to inject a DC voltage onto a coax to power e.g. a mast-head amplifier, finding a bad connection becomes a simple task of finding where the coax is "sensitive to the touch".

bad connection detection circuitBias T-coupler

The coupler is placed between the radio and antenna feed and a small current, derived from a 9V battery and a 4k7 resistor, is injected onto the antenna system. Although only a few mA this current dramatically increases the level of noise produced by a bad connection thus easing the tracing, this done by 'flexing' each connection in turn.

It is advised that a long cable be used on the output of the audio amp so as to 'take the signal up the mast'. It tests relationships when having to rely on someone telling you you've just found it and in the meantime you're onto the next connector!

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