[amsat-bb] Dual band cavity filter?

[Floyd Rodgers]

 Could you give more specifics of what you are trying to do? If it is intermod suppression why not just use one or two comet or diamond duplexers? 
    On Tuesday, July 21, 2020, 06:32:48 PM CDT, Robert Bruninga via AMSAT-BB <amsat-bb at amsat.org> wrote:  
 
 1-in 2-out is fallback but only if impossible for 1 in and 1 out.

Do you think if we make the UHF closer to the 3rd harmonic of the VHF that
we can make it work with one cavity?
I assume that due to stray effects that the UHF peak will NOT be exactly
the 3rd harmonic so there can be some isolation maybe.

Since the radio is dual band full duplex, I wonder if it can protect itself
if one TX on vhf and RX on 3rd harmonic.  ALl that power into the UHF front
end?

I'd start building but just today in my Chemo saga, my right arm is
almost useless  I have to move my right hand from mouse to keyboard with
my left hand.  So whipping up something is a dream at this point.  This has
come on suddenly...

Bob
P

On Tue, Jul 21, 2020 at 5:53 PM GEO Badger <w3ab at yahoo.com> wrote:

> 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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_______________________________________________
Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available
to all interested persons worldwide without requiring membership. Opinions expressed
are solely those of the author, and do not reflect the official views of AMSAT-NA.
Not an AMSAT-NA member? Join now to support the amateur satellite program!
Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb