Paul, The Motorola TDE7780A duplexer is simply a relabeled Celwave 633-6A-2N unit, and it is designed for 450-470 MHz. It is the standard duplexer furnished in GR300, GR1225, RKR1225, and CDR700 repeaters. The duplexer is okay for non-critical use at low-density RF sites, such as at construction sites. The compact mobile notch design has very little bandpass action, since its operation is based solely upon the notch.
I have tried to tune such duplexers down into the Amateur 70cm band, with little success. Although the duplexer APPEARS to tune down just fine, the reality is that the insertion loss begins to climb as the tuning progresses further out of band. For example, one such duplexer had rather impressive numbers at its original frequencies of 451.700/456.700 MHz: insertion loss just under 1.00 dB at the pass frequencies and rejection of greater than 89 dB at the notch frequencies. These numbers are better than Celwave's specs. When I tried re-tuning the same duplexer to the 444/449 MHz band, the insertion losses exceeded 3.0 dB and the rejection notches barely reached 70 dB. These figures were obtained with a calibrated network analyzer. There are folks who will dismiss this poor performance and claim that they have tuned such duplexers into the Amateur 70cm many times, and they always "worked fine." Yeah, right. If the application is not critical and mediocre performance is acceptable, go for it. However, for optimum performance in the 70cm band, the Celwave 633-6A-1N duplexer is needed. The difference between the -2N and -1N duplexers involves the size, shape, and position of the internal coupling loops- which are not user-adjustable. The tuning screws are simply variable capacitors that tune the helical resonator coils, while the coupling loops are selected and installed during manufacture based upon the intended band of operation. To test my observations, I bought a Celwave 633-6A-1N duplexer for a portable UHF repeater in the 70cm Amateur band. This duplexer was factory-tuned by Celwave to my coordinated frequencies. I tested the duplexer on my network analyzer, and was pleased to note that its performance was better than Celwave's specs. When combined with a Motorola R1225 full-duplex transceiver in a Pelican case, it makes a very compact, lightweight, and capable repeater for tactical use. The Bottom Line: The TDE7780A duplexer is not a good choice for a 70cm repeater. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Paul Sent: Friday, February 05, 2010 4:10 AM To: [email protected] Subject: [Repeater-Builder] RFS TDE-7780A Okay guys, I am building a tactical, mobile UHF repeater out of a GE MVP and have been looking for a duplexer for the package. I have seen several of the RFS TDE-7780A listed on epay. Do these work reasonably well in the Ham band? My freqs are 445.800 - 440.800. I hope to have 15 to 20 watts out of the MVP after conversion. Two last questions, what cable does the collective recommend for the antenna feedline, between the duplexer and the antenna? Any recommendations for a antenna? The plan is to deploy this repeater when additional radio coverage is needed by placing it on a mountain top or high structure. It would be self contained in an Ammo can or two. Thanks for the input and guidance. Paul KI4ADT
