[CQ-Contest] Bandpass filter

Joe nss at mwt.net
Tue Jul 10 07:35:58 PDT 2012

That is super complex. but, can these stubs like seen here,

Be made switchable using say radio shack DPDT slide switches.

I'm thinking low power barefoot rigs, in a Field day situation. Where I 
make out of double sided PC board stock a square rectangular box just 
big enough to hold the connections and the switches.

Where the operator is say on 20 meters and is being effected by the 15 
meter station nearby. He switches in the stub for 40/15

Possible like drawn here?


The Original Rolling Ball Clock
Idle Tyme
On 7/5/2012 6:49 AM, Tom W8JI wrote:
>> 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
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