George, A good question is never a dumb one. Although it may not be perfectly clear to everyone, Celwave marks their duplexers with "LOW" and "HIGH" to indicate the frequency that it will PASS. Thus, in a GMRS repeater system, the 462 MHz output of the transmitter will connect to the "LOW" connector, and the 467 MHz input to the receiver will connect to the "HIGH" connector.
The Celwave 633-series duplexers are notch-only devices which are suitable only for TX-RX spacings above 3 MHz. The internal coupling loops are adjusted during manufacture for a specific frequency range, spacing, and insertion loss. Even though the tuning screws can be adjusted to move the band of operation a few MHz, the performance falls off very rapidly once the tuning is moved more than 2 or 3 MHz. As others have noted, the internal adjustments have been optimized for specific frequencies, and the tuning cannot be inverted without major degradation. At one time, the connectors on such duplexers were labeled "RX" and "TX", and some clueless tweakers actually tried to change the input and output tuning instead of simply switching the connectors! Moreover, the design of notch duplexers is asymmetrical, so it may not be possible to invert the tuning. The tuning procedure for the Celwave 633-series duplexer calls for adjusting the screws on the three "LOW" side cavities for maximum rejection of the receive (high) frequency, and for adjusting the screws on the three "HIGH" side cavities for maximum rejection of the transmit (low) frequency. Bear in mind that notch (mobile) duplexers have no adjustment for pass frequencies, and what you get for bandpass insertion loss is what you get. In other words, tune for the notch only. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of George Henry Sent: Tuesday, July 10, 2007 6:56 AM To: [email protected] Subject: [Repeater-Builder] Notch duplexer dumb question I have a Motorola TDN7407A notch duplexer (in reality, a Motorola-branded Celwave 633-6A-2N) that I want to use for a low-power GMRS repeater. The ports are labeled HI and LO.... now for the dumb question, do those labelings refer to the NOTCH frequency or the pass frequency? Unfortunately, it is not labeled with the current tuning, and I don't have easy access to anything to sweep it with.... George, KA3HSW / WQGJ413
