Spontaneous Increase in Feed Line Impedance

[Paris]

I was testing a new Wilson Silver Load antenna connected to 16' of Belden coax with an MFJ 259 Antenna Analyzer when I noticed something strange. The impedance would start out at 50 ohms, and slowly, over five minutes or so, creep up to 60 ohms. I disconnected the meter, let everything sit for a while, and then took another measurement, but it happened again. I repeated this twice more with the same result.

What causes this? Is it the MFJ Analyzer not measuring the impedance accurately, or is there something else involved?

 

[bit_pit]

Batteries on the MFJ draining? Is the freq on the display shifting?
Try putting a dummy load on the MFJ and see if it also shifts.

 

[Paris]

.... good idea! I'll try it & see what happens.

 

[Beetle]

The oscillator in an antenna analyzer is not the most stable thing, and if it's in the open air, with wind and changing temperatures, it just makes it worse. Also, and because of the stability factor, they aren't meant to be left on for more than, say, half a minute at a time. They certainly aren't powerful enough to cause anything in the antenna/feedline to heat up.

I agree with the possibility of low battery power, plus the fact that it may have drifted out of the ballpark.

 

[bit_pit]

Paris,
Just as a test, I hooked up my MFJ-269 for 5 minutes to see what happened. It only shifted very slightly ,freq on the display flickered down/up a KHZ or so, and the RS did not change. As a matter of fact, after about 1 minute the display stayed rock solid but showed "SLP", which I have since found out means it has gone into battery save mode...I've never actually left it on for that length of time before.
Not sure if the 259 also goes to Sleep...

As an FYI, the MFJ Website carries calibration documentation for their analyzers (or analysers as we say down here) if you are concerned yours may be off.

 

[Paris]

Thanks bit-pit. I didn't have the right connector to hook the MFJ-259 to a dummy load, so I ran a 10' feed line to an MFJ Loop Antenna, and tried again. As with your experiment, the freq readout only varied 2-3 KHz, while the impedence did not change at all over several minutes. I did notice, however, that when I attached the 259 to an ungrounded antenna, the capacitance of my hand would change the impedence reading. So, I am beginning to think that the increase in the feed line impedence I noticed in the mobile installation has something to do with the way the car chassis works as a ground, but I'm not sure what.