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 <r...@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-boun...@mailman.qth.net] On Behalf Of > kev...@coho.net > Sent: Sunday, April 30, 2017 1:40 PM > To: elecraft@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, kev...@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 >> >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:Elecraft@mailman.qth.net >> >> This list hosted by: http://www.qsl.net Please help support this email >> list: http://www.qsl.net/donate.html Message delivered to >> kev...@coho.net >> > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html Message > delivered to r...@elecraft.com > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to wun...@wunderwood.org ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
