[TowerTalk] Changing Beverage impedance in cold WX

[jimlux]

On 1/16/20 10:08 AM, VE6WZ_Steve wrote:
> Over the years I have noticed my Beverage feed impedance will change during the winter.
> My station is configured to take analyzer sweeps in the shack of any of my 12 Beverage wires.  All 12 wires show the same change in impedance compared to the summer.
> 
> Two days ago I did some experiments to see if this was because the frozen ground was limiting conductivity. In wet soil with the associated minerals and salts, the ionic transfer contributes to conductivity, just like salt water.  When the ground is frozen solid, like a block of ice, it seems possible that this ionic conduction could be diminished.
> 
> To test my theory, I add some radials to both the feed and termination and rescan the Beverage.
> Join me on the adventure to see how it worked out in this video as I brave the -38 C winter cold.  I also show some calculations of the effect of cold weather on Beverage wire conductivity and the termination resistor.  This is perhaps the most interesting, because the term resistors I use are ceramic units which have a significant negative temperature coefficient.  I experiment to confirm the changes in resistance with temperature.
> 


this is quite interesting.. The epsilon of frozen soil is dramatically 
lower than not frozen, and as you say, there's a difference in 
resistivity.  I've seen a fair amount of literature about sea ice vs sea 
water, and I've been working with Lunar regolith simulants with various 
amounts of ice content at HF frequencies recently.

What would be interesting is that as the soil freezes, the skin depth 
will increase, so even though the top layer is frozen, at low 
frequencies (where you're using a Beverage) there might be significant 
RF in unfrozen soil. So the performance changes due to freezing might 
not be as big as you might expect.