[CQ-Contest] Bandpass filter

[Tom W8JI]

> Has anyone built a switching stub box for multi-band operation that
> switches in the proper stubs for the current band?  Any issues or things
> to watch out for?

It's pretty simple to do.

I have a 8 position switch box in my radio room that has stubs on ports. 
This is the same box that selects antenna trunks to different areas.   I use 
steering diodes to activate stubs when certain antennas are called for.

http://www.w8ji.com/Xtal/RR8-1.jpg

http://www.w8ji.com/images/W8JI%20site/wirehiderright.jpg

I effectively have band data available on control lines, because my antennas 
are monoband or grouped by band on my antenna control boxes.

I also have some stubs right at antennas. My N-S-E-W 160 pattern is fed 
through a highpass network, and it happens to have a reasonable match on 80. 
Rather than change the network, I simply added a stub out there that 
activates on an unused port of an RCS8V switch when 160 is selected.

This could be more complicated when a multi-band antenna is used, like a 
tribander or duobander, because the band sensitive information might not be 
available on control lines.

I'd like to re-enforce something....you can get a deeper null if the stub is 
NOT right at the output port of most amplifiers. This is contrary to the 
common (but incorrect) wisdom that a filter belongs as close as possible to 
the device generating the harmonic, but in fact nearly all filters are 
sensitive to location and the optimum location is almost never right at the 
output port of the device except perhaps for VHF or UHF harmonics. I think 
what is true for TVI filters carried over into HF filters, where it is not 
applicable.

I think the common wrong assumptions are either location of stubs and 
filters do not matter at all, or they need to be as close as possible to the 
amplifier.

73 Tom