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 _______________________________________________ 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
