Foundations of Amateur Radio

If you've ever used a spray can of WD-40, you might have wondered what the name means. It stands for "Water Displacement, 40th formula". In my time as a radio amateur I'd never stopped to think what the RG in RG-58 stood for. Turns out that it too has a meaning, "Radio Guide", though I have found some interesting alternative descriptions where the G stood for Government.

This radio guide, really a transmission line, gets a signal from point A to point B. Depending on how you construct that transmission line determines what you'll get at the other end.

Coaxial cable or coax is a length of cable made from several components. There's the outer layer or jacket, that protects the cable from electrical shorting, U/V deterioration and water ingress which causes all manner of problems. Inside that is an electrically conductive shield that forms one half of the transmission line, inside that is a dielectric, essentially a separator or insulator between the shield and the inner most, or central conductor, the core.

Each of these components can change. On the outside the first thing you might notice is the thickness of the cable. The next thing you might observe is how flexible it is. Below the outer surface other things can also be altered. For example, the core could be a solid copper wire, or it could be strands of copper. It could be aluminium, silver or even steel. It might not even be wire. Some coax like Heliax, used in broadcasting, uses a central conductive tube as the core with air as the insulator between the core and the shield.

The dielectric that separates the core from the shield can be made from different materials such as plastics, air and even inert gas such as nitrogen and it comes in varying thickness. Similarly the shield can vary in thickness, material and construction. There are also variations that have multiple levels of shielding, such as for example Quad Shield RG-6, common in satellite television and internet connections that has four layers of shielding.

Other aspects might not be nearly as obvious. If you're running coax down a power line it will need physical strength. If you're burying it in the ground it will need to be protected from water ingress. Temperature and heat dissipation are also considerations and if you're using the coax in a nuclear reactor, its ability to deal with radiation. More commonly if you need to run the coax around a corner, how tight it can be bent is another consideration.

As the materials and dimensions are changed, the characteristics of the coax changes. Each of these are documented and standardised.

The standardisation is both a blessing and a curse. So many options and so much to choose from.

For example, if you compare RG-58 to RG-59 they look pretty similar. If you cut into them you'll notice that they're made from similar materials. If you put them side-by-side, you'll notice that RG-59 is thicker, by about 20%, conversely the core for RG-59 is thinner by about 20%, this also means that the dielectric is about 30% different in thickness. As a consequence, connectors for one might fit on the other, but rarely work well.

These variations mean that while both types of coax are common and priced similarly, they're not interchangeable. RG-59 used to be common in satellite TV installations and is still used in CCTV, whilst RG-58 is common in radio communications.

If you made the decision to actually go out and buy RG-58, you'll come across many variations indicated by extra letters. For example, BC means Bare Copper and TC means Tinned Copper.

The final piece of the puzzle in this tangled offering of transmission line is that each manufacturer has their own way of doing and naming things in pursuit of market share. For example, the coax I installed recently is known as LMR-400, CNT-400, WBC-400 and several others.

If the performance of your coax actually matters that much, I'd recommend that you spend some time looking at your options before handing over any money.

All that behind the name of a piece of coax that runs between your radio and antenna.

I'm Onno VK6FLAB