Well, you kind of messed it up. Tuning the pass band for best received signal is ok but any time you move the pass band even a small amount you mess up the notches big time. Then when you tuned for minimum noise with the repeater transmitter on you probably moved the notches for best noise rejection when you tuned the pass band tuning again. The notches follow the pass band tuning.
What you might try again is to peak the pass band tuning (on receive side cavities) on the weak signal like you did the first time and then don't touch the pass band tuning again. Now turn the repeater transmitter on and tune the "notch" tuning controls on the receiver cans for minimum noise from the transmitter while listening to the weak signal. That should optimize the receive side of the duplexer. The above may cure the problem. If not then the transmit side may also need tuning. You would want to tune the pass band part of for maximum power out but be sure to do this at VERY low power out (a few watts only) or you will cause the tuning rods to arc and have burn spots and it will ruin the duplexer! After peaking the transmit pass cavities for maximum power out then turn up the power and listen to the weak signal again. Now tune the transmit can notches for least noise in the receiver. Again, do not touch the pass tuning once you start to tune the notches. This is a crude way of doing things but with no test equipment you may be able to get it useable. Note, when tuning the notches don't go too far on each tuning adjustment as they may be way off after tuning the pass and you may not have the proper amount of rejection on the notch to start with and may miss the proper tune spot. Go slow. It is best to check desense with the repeater on a dummy load to make sure that there are no antenna problems also. 73 Gary K4FMX > -----Original Message----- > From: [email protected] [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of Jeff > Sent: Tuesday, April 24, 2007 1:14 AM > To: [email protected] > Subject: [Repeater-Builder] Tuning By Ear? > > Hi all. > Installing a 2m repeater back into a higher/better location. > The duplexer, (Wacom 641), was tuned and seemed to be operating > perfectly. > When moved to the new location, there was a lot of TX noise and > desense introduced into the system. > With a weak and STABLE portable station transmitting into the input > frequency, and listening on an HT connected through the RX side, I > noticed a fair amount of attenuation. (Signal straight from antenna > was full scale, thru duplexer was about a 7). > SO, I (by ear and signal) retuned the RX band pass filters to get an > almost full scale signal. > When the repeater was put back on the air, The densense problem was > better, so I listened to the noise level on his signal and carefully > adjusted each band=pass filter for minimum noise. > This seemed to help the desense problem. When I adjust transmit > power from 0 watts to 35 watts, I hear no audible noise added. At 40 > watts, noise begins to enter the received signal. > > Now, I'm concerned about WHY this happened, mechanical changes, > impedance mismatches, etc., and if I could have messed anything else > up by doing this "by ear". > Also, I'm thinking I could do the same with the TX pass filters, but > I also know I could really mess up VSWR and other things without > having proper field test equipment. > Any thoughts? Should I leave it as is, or could I possibly do some > tuning to make the duplexer better? > Repeater is 35 watts out, and currently pretty well "balanced" I > think.. If a station can hear it decently with a 5 to 7 S-meter > reading on a moble, 1/4 antenna, they can get in with a slight bit > of noise at 5 watts. Is this suitable, acceptable, or should there > be more receive? > > Thanks, 73 > Jeff - N5VAV > > > > > > > Yahoo! Groups Links > > >
