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[aprssig] APRS Cave Radio Test Crystal Grottoes, MD

Jim Sanford wb4gcs at wb4gcs.org
Sun Dec 9 14:14:46 UTC 2012


Bob:
I can report similar experiencess, although contrary vhf vs uhf.

Specifically, our 2m and 70cm repeaters in Hampton, VA were on the top 
of the Chamberlin hotel, on Ft. Monroe, adjacent the Hampton Roads 
Bridge/Tunnel.  I had noted for years that the 2m propogated several 
hundred feet int the tunnel at each end.  I speculated that it was a 
waveguide effect, and predicted deeper coverage when we put the UHF on.  
Sure enough, you could talk on the UHF repeater, /all the way through 
the tunnel/, unless there were a bunch of large trucks in the tunnel.

I suspect that your different observations relate to waveguide 
conductivity --- the metal in the walls, or at least rebar in the 
concrete, had to be more conductive than the rock you were in.

All good fun!!

73,
Jim
wb4gcs at amsat.org

On 12/9/2012 8:59 AM, Robert Bruninga wrote:
>> 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!
>
> Bob, WB4APR
>
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