[aprssig] HF underground (update)

Robert Bruninga bruninga at usna.edu
Tue Nov 20 13:08:28 EST 2012

I just added a map showing the best locations for 5 vertical HF links
(every mile topside) and a concept drawing.  Now all we need is someone to
lay two dipoles on the ground about a mile apart and see what the topside
link looks like on say 80m.


Again, these are LONG wire antennas, not practical in mud-crawling
caves... but here we are playing in Mammoth where it would be easy.

Bob, WB4aPR

-----Original Message-----
From: aprssig-bounces at tapr.org [mailto:aprssig-bounces at tapr.org] On Behalf
Of Jim Lux
Sent: Tuesday, November 20, 2012 9:07 AM
To: aprssig at tapr.org
Subject: [aprssig] HF underground

On 11/20/12 4:00 AM, aprssig-request at tapr.org wrote:

Cool project..

> HF VERTICAL LINKS:  I just realized that since HF penetrates a few
> hundred feet vertically, we can use HF to link up to the surface every
> 7 hops or so.  Just an FT-817 with a DIPOLE above and below ground
> hooked to a TNC can provide the link  Once above ground, then hops
> continue on VHF.

The attenuation is quite high for HF propagation underground (1-10
dB/meter). And, while going lower gets lower loss, you also run into
antenna efficiency issues if you don't use a full sized antenna. Most
people propagating RF in this kind of application use loop antennas, by
the way.

Bear in mind also that parts of that cave are full of water, and I
suspect that the rock strata are also quite wet. Propagation through
water is worse than through rock.

> Each vertical link takes up 2 hops, but it sure beats having to do 14.
>   We need some people experimenting with HF packet FM (with squelch) on
> 80m (below ground since it would not be legal above ground)...  I
> gotta do some research on HF links...

The FCC doesn't care where you radiate, above or below ground.  Either
your license and service allow a particular modulation or it doesn't.
And your below ground transmitter will radiate into the above ground
environment.  In any case, isn't one of your end points above ground?
Is it receive only?

You could probably apply for a Part 5 or STA license, if you want to use
that modulation.

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