APRS in a Cave

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).

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

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.


Leave a Reply