[TowerTalk] Soil Probe Calculator

[Brian Beezley]

 > Thanks very much for this, Brian.

 > Can't wait to try it out!

 > 73, Jim K9YC


Jim, Rudy Severns, N6LF, is developing a new 24" soil probe. In the 
process he discovered that common-mode current on the long cable he runs 
to his desktop computer from his older VNA can seriously disturb the 
measurements. He used a common-mode choke, but it was inadequate below 3 
MHz. I thought of suggesting one of your wideband compound chokes, but I 
finally suggested that he just buy a NanoVNA and make isolated 
measurements. Rudy will be writing up his new probe for the ARRL Antenna 
Book. He will emphasize common-mode issues, which can be insidious.

I'd like to attach a photo of Rudy's new probe, but I gather that's not 
possible. Here's a link to a post on the NanoVNA users group that 
includes the photo:

https://groups.io/g/nanovna-users/message/26166

Rudy's new probe uses 3/8" aluminum rods spaced 4". Rod length (tip to 
crosswire) is 26" and exposed (soil surface to wire) is 2" when fully 
inserted. None of these dimensions is critical.

Here's an image of what a poor choke yields. As a test, Rudy wound a 
narrowband 1 MHz resonant choke and permittivity rose beyond 110.

http://ham-radio.com/k6sti/badchoke.gif

I'll delete this image in a day or two. I don't want it cluttering my 
website.

Brian