Just one added point - for tri-bander stacks, spacings of less than .5
wavelength on 20M are quite common because the spacing used is necessarily a
compromise between three bands (or at least 2, these days). Fortunately,
calculating maximum forward gain is not what we use HFTA for anyhow.
73, Pete N4ZR
At 01:17 AM 8/21/2006, K4SAV wrote:
>I need to correct a couple of statement I made about HFTA. I said HFTA
>always gives 3 dB gain for any antenna stacking distance. That is not
>correct. It gives 3 dB gain for very closely stacked antennas, less than
>about 0.5 wavelengths. For other wider spacings it seems to function
>correctly. Gains for closely spaced stacks, can be manually corrected
>with data from NEC, but you will discover that anything less than about
>0.4 wavelengths yields so little gain as to not be worth the effort of
>stacking. N6BV recommends stacking no less than 0.5 wavelength for HFTA
>because of the gain errors produced.
>Also the gain error is because of the math models used for the stacking,
>not the ray tracing algorithm.
>>Be careful when using HFTA to model stacks. HFTA does not model antenna
>>interaction. All rays are assumed to go outward from the antenna
>>between 0 and 34 degrees elevation. No rays directed upward at the
>>other antenna are modeled. So HFTA will always give 3 dB gain for ANY
>>stacking distance. This is not the way it works in the real world. NEC
>>will show the correct amount of gain from the stack.
>>The big advantage of HFTA is that it can handle irregular terrains where
>>NEC cannot. Your ground can also affect the optimum stacking distance.
>>In other words, an optimum stack over flat ground is not necessarily an
>>optimum stack over sloping ground. You can use HFTA results up until you
>>get to the point where you start to loose gain because of a stacking
>>distance which is too close. You can determine this distance using NEC.
>>If you do not have flat ground, using HFTA is to evaluate the stack is a
>>good plan, because in some cases the terrain can have a huge affect. If
>>you go closer than that stacking distance, calculated by NEC which
>>starts to decrease the gain, you can manually correct HFTA gain numbers.
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