At 06:46 PM 5/10/04, Trentkd5ia@aol.com wrote:
I'm confused. When I model my future tower using YT, the best performance
(pattern and gain) occurs with a spacing of 0.5 wavelength. This spacing
outperforms 0.6, 0.9 and 1.0 wave spacings on my flat ground (Texas
panhandle). So, why would 50 feet spacing on 20 meters be better than 35
feet? Plans are to have 4 elements for 20 meters at 70' and 105', and
hopefully someday another one at 140'.
Trent I am not familiar with YT, but I have used HFTA to
for modeling my terrain. If YT works as HFTA does, it simply
assumes fixed point source antenna gain and shows the effects
of ground reflections, etc. HFTA does not actually compute your
gain, it just assumes what you tell it. A modeling program like
Eznec or YO will actually compute the interactions of multiple
antennas in a stack and allow you to determine the optimum
stacking distance to maximize system gain (in free space).
Optimum stacking distance is a function of your antenna
pattern. Antennas with narrow beamwidths need wider stack
spacing to optimize gain. An old rule of thumb I recall from
somewhere is to take the square root of the boom length in
wavelengths. If we follow this guideline, the 20-4CD with
a boom length of 31.5' should be stacked at about 47' for
optimum gain. SQRT (31.5 / 70) = .67 wavelengths = 47'.
Of course you should really use a program like YO to first
determine the best stacking distance and then optimize antenna
dimensions in the stacked configuration, which may be quite
different than if they were optimized as a single Yagi. The
interactions between antennas in a stack will be greater the
closer you space them, so it becomes even more important to
optimize them if you use close stack spacings.
As N4ZR said previously, I am not aware of a program
which will automatically optimize over actual terrain, although I
believe AO may be capable of it (manually) in combination with
YT. I don't have either AO or YT but perhaps someone who
does will comment since I am interested to know also.
73, Bill W4ZV
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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