Thanks Eric; I feel more comfortable with my plan now. Mark - N9WYS
-----Original Message----- From: Repeater-Builder@yahoogroups.com On Behalf Of Eric Lemmon Mark, I assume that Celwave uses a network analyzer when setting the coupling loops, and tweaks them for maximum return loss at the pass frequency. I have never tried to do this myself, since I seldom use or tune mobile-type notch duplexers. A network analyzer has no peer for tuning duplexers or ferrite isolators. A pass hump is rather vague and broad when viewed on a spectrum analyzer, but is a very sharp and prominent dip when viewed as return loss on a network analyzer. Before I got my network analyzer, I used an Eagle return-loss bridge. Although it (the Eagle RLB) is a bit cumbersome to use, it is well worth its cost when the return-loss function is not needed on a daily basis. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com On Behalf Of n9wys Eric, I'm curious how this phenomenon would evidence itself on a network analyzer. I ask because I was planning on doing just this (use a notch duplexer rather than Bp/Br) for a low power repeater application I am working on for the future... and the guy who tunes my duplexers locally uses a network analyzer to do the work. I have seen them tune up "well" in the past, but you now have my curiosity piqued. Thanks in advance! Mark - N9WYS -----Original Message----- From: Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> On Behalf Of Eric Lemmon When dealing with a notch-only mobile duplexer, it may be a good idea to mark the connectors "Pass High, Notch Low" and "Pass Low, Notch High," to avoid any misunderstanding about what each side does. About nine years ago, I ordered a Vertex VXR-5000 UHF repeater from a major Amateur Radio supplier (who shall remain nameless) along with the Vertex VXD flat-pack notch-only duplexer. I had ordered the package with the conventional high-in, low-out frequencies, and the vendor did not charge for tuning the Vertex duplexer. When I checked the tuning of the duplexer with my spectrum analyzer, I found that the vendor technician had tuned all of the screws for maximum pass instead of for the deepest notch! It's a good thing I caught this before putting the repeater on the air, because I could have fried the RF amp in the receiver's front end. As for moving a mobile notch-only duplexer from the commercial band down to the 70 cm Ham band, that usually will not work well. Even so, many Hams retune such duplexers to the 70 cm band and believe that they are performing well, when in fact they are not. I proved this to myself by ordering a new RFS/Celwave PD633-6A-1N duplexer, factory-tuned to my 70 cm pair, and comparing its performance to the Celwave PD633-6A-2N (Motorola RFE4000A) duplexer that I had tried to tune down from the commercial frequencies. What an eye-opener! The notches were almost the same, but the pass loss was much greater in the -2N duplexer when operated at 70 cm. A technician at Celwave explained that the coupling loops (under the black plastic hole plugs) must be adjusted for optimum performance at the intended frequency of operation, and that is something that is difficult to do without the proper test equipment and tools. 73, Eric Lemmon WB6FLY
