[TowerTalk] Loading a Pro-AM 160M on other bands?

Jim Lux jimlux at earthlink.net
Fri Feb 16 15:33:13 EST 2007

At 10:50 AM 2/16/2007, Cecil Moore wrote:
> >From: kd4e <kd4e at verizon.net>
> >Subject: [TowerTalk] Loading a Pro-AM 160M on other bands?
> >Date: Fri, 16 Feb 2007 12:03:31 -0500
> >Is the coil and feed system on such a vertical
> >such that trying to load it on 80, 40, 20, etc.
> >via an antenna tuner is a waste of time?
>Every loading coil has a self-resonant frequency somewhere above the
>frequency it is used on for mobile operation. It occurs at the frequency
>where the distributed capacitive reactance is equal to the inductive
>reactance. A mobile loading coil should not be operated at or above
>its self-resonant frequency. As a data point, my 75m bugcatcher coil
>is self-resonant around 6 MHz, making it not useful on 40m.
>160m loading coils are large with lots of distributed capacitance. Odds
>are, they are all self-resonant before the 80m frequency is reached
>and certainly before any of the other HF frequencies.

So that it would look like a series capacitor above the self resonant 
frequency, right?  On a high enough frequency, the series X from the 
C might be pretty small...

>  So, to answer
>your question, operating a loaded mobile antenna far above its resonant
>frequency is usually an extreme waste of RF energy.

Before leaping to "usually" and "extreme", you might want to qualify 
it with some specific examples... After all, a trapped multiband 
antenna relies on a series of self resonant circuits along the antenna, no?

Jim, W6RMK

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