[aprssig] APRS Cave Radio Test Crystal Grottoes, MD

Robert Bruninga bruninga at usna.edu
Sun Dec 9 08:59:57 EST 2012

> Bob. I discovered years ago installing radio comms  on large cruise ships
> that VHF radio signals propagated up down and through the ships where
> the passages acted as waive guides. The caves possibly do the same.

Good point.  During my 3 year tour on the LCC-19 in Westpac in the
early 80's, I hooked up an autopatch to an unused telephone in a radio
room and then carried my HT.  The XO could call me anywhere on that
phone (even if I was in the rack).  My longest phone call was over 75
miles from the top of Mt FUJI back to the wardroom to make
reservations for supper.

The waveguide theory works because everything is a conductor.  From
what I read, caves operate more like perfect dummy loads since the
walls are more like the absorbers in an anechoic chamber.

Even if the range is only a hundred feet, some may wonder why bother.
The reason is that some caves, just getting in that first 100' may
take hours.  THen once in, they can walk around freely.  Being able to
communicate from a base camp on the inside to the outside is valuable
during cave rescue.

Come to think about it, a few hundred feet of light wire to link
between two digipeaters through very circuitous crawl passages might
also be a handy tool in the tool box.

Also, come to think about it, for carrying these digis, I need to come
up with a lighter weight housing instead of 4" PVC!


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