Dumber Faster

Y’know how a quarter-wave stub of coax feedline has interesting properties? Like, if you attach one end to an antenna analyzer and left the other end open at 1/4λ down, then the open circuit looks like a short on the analyzer; zero Ohms resistance. And if you short the end, it looks like an open; infinite Ohms resistance. This is a great way to help you figure out the true velocity factor of a piece of cable; find the lowest frequency where this effect happens, then measure the cable for its physical length, then compare that length with the length of 1/4λ in free air.

Velocity Factor = 1/4λ(air) / 1/4λ(cable)


Well, I wasn’t trying to calculate the VF of a piece of coax; I was trying to cut 1/4λ of coax at 20m (14.2 MHz) so I can use that stub to tune a new vertical dipole I’m going to raise. So I brought out my new MFJ-259C analyzer on its maiden run. This rev of the meter shows you the complex impedance in R and X values, which my old analyzer didn’t do. Really helps you dial in the impedance matching.

1/4λ effects! Physically open line, shorted electrically!

So I multiplied 14.2MHz by 4 to get my quarter wavelength, then tuned the meter to 56.8MHz and noticed the cable I’d cut a few nights ago was horribly long. So I started snipping. Snip, snip, snip, snip, snip. Chopped off about 2.7ft of coax before I could get it to look like a 1/4λ at 56.8MHz. Success!

Snip-snip-snip! Sometimes your workbench is your floor.

No. Not successful at all. Stupid. Absolutely stupid. I failed at logic.

I should’ve set it to 14.2MHz and adjusted from there. But no, I’m kinda dumb. Why the hell did I think to quadruple the frequency for a quarter wave? Failure in logic. Can’t un-cut the cable now. Maybe I can stretch it? (No, I can’t do that.) The cable’s now too short for even the 6m band at 50MHz. I’d have to use up another PL-259 connector to make another cable.

I guess for what I need, this will be, eh, sufficiently short for the analyzer to be electrically close to the antenna as I tweak it. I got 10-ish physical feet to raise/lower the antenna, so hopefully it’s short enough to eliminate/reduce any effects of feedline. My hope is anything below 1/4λ should be enough. Who knows?

So let me tell you this much: finding the right tool for the job will help you do stupid things more efficiently.

Radio is hard.

Published by Shawn

He's just this guy, you know?