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" [email protected] To: "amsat bb" [email protected] 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. _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb