[CQ-Contest] A bandpass filter design

[Jim Brown]

On Fri,4/28/2017 12:37 PM, Jukka Klemola wrote:
> PI4CC have a very good single stub system.
> https://www.pi4cc.nl/tech-info/stubs/
>
> PI4CC have most likely believed the people claiming the double stub 
> would be just 6dB better than a single stub.

That would be true if the two stubs were placed at the same point along 
the line, but each stub provides about 30 dB if they are separated by a 
quarter wave at the harmonic frequency.

>
> PI4CC result is somewhat better than any 2 stub arrangement would be.
> I have an uneasy gut feeling about the presented filters, how they 
> might drift over time.

I don't doubt their ability to design and build good filters, but 
shoving them into a paint can is a really bad idea, because it limits 
air flow around the circuit elements, limiting power handling. At the 
very least, the paint cans should be perforated (that is, lots of small 
holes, high percentage open) so that they allow air flow but provide 
good shielding.

Also, stub placement with respect to the power amp and with respect to 
the antenna matters a LOT, and can be the difference between almost no 
suppression and excellent suppression. That is, a difference as great as 
60 dB! Anyone using stubs for harmonic suppression should study the link 
I posted about stub placement. http://k9yc.com/LocatingStubs.pdf

73, Jim K9YC