[Amps] RF choke switching

[Larry Benko]

  Using 2 chokes in series and not switching them is not a "carte 
blanche" solution to not having any series resonances.  It is probably 
easier to get 2 chokes to work over all the bands but be wary if you 
don't make measurements.  One choke might be 5+j500 and the other choke 
might be 10-j500 (above the series resonance point) resulting in a low 
impedance for the 2 chokes together.  The only real solution is to 
prototype the choke arrangement in the chassis and measure the RF 
impedance to ground over the range of frequencies you want to operate.  
This is easily done with any of the new small analyzers such as the 
AIM4170, N2PK, etc.  These devices may not be super accurate at an 
impedance of 20k ohms but they show something like 20k +/- a kilo ohm 
and would show series resonances very well.  You would need to do a 
calibration to the end of a short length of cable to get good accuracy.

Larry, W0QE

On 6/10/2011 6:04 AM, Carl wrote:
> The Henry 8K uses 2 chokes and so do several commercial 2-30MHz amps. Some
> switch, some dont.
>
> Carl
> KM1H
>
>