Re: [TowerTalk] Antenna modeling

[Pete Smith <n4zr@contesting.com>]

At 12:07 PM 12/8/2004, Jim Lux wrote:
> > #3. When modeling an antenna with traps how do you know what values to 
> > plug in as loads for the traps?
>
> Measurement? Ask mfr? Ask other hams who have one? Model it from physical 
> dimensions, and adjust trap values until the SWR curves look like the ones 
> in the spec sheet.


This what I have done with linear loading, as well.  Simulating it with a 
lumped inductance and the right length elements gives me what I choose to 
believe is a sufficiently high-fidelity model for interaction and similar 
large-scale phenomena.  Some modern antennas -- the F-12 C'3s come to mind 
-- probably won't give you believable feedpoint impedances anyway -- but 
the patterns seem to be pretty credible.

Some very effective tri-bander stacks have been designed using single-band 
monotaper models of antennas like TH-7s, bypassing the whole tra\sp 
exercise altogether

Modeling antennas with LC traps could be a real pain in cut and try mode, 
given all the possible combinations that will resonate at a particular 
frequency.  In that connection, AC6LA's MultiNEC is a really neat tool, 
because it will let you automatically alter multiple parameters in a design 
singly or all at once, producing hundreds of runs from which you can select.

73, Pete N4ZR




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