When I say transmission lines, most of you will think of coaxial cables, which are the most common and most popular form of transmission lines in use today. Two points: first, they’re not the only kinds used in ham radio and in our shacks; second, transmission lines are also circuit elements widely used in radio frequency design. Having said that, let me go back and start from the basics. As always, this is to ensure we’re all on the same pages.
Transmission lines are what we think of when we want to get a signal from one point to another in our shacks or around our yards to get to antennas. Ideally, you’d want no loss of signal and you’d want perfect shielding to keep the signal from radiating minute amounts of power into other circuits. The strong point of coaxial cables is that they’re good at both of these. A diagram of a typical cable shows the important dimensions.
The impedance of the cable is set by the dimensions it’s built to, but ordinarily you won’t build coaxial cables, you’ll buy the cable.
The other common type of transmission line is formed by two parallel conductors. These are available in different impedances, and the impedance of a parallel conductor line is given by a very similar equation.
Since the theme of coaxial cables is “easier to live with and work with,” it’s worth mentioning connectors. Over the years, manufacturers have migrated from soldered-on connectors to those that are crimped on. This is because crimped on connectors have gotten better over the years and generally offer higher quality (higher repeatability) in the assembled connectors. Much as you hardly see soldered on power connectors anymore, and they’re virtually all crimped, the same trend is in the RF world. My station is still exclusively soldered-on connectors, but if I was starting over, or anticipating doing a lot of cable replacement, I’d seriously look at buying crimp on coax connectors and a good crimping tool.
There’s a world of “connector nerds” out there who are against the use of the so-called UHF connectors (PL-259 plugs and SO-239 sockets) and it’s true that if you apply that equation for the impedance of a coaxial cable to the dimensions of UHF connector you come with a lower impedance than 50 ohms (around 30, IIRC). The argument, undeniably true, is that difference causes an impedance bump on the transmission line that decreases performance.
I think it's good to ask yourself if that matters. My approach is to always think in terms of electrical length and how big the impedance bump is. The length of a mated pair of UHF connectors is around 1.3 inches. I’d like to make sure that’s under 1/20 wavelength – and 1/20 wave is admittedly arbitrary and probably because I can’t recall the last time I saw a bump that small that mattered. The formula for the length of a quarter wave vertical, in feet, given f in MHz is 234/f; 1/20 wave is 1/5 of that, and since 1.3 inches is 0.1083 feet, that gives us