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.
bob finger wrote:
>Run HFTA. You will be amazed to see that the little one at 35 feet will
>be MUCH better to some areas of the world at times. Here on the east
>coast there are times when the 40 footer will be 8-10 db better into EU
>and SA and AF than will the same antenna at 70 feet. It is the
>difference in being first in or among the last to be heard. Everyone
>who contests should try to have one antenna that is at 40 feet or so.
>When the angles are high that beam at 70 feet will have a null right
>where the signals are coming. HFTA will graphically display that so you
>can see what will be going on. 73 bob de w9ge
>Edward Sylvester wrote:
>> But if I don't combine the antennas and simply have them completely
>> separate, is there an advantage here in terms of vertical take off angles
>> because of the different heights of the antennas (i.e. lower antenna better
>> for short path and higher antenna better for long path and short path to
>> farther places)?
>> Or am I just spinning my wheels and reaching a point of redundancy?
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