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 <*> 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/
