[TowerTalk] Antenna software

jimlux jimlux at earthlink.net
Sat Sep 6 18:11:29 EDT 2008

N7mal wrote:
> Scott MacKenzie wrote:
> ""Simple to use and reasonably accurate""
> "Reasonably" is the key word for all the modeling programs available. 
> They are a very useful as a guide but they are not the 'end-all-be-all' 
> for defining an antenna. The modeling software gives you a fairly good 
> starting point but as has already been said the final result requires 
> getting your hands dirty: lengthening and shortening etc etc etc.
> 73

Actually, modern modeling tools can very precisely predict the behavior 
of the antenna being modeled. It's not unusual in a professional setting 
to "build to print" from the model and if the performance differs, you 
assume that the test setup is faulty, or the construction of the antenna 
had a problem.  Done right, no cut and try should be needed.

However, to get this level of modeling performance with simple antennas 
typically used by hams might require more work to do the model than to 
actually build the antenna and cut and try.  For instance, rarely do 
hams actually erect an antenna in a place where the surroundings aren't 
in the near field where they'll have an effect (just how do you enter 
your house in a NEC model?).

I am aware of some ham antenna installations where they have spent 
literally years carefully building the models for towers, guy lines, 
support structures, etc.; and for those folks, they can probably model a 
proposed design, then build the same design, and get exactly the same 
performance (within tenths of a dB).

If you have a situation where you have multiple antennas and you want to 
understand interactions, before going to substantial expense to build 
and test, the modeling tools are essential.

If you want to "get a feel for" the impact of moving wires or changing 
supports for inverted-Vs and dipoles and the like, the simple modeling 
tools are great.  If you want to know if that rain gutter is going to 
interact, the tools are great, because you're really looking for a sort 
of qualitative answer, not a "the gutter will change the gain on this 
azimuth by 0.1 dB" sort of thing.

If you read about some new "miracle" antenna, you can zap it into a 
model and immediately know if they are "breaking the laws of physics". 
(note well, snake oil antenna designers always claim, "my antenna is so 
special, it can't be modeled".. nonsense... ask them for specific 
reasons why the model doesn't work..)  Sure, your quick and dirty model 
might not get you to 0.1db or even 1 dB, but if someone is claiming 10dB 
improvement from their design, even a crude model will show *some* changes.

Jim, W6RMK

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