Are you then saying that some of my cavities I have for 150Mhz, I can use without any mods for around 450Mhz. If this is correct, I would be very happy, as I can use them, instead of them sitting around waiting to be used.
Kev. ----- Original Message ----- From: "skipp025" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Friday, June 24, 2005 11:41 AM Subject: [Repeater-Builder] Re: how broad are cavities? > The shame about the dumpster part of your post. A 1/4 > wave cavity at 150MHz makes a 3/4 wave cavity at 450MHz > skipp > >> Don Pomplun <[EMAIL PROTECTED]> wrote: >> Thanks for the insight, Paul. >> The fact that they are spec'd by fairly wide ranges was the info I was >> hoping for. I called a friend today whose system switched from the 150 >> band to a 400 MHz trunked system a few years ago to see if they had > any of >> the old cavities laying around. He said that they had just finished >> cleaning house last week and all the old junk went to the dumpster, > but he >> would do some diving to see what was still there. >> Regards, >> Don >> >> At 12:03 PM 6/22/2005 -0400, you wrote: >> >Don, >> > >> > > 1. Cavities are roughly tuned by their >> > > physical dimensions, and then tweaked by rods, plates, >> > > etc., eh? >> > >> >The length of the cavity (more importantly the inner >> >conductor of the cavity) sets the frequency. Usually >> >cavities are designed to cover a RANGE of frequencies... >> >the tuning rod coming out the top adjusts the length of the >> >center conductor inside the cavity. The tuning range >> >varies depending on manufacturer and model... some typical >> >ranges are 136-150, 150-174, 136-174... and many others. >> > >> >You might want to check some of the articles referenced on >> >the following page for more on cavity and duplexer theory: >> > >> >http://www.repeater-builder.com/rbtip/ant-sys-index.html >> > >> >There is a section on Cavities, Duplexers, etc. part way >> >down that page. >> > >> > > 3. How awful a compromise would it be to use available >> > > cavities from a 2 meter system at railroad frequencies >> > > (160-161 MHz) ? >> > >> >If the cavities will tune that high, they probably work just >> >fine. If the are now on the 2 meter band (below 148 MHz) >> >they may or may not tune at 160 MHz... it depends on >> >whether the center conductor can be adjusted that far by >> >means of the tuning rod. >> > >> >If you are actually referring to a duplexer (several >> >interconnected cavities to allow use of a single antenna >> >for transmitting and receiving) there may be other factors >> >to consider. The cavities themselves may tune where you >> >want them, but the length of the coaxial cables >> >interconnecting the cavities may need to be optimized for >> >that frequency to get best performance. >> > >> >The cavities may have more than one adjustment... if they >> >are bandpass-bandreject cavities, there will be one >> >adjustment for the length of the center conductor (usually >> >a big knob / tuning rod) and another adjustment of some >> >sort to set the frequency of the notch relative to the pass >> >frequency. >> > >> >If you can tell us what specific cavities (or duplexer) you >> >are looking at, some of us can probably provide more >> >specific information. >> > >> >Paul > > > > > > > > Yahoo! Groups Links > > > > > > > > Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
