[Antennas] Ground Planes

Sat Apr 9 19:55:45 EDT 2005

Hi Bill:

  I've read your response several times and am not quite sure what you mean.  Is is safe to say that two radials 6 inches above the ground will be less effective than two radials at 64 feet feeding an elevated 40 meter vertical?  I suspect most feel this is a true statement.  This was all I was saying.  The comment about being 1/2 wavelength off the ground was made to try and identify where ground detuning of the radials ends.

   Generally, any ground mounted vertical will require a minimum of 50-60 radials to to approach an 80% efficiency, so when you stated that a modeling program suggested that just two radials six inches off the ground were better than a copper plated back yard, I just assumed the modeling program was missing some data.  

Ron W7MRR

 --- On Wed 04/06, Dr. William J. Schmidt, II < bill at wjschmidt.com > wrote:
From: Dr. William J. Schmidt, II [mailto: bill at wjschmidt.com]
To: pacificcommonwealth at excite.com, antennas at mailman.qth.net
Date: Wed, 6 Apr 2005 22:16:41 -0500
Subject: Re: [Antennas] Ground Planes

Ron... a couple of issues here... 1... <<they [radials] need to be at least 1/2 wavelength about the ground on the frequency of use>> Well that all goes back to the assumptions you make about the conductivity of the soil, and how you would model it. If ground assumptions were perfect like in NEC2, you would come to this conclusion. If not [in reality], as can be better modeled NEC3 or NEC4... you would come to a completely different conclusion based upon the true conductivity of the soil. I would say you are correct if the antenna were over salt water. 2. Those two elevated radials are more efficient that the underground copper lattice work. The same model shows that the buried 120+ radial system is even more efficient. 3. <<Multiple radials of varying lengths would be best to enhance the conductivity of the ground return.>> Untrue. Since the current density limit can be truly extrapolated to infinity [e.g. zero current exists at the end of a radial of infinite length ONLY], I would argue that it makes no difference how long the radials are... SO LONG AS they are greater than some nominal length... that would not detract from providing "a high percentage" of efficiency. Therefore, you would always make the radials the maximum length, and as long as is "feasibly" possible. Again, read Severens paper. >From the charts, I would say that 0.25 wavelength is marginal, 0.5 wavelength is great, and 1 wavelength is excellent... anything longer than 1 wavelength is not of value. Remember, the radials are not meant to be resonant. Sincerely, Dr. William J. Schmidt, II K9HZ Trustee of the North American QRO - Central Division Club - K9ZC Email: bill at wjschmidt.com WebPage: www.wjschmidt.com "If computers get too powerful, we can organize them into a committee; that will do them in." -- Bradley's Bromide ----- Original Message ----- From: "Ron W7MRR" <pacificcommonwealth at excite.com> To: <bill at wjschmidt.com>; <antennas at mailman.qth.net> Sent: Wednesday, April 06, 2005 2:23 PM Subject: Re: [Antennas] Ground Planes > > > Bill: > > Your modelling software may need some parameters. For elevated radials > to be totally effective [read this to mean not affected or detuned by the > ground], they need to be at least 1/2 wavelength about the ground on the > frequency of use. On 40 meters, this would be 64 feet or so. Less than > that and more radials would be required to acheive the same result and it > would take considerably more radials than two within six inches of the > ground. > > Generally speaking, I would guess you would need to approach the 120 > radials needed to ground mount a vertical if they were only 6 inches above > the ground. In addition, the wavelength of the radials when mounted on or > near the ground becomes less critical, except that longer is better than > shorter for the same number of radials. Closer than 1/2 wavelength, the > ground detunes the formual electrical length so that cutting them to a > specific formula length doesn't really deliver what it implies. > > Multiple radials of varying lengths would be best to enhance the > conductivity of the ground return. Lastly, the ground conductivity will > greatly vary the effectiveness described above and, as you know, > conductivity varies greatly throughout the world. A lot of variables and > a subject not easily explained. > > Ron W7MRR > > > > --- On Tue 04/05, Dr. William J. Schmidt, II < bill at wjschmidt.com > wrote: > From: Dr. William J. Schmidt, II [mailto: bill at wjschmidt.com] > To: antennas at mailman.qth.net > Date: Tue, 5 Apr 2005 22:38:18 -0500 > Subject: Re: [Antennas] Ground Planes > > Interesting. I just modeled your system... two radials elevated six > inches off the ground on the diagonal is more effective than copper > plating the entire yard on 40-10 > meters... Sincerely, Dr. William J. Schmidt, II > K9HZ Trustee of the North American QRO - Central Division Club - > K9ZC Email: bill at wjschmidt.com WebPage: > www.wjschmidt.com "If computers get too powerful, we can organize > them into a committee; that will do them in." -- Bradley's > Bromide ----- Original Message ----- From: "Mel Vance" > <icra at galaxynet.com> To: <antennas at mailman.qth.net> Sent: Tuesday, > April 05, 2005 10:18 PM Subject: Re: [Antennas] Ground > Planes >> of my test equipment when I hit 240 radials... and > basis my threshold, >> about 120 radials was really optimum given the > cost of the wire. >> >> Meshing your back yard... while it may give > you wood... isn't really >> cost practical. >> > Radials are be! > st...if you have 100 some feet dia to put them in..... > > With a > yard just 20 feet by about 100 feet, and cluttered with sheds, > > trees, ect, no way to > jam 120 radials in there....hard to fit more > than 10 into the yard > available and clear trees, > sheds, and > other stuff > > A mesh system should give a better ground than the > hand full of radials I > have room for. > Also, my goal is not just > for an antenna ground plane...due to very heavy > clay, getting a > > good ground off a ground rod is very difficult. The mesh would get around > > that problem. > > Mel > N7OKL > > > ______________________________________________________________ > > Antennas mailing list > Home: > http://mailman.qth.net/mailman/listinfo/antennas > Help: > http://mailman.qth.net/mmfaq.html > Post: > mailto:Antennas at mailman.qth.net > > ______________________________________________________________ Antennas > mailing list Home: ! > http://mailman.qth.net/mailman/listinfo/antennas Help: http://mailm > an.qth.net/mmfaq.html Post: mailto:Antennas at mailman.qth.net > > _______________________________________________ > Join Excite! - http://www.excite.com > The most personalized portal on the Web! > ______________________________________________________________ Antennas mailing list Home: http://mailman.qth.net/mailman/listinfo/antennas Help: http://mailman.qth.net/mmfaq.html Post: mailto:Antennas at mailman.qth.net

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