[Amps] Pi tanks vs Pi L + LOW PASS

[Richard]

>> 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. 

*  In my experiences, this is not rocket-science.  The idea that only 
recognized amplifier experts can understand the esoteric secrets of 
proper layout smells kinda like bologna. 
-    After I added 160m coverage to a SB-220, I was curious about 
harmonic radiation.  I measured second-harmonic radiation in the 
near-field at c. 58db down.  The L-network antenna tuner that was feeding 
the 160m halfwave end-fed antenna was probably responsible for some of 
the attn.

>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. 

* For better vhf harmonic suppression, connect a 50pF doorknob ceramic 
cap from the output connector to chassis gnd. The Xc will also help to 
cancel the intrinsic L in the T/R-relay when operating in bypass-mode.  
The inductance of the lead connecting the cap to the output terminal 
forms the L part of the vhf filter.
>
>> 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.
>
*  When a roller coil-form is made from a material which does not have a 
low D-factor - such as Delrin, PVC or Nylon, especially when feeding 
high-Z loads, the coil form can catch fire.  Better materials are: 
alumina [ceramic], ABS, G10 [glass-epoxy], Micalex, and Rexolite.

>...

>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. 

*  160m and 80m tank inductors also have such vhf resonances.  
>
>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!

*  One solution is to use a separate, 10m tank-L made from larger surface 
area conductor material -- with the roller-L set to zero turns on 10m.

>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


-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
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