[aprssig] APRS Cave Radio Test Crystal Grottoes, MD

Joel Kandel kandelj at bellsouth.net
Sun Dec 9 06:11:53 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. 
Joel, KI4T

Sent by Smoke Signal

On Dec 8, 2012, at 9:07 PM, Robert Bruninga <bruninga at usna.edu> wrote:

> The test today by WB4APR and KD3SU successfully demonstrated APRS
> communication throughout the Crystal Grottoes Caverns in Boonsboro MD
> (a small "900 foot" cave but full of the most beautiful and continuous
> formations I have ever seen).  Greg has an Album of photos:
> http://kd3su.us/photos/piwigo/index.php?/category/5
> We parked our two cars very near the entrance, one beaconing on UHF
> and the other on VHF every 12 seconds.  Then we paid entrance fee and
> went inside. The UHF was lost within the first tens of feet.  The VHF
> was strong  more than 150' through 3 very acute bends (think "Z"'s)
> and two level changes. The passageway was between 2 to 4 feet wide,
> and 6 to 10' tall all that distance.  Then we began to notice weak
> signals and found a ledge where we could set our first TH-D72 digi and
> made another 120 degree acute turn.  Down this long but straight
> passageway again from 2 o 4' wide but as high as 20' or more, we were
> shocked to lose the signal if we even made a 6 or 10 foot excursion
> off the straight path*.  So, since we were again going to make another
> acute 120 degree turn, we dropped our second CAVE-2 digi but this was
> less than 100' away from DIGI-1.
> The rest of the cave passages were all within about 100' of this
> location and so we had solid coverage.  Greg went back out of the cave
> and came around to the exit and we had VHF good voice contact through
> about 150' of  again, acute switch-back passageways maybe 3 to 5' wide
> and 6 or 7' tall.
> Lessons learned:
> 1) This test shows that a single link can make it about 100' or so
> through very constrained and convoluted small passages.  This will be
> useful in Mammoth where sometimes such small byways connect larger
> through tunnels..
> 2) We hope that the very large and more or less straight Mammoth cave
> passages 20-30' wide and 10' to 30' highby hundreds of feet long will
> provide great distances per link, and these can be linked by any
> trouble areas as shown in #1 above.
> 3) We were operating on borrowed time from the tour guide and so we
> had NO TIME to do any keyboarding or to set our positions, or to
> change paths or really do anything but keep moving (and pausing evry
> 12 seconds to confirm we were still hearing the 5W beacons from my
> car..
> 4) Unfortunately, Since we do not have PREEMPTIVE digipeating, then
> the only way a packet could make it through the cave and into the
> APRS-IS was only if the up-to 6 hop path matched exactly our position
> in the network.  And since we were using HOP3-3 inside the cave, and
> had my car set with an alias of LOT in the parking lot, we were using
> the path HOPn-N,LOT,WIDE2-2 inorder to have a chance of getting into
> the APRS world.
> BUT, If n-N did not match to -0 at the LOT digi, then it went nowhere.
> And even if it did get digipeated by the LOT digi, (also running 5
> watts to match our handhelds so that we could assume our paths were
> bidirectional based on what we could hear).  Then this 5W parking lot
> digi had to be heard by a mountain top digi (that could also hear
> Baltimiore/Washington DC) hearing one of the highest density APRS
> regions in the country.  And then be successful twice to get to an
> IGate.  None did.
> My HT was set to HOP2-2,LOT,WIDE2-2 and I never did set to HOP1-1 and
> so the only time any of my packets coiuld have made it past the LOT
> digi was after we got the CAVE-2 digi in place towards the end.
> SO, although every packet made it to the entrance, linking into the
> APRS-IS cannot be pracitcal until we have a pre-emptive digi at that
> location.  We cannot expect the underground team to beable to predict
> exactly how many hopsto nail the gateway dig with everything else
> going on.
> Frustrations and Risks:
> * Trying to program 3 THD72 digis, 3 D7's and two D710's in a parking
> lot in 40 degree weather from their keyboards for this test was dumb.
> Every radio had to have ALL of these settings changed from normal:
> Turn on UITRACE to HOP and change UITIME to 10 seconds from 30
> Change SYMBOL and BText.
> Set manual LAT/LONG, select MANUAL,
> Chances are very high that not all radios got set right.  This is why
> we need lots of people who know their radios well and can each be
> responsible for their own settings.  .
> * Turning on the radios to test at the entrance was like a chain
> reaction.  There was so much beeping we could not tell at all what was
> happening.  Oh, also don't forget to enable the TNC!  THe plan was to
> just turn on one radio/digi at a time until signals were weak, place
> the radio as a digi, set its path and settings and then continue.  But
> since the tour guide was responsible for us, and was on a limited
> schedule, this plan cannot work.
> Photos including ones of the digis and a map of the cave will
> eventually get tot he web page.  TO hold the HT digi's upright, I used
> a 4" to 3" PVC pipe adapter and a 3" pipe cap.  The full size HT
> antenna poked up through a hole in the pipe cap, and the bottom oft he
> 4" adapter had a 3/4" thinck base.  Inside was a ring of foam to hold
> the radio from banging around.  A 19" counterpoise wire was screwed
> under the belt-clip screw and dangled out the bottom.
> In Greg's photos, the cave entrance is simply a 10' set of stairs from
> just inside the door of the building.  You can also see the digi's,
> though we did not bother putting the top's on any of them since we had
> to have access to the digi keypad for fixing set-up errors.  The big
> photo near the bottom is their biggest room, about 15-20' wide and
> maybe 12' high.  The door at the bottom is the exit.
> One of the digis, with its full size2m whip and 19" counterpoise would
> not hear packets that a standard D7 with rubber duck was hearing fine.
> It couild have been a null, but I want to test ALL radios on Monday
> with a sig gen to make sure all were up to snuff.
> Bob, WB4APR
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