Why do my antenna tuner settings change over time?

Yesterday, a ham wrote to me:

I’ve been a ham since I was a young teen in 1965, but only recently became active again, in retirement. I know you are an antenna guru, so I thought I’d run a question by you, if you don’t mind.

I have a couple of new G5RV’s made by W8AMZ, one for 80-10 and another for 40-10, polarized in opposite directions, up 55 and 45 feet in different parts of my city lot here in Ann Arbor. The rig is a Icom 751A and I’m using a Daiwa CL-680 antenna tuner.

I’m able to get close to a 1.0 swr on most bands on both antennas, after jockeying the capacitance and inductance settings (after first listening for peak signal strength). It’s a bit of an annoying process, as the low SWR setting I get in low power will vary as I increase the output to full power, and being slightly off will result in a high SWR.

My question for you is why the settings for a minimum SWR on the antenna tuner for either antenna vary more than slightly from day to day, rather than remaining fixed? As I understand it, the tuner’s function is to make the impedance between the tuner and the rig look like 50 ohms, despite the fact that the transmission line to the antenna has a section of 450 ohm ladder line, a balun and 52 ohm coax feeding it (both > 90 feet). Seems like that shouldn’t be much of a moving target.

I opened the Daiwa tuner and both variable capacitor plates are consistently 1/2 meshed at the “5″ reading (on a scale of 10) and the knobs are on solid, so it isn’t a matter of the capacitance scales being inaccurate/varying.. Even the best setting on the inductor (which has 18 settings) will vary +/- a setting over the course of a month. Can you explain or point me to a reference that would explain why, for a given frequency, the capacitance/inductance required for the lowest SWR would vary over time? ¬†Thanks.

I replied:

Actually, I’m not an antenna guru at all, nor do I have any experience with G5RV antennas, but I’ll take a swipe at answering this.

Several things could cause the impedance of an antenna system to vary:

  1. The antenna system itself. If the connections are not solid, the impedance could vary. Another thing that could cause the impedance to vary is whether or not the antenna is wet. Also, make sure that the connection between the coax and the ladder line is sealed. If water gets in there, then weird thing might happen.
  2. The metal objects near the antenna. If the ladder line comes close to metal objects, then that could affect the antenna impedance.

Having said all that, I’m not a big fan of G5RV antennas. The original design was not meant to be a multi-band antenna. That bit of ladder line is actually supposed to be a tuning stub to transform the impedance of the doublet to 50 ohms on 20m. SWR on the other bands could be quite high, and that could make the coax very lossy on the other bands. If I were you, I’d either get more ladder line and run that directly into the tuner, or ditch the G5RVs entirely and build some fan dipoles. Fan dipoles are dipole antennas with elements for each of the bands that you wish to operate.

I also said that I’d post his question to my blog. I’m sure that some of you will have other and better ideas about his conundrum.