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## [TowerTalk] High Antenna Modelling

 To: [TowerTalk] High Antenna Modelling jfarr62@home.com (John D. Farr) Sat, 2 Feb 2002 18:11:21 -0600
 ```Tom: The pattern is also a strong function of polarization and is a strong function of the dielectric constant of the earth for vertically polarized antennas. John ----- Original Message ----- From: To: Sent: Saturday, February 02, 2002 9:12 AM Subject: Re: [TowerTalk] High Antenna Modelling > > The pattern enhancements (peaks) and cancellations (nulls) > are a result of combining the incident wave from the antenna > with the ground reflection. It is strictly a function of geometry > and wavelength (frequency). > > For angles where the reflections are IN PHASE, there is an > enhancement. For angles where the reflections are OUT of PHASE, > there is cancellation. Note that there is a 180 degree phase shift > at the ground reflection. > > See the chapter on "The Effects of Ground" in the > ARRL Antenna Book for a detailed explanation. > Figure 2 depicts the phase difference between > the incident and reflected waves using the concept > of an 'Image Antenna' at an equivalent distance below > ground. > > At 6 wavelengths above ground, the peaks and nulls alternate > every 2.5 degrees over perfectly flat ground. In real life, small > changes in slope can cause the nulls to 'fill in', if not completely, > at least enough to become harder to detect when the peaks > and nulls are close together, i.e., when the antennas are more > than 4 wavelengths high (> 140 ft on 10M, 190 ft on 15M, etc). > > Tom N4KG > > > On Sat, 02 Feb 2002 Don Havlicek writes: > > > > Comments inserted below: > > > > Pete Smith wrote: > > > > > > At 09:13 PM 2/1/02 -0500, Chuck wrote: > > > > > > > >In getting back to the basics, I submit that at heights over 4 to > > 6 > > > >wavelengths above ground, the antenna is in "free space" and > > > >that's why the "nulls"are filled in. Also, at these heights, in > > free space > > > >means you have no gain from ground reflection, so they are 5-6 db > > > >down from lower antennas. How does one define free space? How > > > >high above ground would you have to be to be considered in this > > category? > > > > > > A lot higher than 4-6 wavelengths. I ran a 10m yagi in EZNEC at > > 200 feet > > > above ground, and both the deep nulls and reflection gain were > > still fully > > > evident. > > > > Obviously, the mathematical functions utilized within the modelling > > program do NOT vary when the height above ground [or HAAT] is > > 'excessive'. > > > > This is one of the reasons that we must NOT trust the software to > > give > > EXACT answers, but rather to give 'idealized' situational results of > > the > > MATHEMATICS involved [not shouting ... just emphasis]. > > > > Don-N8DE [BSc, MAt, ABD - 'all but dissertation' - Mathematics] > > > > > > > > 73, Pete N4ZR > > > > > > > > ________________________________________________________________________ > > Where do you get ICE bandpass filters & beverage matching boxes? > > The > > same place that pays for the hosting of this list: The eHam Store. > > Order online at http://store.eham.net. > > ----------------------------------------------------------------------- > > > > ----- > > FAQ on WWW: http://www.contesting.com/FAQ/towertalk > > Submissions: towertalk@contesting.com > > Administrative requests: towertalk-REQUEST@contesting.com > > Problems: owner-towertalk@contesting.com > > > > ________________________________________________________________ > GET INTERNET ACCESS FROM JUNO! > Juno offers FREE or PREMIUM Internet access for less! > Join Juno today! For your FREE software, visit: > http://dl.www.juno.com/get/web/. > > ________________________________________________________________________ > Where do you get ICE bandpass filters & beverage matching boxes? The > same place that pays for the hosting of this list: The eHam Store. > Order online at http://store.eham.net. > ----------------------------------------------------------------------- > > ----- > FAQ on WWW: http://www.contesting.com/FAQ/towertalk > Submissions: towertalk@contesting.com > Administrative requests: towertalk-REQUEST@contesting.com > Problems: owner-towertalk@contesting.com > ________________________________________________________________________ Where do you get ICE bandpass filters & beverage matching boxes? The same place that pays for the hosting of this list: The eHam Store. Order online at http://store.eham.net. ----------------------------------------------------------------------- ----- FAQ on WWW: http://www.contesting.com/FAQ/towertalk Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com ```
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