On Jan 5, 2010, at 8:22 PM, Greg Niles wrote: > Thanks for the reply Richard. I have the GE 110 watt PA I only antipate > needing around 35 watts not going to build a power house just a locat > repeater. I know I can detune the PA was just trying to cut out the PA for > simplicity purposes if that is possible. > > Greg K9GJN
Greg, Do you want 35W of RF to the antenna AFTER the duplexer? You'll have to put in more than 35W to get 35W out to the antenna in a duplexed system, of course. Or are you really saying that you want 35W into the duplexer where you'll probably only have 10-15W at the antenna after that? (Okay that's an exaggeration but have you forgotten that a duplexer is relatively a very lossy device in your TX chain?) The 110W VHF MASTR II hooked into a properly designed and built antenna system, with low-loss hardline feeding the antenna (and usually backed off a bit from 110W... no need to run it flat out, but you can... they're rated for it), proper low-loss jumpers to the duplexer/antenna system, etc... usually yields just about the right amount of power to make a system "balanced" with 50W mobiles accessing it. Unless you're purposely trying to restrict the coverage area to where a modern 50W mobile with a typical modern sensitivity receiver can't hear the repeater very well where it can still access it, I'd leave the 110W PA on it. For a MASTR II, the stock .25uV sensitivity of the receiver (if you tune carefully and get factory "spec" out of it) will mean you usually need to do some work on the receive side (adding a very low noise pre-amp and appropriate large high-Q bandpass cavity in RF noisy environments...) to bring the receiver sensitivity up to where HT's can get into it pretty nicely in most of its coverage area in flat-land at 110W, and you'll get those HT's in fine in virtually ALL of it's coverage area if it's up high (mountain, really tall building, etc.) GE built it as a 110W repeater for a reason... think about the "users" at the time... relatively deaf trunk-mounted radios (made up of the EXACT same components as the repeater) in Public Safety vehicles with external antennas... also often running 110W non-continuous-duty... Anyway, to get back to your question... the UHF 100W PA is "hackable" where you can easily remove the driver stages and get a lower wattage PA - useful for links, etc... that you know don't need the power. But the VHF has a feedback circuit between the driver section and the first amplification section that must be dealt with to do a similar modification. Just run 'er flat out or up around 85-95W... also consider putting an isolator on it, especially if yours doesn't have the late-model Z-matcher in the Low-Pass-Filter near the PA's output. The VHF is especially sensitive to not being driven into a 50 ohm load and they tend to burn up when they're not. The factory Z-matcher was supposed to alleviate this, but their tuning instructions for the thing leave something to be desired. Easier/more effective to just put an isolator on the PA output with proper filtering.... again, eating some of your RF... so 110W starts to look just about right again... -- Nate Duehr, WY0X n...@natetech.com http://facebook.com/denverpilot http://twitter.com/denverpilot
