[amsat-bb] Dual band cavity filter?

[GEO Badger]

Bob,
You've stirred, not shaken, some ideas in my pea brain. 2 band/1 input in-out is the difficult part. That's why it's expensive. Of course the machining adds cost too.
How about 2 band/1 input- 2 outputs?
---  
 Ciao baby, catch you on the flip side  
73 de W3AB/GEO     

 http://www.w3ab.org

You can say "over", you can say "out", you just can't say "over and out". 

    On Monday, July 20, 2020, 06:37:48 AM PDT, Robert Bruninga via AMSAT-BB <amsat-bb at amsat.org> wrote:  
 
 Many of us use dual band radios but when operating on mountains or hills we
need a front end cavity filter.  The DCI dual band helical filter is ideal
but I just learned it is $530!

I'm starting to visualize  a cavity filter made out of a piece of aluminum
drain pipe.  With a capacitive screw at the top for tuning VHF and a
smaller one about 1/3rd the way up for UHF.

My theory is that the lower UHF capacitance will be at a UHF high-Z point
and will tune the UHF nicely and the one at the top will tune the VHF
nicely.  Of course with very much interaction.

Unfortunately for our annual Golden Packet Event, the 3rd harmonic of our
APRS VHF operating frequency is at 432 and our use of APRS UHF talking freq
at 445.925 is quite away away.

Maybe if such a single cavity design could work, we could use the event's
UHF voice coordination frequency closer to the 3rd harmonic to make the
filter easier or possible to tune dual band  (Though we night then also
Desense voice with every packet).

I want it to buildable from Home Depot and common tools.  Wont be as good
as the $530 DCI filter, but maybe close enough for our event.

It wont be robust and it wont be quality, but might work for us AND amsat
dual band operators in the field.

my big assumption is that a 1/4 wave cavity filter will also work on its
3rd harmonic?  We only need to knock down QRM to prevent desense.

Anyone have cheaper than $530 solutions?

Bob
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