[Amps] Pi tanks vs Pi L + LOW PASS

[Tom Rauch]

> Wondering about the harmonic attenuation of a Pi L tank vs a standard
> Pi  tank followed by a good quality low pass filter.

Having wrestled with this stuff to meet FCC TA guidelines, I've 
learned to watch the layout carefully.

On paper the Pi-L looks a bit better, but in real life the actual 
layout is generally significantly more important than the change of 
components or network used. Subtle things like how you route the RF 
leads to and through the capacitors and how things are grounded means 
more than network Q, especially for higher order harmonics.

For bands close to the operating band, like the second harmonic, by 
far the best results come from using a series-resonant trap on the 
output. A simple single-section trap can add 20 dB or more of 
harmonic suppression, and only takes one large component (a doorknob 
cap) and a small series inductor.      

> I am interested in building an amp using a tune and load cap with a
> good roller inductor in between to eliminate the band switching mess
> and adding a good low pass filter.

Rollers sometimes are great to use and they look neat, but they can 
occasionally cause problems.

As for the low-pass, box the low-pass system and make it RF tight. 
Use capacitors that are good at VHF and UHF, not components with wire 
leads. Mounting it outside the RF deck is better, unless you really 
understand shielding and ground loops.

As for the roller.... while it is mechanically and electrically more 
simple wiring in a roller vs using a switch, rollers can create 
problems. 

For one thing, rollers almost never short unused turns at multiple 
points. They generally have one shorting roller that rolls along the 
coil. That means they commonly have "strong" spurious resonances far 
above the operating frequency, and at those resonances can cause the 
system to have very poor harmonic attenuation on some harmonics. 

Another issue is current rating. In order to handle enough current at 
the ten or fifteen meter end, the component will have to overkill 
the lower bands. Be sure to size it for ten meters, and hope 
operating ten or fifteen with hardly any turns cranked in it will 
have sufficient reactance on harmonics to help suppress them!
 
Close to the operating frequency, everything works the same for 
spurious signals. The further off from the operating frequency we 
look at the differences, the more critical component selection and 
layout becomes.

73, Tom W8JI
W8JI@contesting.com