Terry, I understand and appreciate the fact that you are forced to employ a makeshift procedure for duplexer alignment, since you do not have the proper equipment (spectrum analyzer with a tracking generator, or the best choice- vector network analyzer, aka VNA). While it is very true that one can get fairly close with a handheld radio as a detector, close is not always close enough.
The most obvious problem when using a handheld radio is that the antenna connection on a handheld is very seldom optimized at 50 ohms. Even if the rubber duck antenna has a BNC connector on it, one cannot assume that the radio's source and load impedances are even close to 50 ohms. Since a rubber duck antenna is usually nothing close to 50 ohms, the radio-side impedance is more likely whatever will be close to matching the antenna. I found this out the hard way, when both of my Icom IC-2AT handhelds (remember them?) suffered blown final transistors when I used them on a roof-mounted quarter-wave whip on my car. My mobile antenna was carefully tuned to be close to 50 ohms, but the handheld radios were not a good match, and the finals blew. Most commercial portables- the Motorola HT1250 is a good example- must use a special BNC adapter to convert the normal antenna matching circuit to 50 ohms for use in bench testing or to connect an external antenna. So, I suspect that your tuning method is resulting in a mis-tuned duplexer that does not properly match your repeater. If you possibly can find a shop that has a VNA, pay to have it tuned precisely to your repeater pair, and do not touch any of the adjustments afterwards. The next best choice is a shop that has a good spectrum analyzer with tracking generator, and used with a return-loss bridge. The benefit of a VNA or an RLB is that the tuning of the bandpass function for the best return loss is far more sensitive and precise than tuning for minimum insertion loss. The return (reflection) loss is seen as an extremely sharp spike or notch, while the transmission loss is just a broad curve. Too, a VNA has precise 50 ohm matches on all ports. Once you've used a VNA to tune for return loss, you'll wonder how you ever managed without it. No matter what laboratory instrument was used to tune your duplexer, try to avoid the temptation to "tweak" it once it is installed. In every instance of which I am personally aware, the careful shop tuning of a duplexer was negated when a "diddle-stick artist" could not resist the temptation to "improve" the tuning at the site. But, I digress... If you have checked all of the connectors, jumpers, and feedline for problems and found none, I still believe that your duplexer is not tuned correctly. Since you state that the duplexer in question was used on the repeater for more than a decade with no problems, and presumably on the same frequency pair, why are you re-tuning it? You probably have heard the old saying: "If it ain't broke..." 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of terry dalpoas Sent: Saturday, September 19, 2009 5:58 PM To: [email protected] Subject: Re: [Repeater-Builder] UHF Duplexers Eric, The repeater is 70cm amateur (444.625 TX/449.625 RX). These duplexers have been used on this same repeater for I'd say 10-13 years. I was given the repeater by the previous owner because the site was lost and he did not have time to repair the problems that did show up (the PA went out). I had a mobile PA that I turned down and used it and ran the repeater from my house for a few months (ran fine), then disconnected it. It sat in my shop from Oct 2008 until around 1 month ago. It was not as before. I wound up basically rebuilding the receiver, and I rebuilt the original PA. However, this year in Oklahoma, we did get a lot of rain and it did get pretty warm in my shop (along with high humidity. I wonder if the heat and humidity might have done something to the duplexers. I do have another set of these that were on the link transceiver (the link was set up to run full duplex as well), I may retune them to see what I have. Equipment and tuning method. Here is what I was taught how to do it. I use an IFR 1200S service monitor, and the only spare radio I have at the moment is an Icom T7H HT. I start with RX (449.625) first. I connect the IFR to the antenna port, a receiver on the RX port (high pass), and a dummy load one the TX (low pass). I adjust RX pass freq for best SINAD. I then change the receiver and IFR to the TX freq and adjust for the best rejection, or worst SINAD. I do the TX (444.625) side the same way, best SINAD TX pass, worst SINAD RX reject. I tuned a set of duplexers this way before on a 2m repeater using 6 cavity Sinclair BpBr with no problems, this is the first I have messed with a set of Wacom's. Terry KM5UQ ________________________________ From: Eric Lemmon <[email protected]> To: [email protected] Sent: Saturday, September 19, 2009 6:30:57 PM Subject: RE: [Repeater-Builder] UHF Duplexers Terry, Is your repeater in the 70cm Amateur band? If so, and your WP-678 duplexer was not originally tuned for that band, that may be where the problem lies. I have a Remec-Wacom WP-678 duplexer that I bought from the factory for GMRS service, and its insertion loss at the transmit frequency is 1.14 dB, equivalent to 38.5 watts out for 50 watts in. Your duplexer's measured insertion loss exceeds 4 dB. Most Wacom UHF commercial-band duplexers will tune down into the Amateur band, but not all of them. It appears that your duplexer is tuned improperly, or it may be impossible to tune it to your pair. What method and equipment are you using to tune the duplexer? 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@ yahoogroups. com <mailto:Repeater-Builder%40yahoogroups.com> [mailto:Repeater-Builder@ yahoogroups. com <mailto:Repeater-Builder%40yahoogroups.com> ] On Behalf Of terry dalpoas Sent: Saturday, September 19, 2009 2:11 PM To: Repeater-Builder@ yahoogroups. com <mailto:Repeater-Builder%40yahoogroups.com> Subject: [Repeater-Builder] UHF Duplexers Just about finished with the Mastr II UHF repeater. I've run into a problem with the duplexers. They are a set of WACOM BpBr Model WP678. I am putting 48W into them, but only 18.5W out. I believe this should be better. RX will open squelch at around .2-.3 uV through the cans (this I know is OK). Could there be a problem with the TX side of the cans? Terry KM5UQ
