How to melt coax ...

Foundations of Amateur Radio

Recently I made a comment about melting your coax and that this was a bad thing. Today I'm going to talk about some of how this comes about and what kinds of parameters we're dealing with.

Let's start with coax itself. The operating temperature of coax is somewhere around 80 to 90 Degrees Celsius, or 176 to 194 Fahrenheit. Soldering is at 230 Celsius, or 446 Fahrenheit, so for starters, soldering coax is a risky adventure.

For argument's sake, let's assume that you managed to solder your coax without damaging it. What else can go wrong?

Let's have a look at high voltage transmission lines. Why do we move power around the place using high voltage lines? The answer is that in a high voltage line, the current is low. Where the current is low, heating is low, so more of the energy gets from the power-station to your shack and less of it is used to heat up the power line between the power station and you.

So, that means that high voltage and low current is less heat loss. The opposite is also true. Low voltage and high current is more heat loss.

Now if you look at a dipole antenna, you'll know that this contraption is moving energy around at some or other frequency. As it's doing that, there are high and low voltage points and high and low current points.

In a half-wave dipole, the high voltage points are at the ends of the antenna, and the high current points are at the feed-point.

Guess where your coax is?

So, you've got your connection to your antenna located at a high current point, in the place where high current has the potential to create problems, things like heating up your coax and potentially melting it.

So, when does this heating happen?

Well, you need high resistance and high current. This typically happens when you've got a bad connector at the feed-point. In practical terms this means that if you're using QRP, 5 Watts, you're unlikely to come across a situation where this becomes an issue, since the currents aren't that high and a bad connection typically means no contacts.

If on the other hand you're using high power, then make sure that the connection to your antenna is strong, solid, and water proof so it doesn't deteriorate to the point of melting and then killing your radio.

It's best to keep an eye on the SWR meter when you're working, since a high SWR might be indicating that the resistance at your antenna changed for the worse.

Final comment. When you've set-up your station, create a note of the SWR at different frequencies and refer back to that regularly. Spotting a problem early might just prevent some expensive maintenance later on.

I'm Onno VK6FLAB