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## [TowerTalk] Re: [DX] [Fwd: Low yagi question]

 To: [TowerTalk] Re: [DX] [Fwd: Low yagi question] n4kg@juno.com (T. A. Russell) Fri, 4 Jul 1997 07:25:09 -0600
 ```> I did receive one reply from a fellow who claimed that, the yagi >antenna tends to "force" a low take off angle when compared to a >dipole >at the same hight. If this is so, I should enjoy better dx >performance, >along with the quieter reception from the yagi...even with the low >hight >of 40 feet. Is there any truth to this forced low angle radiation from >yagi antennas?? > >Thanks, 73 from AB2CJ > The radiation pattern from any antenna over real earth is the product of it's free space pattern (linear scale) and the ground reflection coefficient (also linear scale). The OLD ARRL antenna books carried charts showing ground reflection coefficients for various heights above ground with a linear scale. Since the pattern of a dipole in free space is equal at all elevation angles, the ground reflection coefficient pattern corresponds to the pattern for a dipole antenna. Yagi and Quad antennas obtain gain by refocusing energy in the desired direction. A 3 element Yagi pattern is typically down 3 dB at +/- 50 degrees in the vertical plane, thus reducing higher angle radiation. Stacked Yagi's (and quads) cutoff at even lower angles, leading to the impression that they "force" lower angle radiation. In fact, all they are doing is reducing high angle radiation. GROUND REFLECTION coefficients depend SOLELY on HEIGHT above ground (in wavelengths). A tribander's pattern will peak at different angles on the different bands because it's (fixed constant) height is different for each band in terms of wavelengths. BTW, a tribander at 40 ft will cover all the higher angles that propagate on the high bands. Separate higher antennas are needed to maximize the lower angles when MUF is marginal. BOTH high and low Yagi's are needed to cover ALL the angles that the ionosphere will support due to pattern nulls in the vertical plane caused by ground reflections. de Tom N4KG -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search.htm ```
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