On 8/9/2016 12:48 PM, Ed Sawyer wrote:
I could be wrong, but I believe that HFTA adds 3dB for stacked
antennas and uses the center point of the stack to do its
calculation. I base that on the fact that you can stack 2 yagis at
the same point (ie zero ft separation) and it will show a 3db gain vs
the one yagi alone. We all know that doesn't play out in real life.
No, as indicated in the HFTA documentation, the software places an
*isotropic source* of the specified gain at the height specified for
each antenna. The software does not model interaction (coupling)
between the two sources so it overestimates gain for closely stacked
antennas (leading to the 3 dB "gain" when the two antennas are at the
same midpoint height).
Using isotropic sources also *overstates* the gain of the higher lobes
as the pattern of a *real* yagi antenna will reduce radiation above
and below horizontal - the radiation at significant angles below the
horizon is responsible for the high angle lobes.
One is best served by modelling the two or three antennas over "real
ground" in a good antenna modelling program and then comparing those
results to HFTA to get some idea of the terrain effects. The basic
antenna modelling program will show the effects of interaction between
(among) the antennas as well as a more accurate elevation pattern over
"flat ground". HFTA can then be used to estimate the impact of
diffraction due to terrain.
... Joe, W4TV
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