[TowerTalk] How to check trap resonance

[Roger D Johnson]

john.brewer at us.schneider-electric.com wrote:
> Can anyone suggest a workable method of determining the resonant frequency
> of a set of traps off of a Cushcraft tribander?  I've got a working GDO, but
> cannot find a dip with this configuration.  Perhaps its my method of coupling
> the GDO to the trap. I've just been placing the GDO coil nearby the feedline
> end of the trap as it sits on a wooden stool.
> 
> Anyone have any tried and true methods for doing this?
> 
> Thanks
> John

In the dark recesses of my memory, I remember seeing an article about
determining trap L/C values by using an RF bridge to take measurements
of the trap above and below the traps resonant refquency. Once the L/C
values are determined, the resonant frequency can be calculated.

Many years ago, I wrote a BASIC program to determine the L/C values to
resonate an antenna on two bands simultaneously. I used it with a 3/8
wave vertical on 160 and 80m. I used a modeling program to determine the
reactance required at the antenna base for each band. These values were
plugged into the program which would then  calculate proper the L and C
values. The result looked like a parallel resonant trap but didn't
operate that way. I think Butternut uses the same technique with their
antennas.

I think the program could be modified to operate in reverse. Knowing the
reactance values above and below Fr, the L and C values could be
calculated.

73, Roger




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