On the first station I had with the internal filter/duplexer, I tried tuning it with the recommended equipment: sig gen, RF voltmeter, tuning probe, dummy loads. It was rather difficult and I wasn't happy with the results, so I figured "I'm smarter than the average bear" and I tried tuning it with my spectrum analyzer and tracking generator. The results were even worse.
Eventually I went back to the recommended method and substituted my own 50 ohm dummy load for the PA during tuning. One thing I did find was that you MUST follow the procedure in the book and not go back and readjust the coils again unless you back them out and start over from the beginning. You'll just end up chasing your tail that way. The tuning is very sharp using the tuning probe; it's very wide using the SA/TG combo. All of the filters are bandpass, so tuning by nature is somewhat broad. Using the tuning probe and "dip" method, like with the front end, you see the suck-out effect in the neighboring coil and you know exactly when you've got it tuned. It's almost like watching the notch tuning on a duplexer. Just follow the instructions and tune each core once. I ended up with just over 2dB loss in the post filter and slightly less in the pre-filter. If you do the math, I think you'll find that your 110/70 ratio is right about on the money, even though it doesn't agree with what you see in the manual. It may also be that Motorola considers those to be maximum power levels with those configurations, not necessarily that you'd get 110 watts without the F/D and 85 watts with it on the same station. If the pre-filter has too much loss, the station may complain and not transmit because it required too much drive from the IPA, but that doesn't seem to be your problem yet. Also remember that the station's power control circuit is measuring the RF voltage at the output of the PA, so the amount of power you see will also depend on the load impedance. The load presented by the internal F/D is anybody's guess, especially after you tune it up. If you had something like a return-loss bridge, you could look into the ends of the filters and adjust the first coil for best return loss / match, and hope the rest work out OK. But if I recall the procedure in the book, you work your way towards the PA of the post-filter so the input (left-most) coil would be adjusted last anyway. The 110w PA is really capable of a lot more power than Motorola rates it for. I've seen stations with the internal F/D making over 110 watts out of the antenna port. The IPA, PA, and internal F/D are all either range-1 or range-2. In your case, range-2 covers about 435 to 475 MHz. The PA has a built-in circulator so it needs either a low-pass filter or the internal F/D after it to get rid of the 2nd and 3rd harmonics that could result. A good external duplexer that has a real bandpass cavity on the input will also do the job. The low-pass filter used on base stations starts cutting off around 600 MHz and it reduces the 2nd harmonic and higher by over 80dB. Those are my thoughts at the moment. You can reach me off the group if you want to get down to the nitty-gritty. Bob M. ====== --- On Fri, 5/8/09, Adam Feuer <[email protected]> wrote: > From: Adam Feuer <[email protected]> > Subject: Re: [Repeater-Builder] First UHF MSF5000 Attempt > To: [email protected] > Date: Friday, May 8, 2009, 5:06 PM > Hi Mark, > > Interesting about your MSR2000. I'm not sure about > it's band splits but > if I'm not mistaken, my MSF is "in band" all the way down > to 438mhz. I'm > suspicious about my tuning of that 4 pole filter because of > my lack of > knowledge on the use of a tracking generator. Although, > when I was done > tuning it, my scope looked very similar to the pics on the > MSF page here > on Repeater-Builder. I tried tuning it > three times and every time I'm > done, I only get 70 to 72 watts at the side of the > cabinet. OH > well....thanks anyway! > > Adam N2ACF > > Mark wrote: > > Adam, > > > > It may be far enough out-of-band for the harmonic > filter that it isn't > > passing as much RF to the antenna connection port, > regardless of what the > > specs call for... I had an MSR2000 that was like > that - it was in the > > commercial UHF band and worked fine, but when it got > moved to 444.5500, it > > kept burning up the PA - melted the connections to the > harmonic filter, even > > with silver solder. (We finally gave up on that > one.) > > > > Mark - N9WYS > > > > -----Original Message----- > > From: [email protected] > On Behalf Of Adam Feuer > > Sent: Friday, May 08, 2009 2:49 PM > > To: [email protected] > > Subject: Re: [Repeater-Builder] First UHF MSF5000 > Attempt > > > > Hmm??? The book at my friends shop said (if I > read it right) that a C74 > > with the filter option was rated at 110 from the PA > and 85 at the > > junction box which is why I'm concerned about my 71 > watts. > > > > > > James Delancy wrote: > > > >> UHF models that are 110 Watts, are rated for that > out the side of the > >> cabinet. This usually results in 160 Watts > out of the PA. Then you > >> are down around 100-110 Watts after the harmonic > filter and the > >> pre/post pass filters. > >> > >> James > >> > >> > >> Maire-Radios wrote: > >> > >>> *well if it was 85 watts with 122.5 watts* > >>> *it would be lower at 110 watts and any > be the 71 watts do the > >>> math should be about the same values.* > >>> ** > >>> *John* > >>> ** > >>> > >>> > >>> ----- Original Message > ----- > >>> *From:* n2acf > <mailto:[email protected]> > >>> *To:* [email protected] > >>> <mailto:[email protected]> > >>> *Sent:* Friday, May > 08, 2009 2:49 PM > >>> *Subject:* > [Repeater-Builder] First UHF MSF5000 Attempt > >>> > >>> Hello All, > >>> > >>> Attempted to tune up > the TX side of a C74CXB today. The odds > >>> weren't really in my > favor as I've never worked on a UHF MSF nor > >>> have I ever used a > tracking generator. I brought the station to a > >>> friends shop and I > used an IFR1200S for the tuning. > >>> > >>> I checked the station > before I began and on 453.900 there was > >>> 82.5 watts at the > antenna port and 122.5 coming out of the PA. > >>> So, I reprogrammed the > station, adjusted the VCO, and then began > >>> to re-tune the 3 pole > and 4 pole filter. I also changed the > >>> output power of the PA > to 110 watts because I believe that is > >>> what the station is > rated for. The new TX frequency is 441.950. > >>> > >>> When I was done with > both filters, I checked the power at the > >>> antenna port and it's > only 71 watts. I believe it's supposed to > >>> be 85. > >>> > >>> The PA looks very > clean on the spectrum analyzer and the carrier > >>> is right on frequency. > Again, I'm not the best at reading the > >>> scope while using the > tracking generator but I did it the best I > >>> could for the first > time. Is my output power OK at 71 watts or is > >>> something wrong? > >>> > >>> Thanks in advanced! > >>> > >>> Adam N2ACF
