[amsat-bb] Re: 1/2 Wave "cavity" filter design?

[i8cvs]

Hi Bob,

If the the classic 1/2 wave cavity filter you need is for 144 MHz you will
find the answere to all your questions in my article:

"Filter and Receiver Front End Protection Device for 144 MHz EME "
published with all building details into the AMSAT Journal May/June
2010

If it is for 70 cm you will find the description ,drawings and application
in the following address.

http://www.k3pgp.org/432filter.htm

BTW in a separate email I have sent to you a pdf file of my article for
144 MHz.

Have fun

73" de

i8CVS Domenico

----- Original Message -----
From: "Robert Bruninga" <bruninga at usna.edu>
To: "amsat bb" <amsat-bb at amsat.org>
Sent: Sunday, July 21, 2013 6:33 PM
Subject: [amsat-bb] 1/2 Wave "cavity" filter design?


> Given the classic 1/2 wave cavity filter, what is the relationship
> between inner cavity and center conductor sizes?  Of course bigger is
> better, but what is "OK".
>
> This is a 1/2 wavelength tube with a center conductor shorted at each
> end.  The input is loop coupled at one end, an output is loop coupled
> at the other end.  In the middle is a tiny variable capacitor (usually
> just a screw)  to tune to resonance.
>
> In the limit, as the cavity size shrinks, you can end up with what
> could be considered as just a piece of 1/2wave coax.
>
> Im looking for a really cheap Home-Depot plumbing design that 15
> people can reproduce to give them good front end antenna filtering
> when operating on mountain tops adjacent to other RF souces. (say
> within 100 yards, not permanent installations which of course should
> spare no expense at getting the best cavities possible).
>
> We just finished our 4th annual Golden Packet attempt from Maine to
> Georgia along the Appalachian trail and many stations were plagued
> with front end overload. http://aprs.org/at-golden-packet.html
>
> I'd like to come up with a 3/4" copper pipe design that is robust,
> provides sevral dB of out of band rejection.  Im trying to understand
> the parameters that drive the size of the center conductor.  Normally
> bigger is better for better bandwidth, but I think smaller will give
> me steeper skirts and better rejection?  I dont mind say 2 or more dB
> insertion loss, because as it is, front end-overload is making us
> totally deaf and anything would be bettter.
>
> Lastly, I think such a 1/2wave filter will also pass as a 1.5wave pass
> filter on UHF.  We need dual band, since we use dual band rigs and
> coordinate on UHF voice from the same antennas and coax used for the
> VHF packet.
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