APRS communication in Crystal Grottoes Caverns in Boonsboro MD

APRS communication in Crystal Grottoes Caverns in Boonsboro MD

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: Gallery Link

Crystal Grottoes (Prius Digis!)

Setting up the APRS Radios

Deep inside the cave

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:

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.


APRS map of cave-2

Inside the Cave setting up a radio

Link # 1