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

[Walter Underwood]

Right, an NVIS antenna is effectively a Yagi pointed at the sky. So put a reflector on the dirt.

The Loma Prieta was a 6.9. The Cascadia area could produce a 9.0. Richter is a log10 scale, so that is 100X as strong. 

wunder
K6WRU
Walter Underwood
CM87wj
http://observer.wunderwood.org/ (my blog)

> On Apr 30, 2017, at 3:49 PM, Ron D'Eau Claire <ron at cobi.biz> wrote:
> 
> 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.
>> -----------------------------------------------------------------------
>> Bill Frantz        | I don't have high-speed      | Periwinkle
>> (408)356-8506      | internet. I have DSL.        | 16345 Englewood Ave
>> www.pwpconsult.com |                              | Los Gatos, CA 95032
>> 
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