Foundations of Amateur Radio

Recently I spent some time discussing the losses associated with connectors between your radio and your antenna. The traditional wisdom, and I use the word "wisdom" ironically, says that each connector introduces loss into the feed line. There is an understanding that the more connectors you have the worse it is and the more loss you have.

Jim W6LG did the test, connected up 30 odd connectors and measured. His measurements were done on 14 MHz and on 50 MHz, using 50 microvolts and 1 kilowatt. No discernible difference.

Of course after I mentioned this out loud the questions started. Why didn't he test this at a usable frequency, something like 145 MHz, or in the GHz band? Then there were those who said that this wasn't a real test and that it should be tested with coax in between the connectors.

I discussed this all at some length and one idea we had was that perhaps the intersection between the coax and the connector was the problem, that each transition between coax and connector and back was introducing the loss.

I wondered if there was a way to test this.

Turns out that somebody already did. Back in July 2015 Jim K9YC decided that this needed to be tested. That's right, another Jim.

He set up a test with a dozen 100 foot lengths of low loss coax, that's just over 365m of coax. This included two dozen PL259 connectors and 11 barrel connectors. He tested using a calibrated HP generator/voltmeter rig. The total loss was and I quote: "one dB or so less than the loss specified for the cable by the manufacturer". So, the run with connectors was actually better than a single run of coax.

In case you're wondering, he tested this up to 500 MHz.

Jim K9YC points out that there is a grain of truth in the loss when using junk connectors which can introduce excessive loss and can overheat because the centre conductor is too small.

I should mention that this might now debunk the connectors and loss issue, at least up to 500 MHz, but there is something to be said about reducing the number of failure points along the way. Having 35 connections instead of two is an added risk of water ingress, loose connections, short circuits in the connector and potential for other unexpected things like an intermittent connection.

In the broader scheme of things, on a field day, or a temporary antenna set up, there's clearly nothing wrong with using some connectors to join together some coax. It also means that my investment into coax terminated SO239 connectors was based on poor information, though it does mean that I don't need to carry nearly as many barrel connectors around.

Perhaps it's time to, as Jim puts it, put this "old wives' tale" to bed.

In the same document Jim discusses many other questions in relation to coax and stub filters in your HF station. I came across the document while I was looking for information about coax stub filters, since I just participated in another contest where two stations in the same location were interfering with each other and I want to be prepared for next time.

There's a lot to discuss in relation to coax stub filters, but in essence you create a quarter wave and half wave lengths of coax that are resonant at a range of different frequencies and the combination of these will either pass or block the band you care about. Given that I have a roll of Quad Shield RG6 lying around, I thought I'd try my hand at making a set of these for my next outing. No doubt I'll share my adventures with you as I explore and dig through the pile of information.

Coax and connectors, stubs and filters, it's all in a days experimentation in this amazing hobby we call amateur radio.

I'm Onno VK6FLAB