[TowerTalk] Beverage impedance change in cold WX solved

[VE6WZ_Steve]

A few days ago I posted a video explaining some experiments I did adding short radials to my feed and termination grounds on the Beverage.
During that week temperatures went as low as -41 C at the station.

The mystery has been solved.

I wondered if the frozen ground was limiting conductivity and upsetting the ground resistance part of the impedance. The experiments showed that adding radials to stabilize the frozen ground had no effect.

In the video I show how the real resistance of the 920 foot galvanized steel wire might drop 10-20 Ohms when the temperature drops 50 deg C from the summertime.  This would mean that I need a LOWER termination resistor to match the surge impedance.
However, I also show that the ceramic termination resistor I use has a very high negative temperature coefficient (-1300 C). This results in an INCREASE of  30 to 40 ohms in my 470 Ohm termination resistor.  Therefore, the termination could be 40 to 60 Ohms too low!!  I show in the video that the resistor does indeed increase by 40 Ohms when I chucked it out my back door at -30C for an hour.

Today I was at the remote and it has warmed up to -1C (35 deg C warmer) and the analyzer sweep of the wire has mostly returned to it summer time reading.

This seems to indicate that the main cause of the Beverage miss match is the temperature coefficient of the wire and the termination resistor. Obviously the ground is still just as frozen today as it was last week at the feed and termination and under the wire length!  Perhaps I will look into changing the term resistor with a metal foil unit that has a positive temperature coefficient.  Carbon composition units also have a negative TC, but not as severe as the ceramic units.  Those ceramic units are almost like thermistors! (well….not really)

However, as I said in the video, this is like a “solution looking for a problem” because few Hams experience these extreme temperature changes, and even when we do, it's just for a short time.  Also, Beverage modelling shows that the termination resistor can effect the F/B, but it has limited effect on the RDF which is what really matters.  This exercise was done mostly out of interest and to try and solve the puzzle.

I made a new video showing the sweeps I made today if interested.  The video includes the original content so if you've seem that just jump to the end.

YouTube video here: https://youtu.be/3Oft826Q8tA <https://youtu.be/3Oft826Q8tA>



73, de steve ve6wz