[Lowfer] Cave RFadio

[Tom Clifton]

The converted SSB CB transverter was a design by Ian Drummond and was used perhaps 10 years ago in the European 73khz and 135khz amateur allocations if I have my history right.  I have been out of touch with that group for a long time, but do believe that current designs (even in the USA - by non-hams) are 135khz and 185khz with small diameter loop antennas pretty much laid on the ground. Probably running 5 watts.   The lower thr frequency, the better the ground penetration.  No way are they part 15 compliant, and I'm not risking my ham license or commercial license by using this equipment.  Some of these guys had part 5 licenses for  "developing cable locating equipment" and "tunnel radio systems" but I suppose that the most polite term for them is pirates that happen to be cavers....

The really serious guys are using 4.096 khz (yup - "audio") which gives outstanding ground penetration and is used for radio location. They can set up a transmitter and loop antenna in a cave and do a surface locate within centimeters.  Cave divers in Florida use this quite a bit.    Carroll Cave in central Missouri  used a Drummond design to figure out where to bore a new access shaft that permitted researchers to avoid tens of hours of long, difficult and dangerous crawling to do whatever it is that they do.  Likewise, Cavers in Oklahoma worked with the Oklahoma State Highway Department to do precise locates of caves passing beneath interstate highways before doing construction work.

It is an interesting subject, and part 5 licenses can be obtained.  Here in St. Louis we are blessed with plenty of caves within a 100 mile radius. It would be an interesting thing to study antenna efficiency and do field strength measurements (both above and below ground) to see what a 5 watt radio with an 18 inch loop laid on the ground can do...