[Elecraft] NVIS HF vs VHF line-of-sight & CB

[Ron D'Eau Claire]

Ground losses mount rapidly as a horizontal antenna is lowered closer to the
earth. So, while the pattern may show the main lobe straight up, the amount
of RF lost in the earth below increases. 

EZNEC confirmed to me that about 0.2 wavelengths up is the optimum height
for the strongest vertical lobe (NVIS pattern). That fits with the fact that
0.2 wavelength spacing between the driven element and reflector of a Yagi
produces the maximum gain. Running a wire near the ground helps too, since
the earth is, at best, a poor dielectric instead of an efficient reflector. 

Well supported towers have withstood some serious quakes, including our
land-mobile repeater towers in the Loma Prieta earthquake that broke the
S.F. Bay bridge and knocked down a good part of downtown Santa Cruz back in
the 80's. 

However a tower, no matter how robust, is of little use if operations need
to move elsewhere. They are darn clumsy to move by hand in an emergency. 

73, Ron AC7AC

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of
kevinr at coho.net
Sent: Sunday, April 30, 2017 1:40 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] NVIS HF vs VHF line-of-sight & CB

I lose my antennas about once each year to straight line winds. Flying
branches knock them down.  But if the wires are still intact I can run the
nets with the lowered antennas.  Using less than optimal antennas works;
just not as well as perfect antennas.  In an emergency I really don't care
about perfect performance, I simply want to contact someone for assistance.
By all means put up the best antenna you can just remember any antenna can
make contacts.

Years ago I wrote some software to display antenna propagation patterns as a
half wave dipole was lowered from 1 wavelength above ground until it was on
the ground.  The results were pretty interesting.  By using the program I
found many heights would work well depending on where I wanted to contact.
At less than 1/10 wavelength above the ground the radiation patterns got
rather odd but still worked for in-state comms.  
Once I had modeled what was going to happen I tested it by dropping my
antennas to different heights and tested comms.  Theory and practice
correlated quite nicely.

     Kevin.  KD5ONS

P. S. The application also modeled 1/4 wave verticals and loop antennas.  I
never found the time to model the Yagi-Uda, the math got too hairy.  Using
Euler's equation a few times got me through the three antenna types I was
able to model.  Beating on the Bessel functions would have taken longer than
the time I had allotted to me.

      K.


On 4/30/2017 1:22 PM, Bill Frantz wrote:
> I have always wondered how towers hold up during earthquakes. Being 
> able to work with ad-hoc antennas seems a good attribute for any 
> emergency plan.
>
> 73 Bill AE6JV
>
> On 4/30/17 at 11:34 AM, kevinr at coho.net wrote:
>
>> Please try NVIS on 40 or 80 meters. You'll find you can cover most of 
>> the state with its use.  Plus the antennas can be ad hoc - tossed 
>> into trees or even an old fence line.
> -----------------------------------------------------------------------
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