[aprssig] HF underground

Jim Lux jimlux at earthlink.net
Tue Nov 20 09:07:02 EST 2012

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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