Don, The length of that jumper cable must consider both the length of any cable between the TX output jack and the power amplifier itself, as well as the length of the coupling loop inside of the duplexer cavity. If the output of the PA and the input of the duplexer were purely resistive, the cable length would be irrelevant. However, the load impedance of most PAs will vary significantly with the drive level, and the input impedance of a duplexer cavity is always reactive. Therefore, there is no pat formula for determining the optimum length of the jumper cable.
In most instances, the TX jumper cable acts as an impedance transformer of sorts, and the optimum length can be determined by a laborious cut-and-try method, or by experimenting with the addition of elbow adapters. However, a simpler approach is to install an impedance matching device, sometimes called a "Z-Matcher", at the output of the PA and adjusting it for maximum forward power. If you have a vector network analyzer, you can then measure the transformation value of the jumper and Z-matcher combination, and fabricate a new jumper cable that is equivalent. This may not be cost-effective, since the round trip back to the shop probably will cost a lot more in time and fuel than a Z-matcher costs. A VNA is not something that most installers want to carry around in the service truck. If your repeater changes to a lower power level when on a backup battery, be aware that the cable length that is a good match at full power will no longer be a good match when running on the backup battery. That may not be a problem for you, but it is something to keep in mind. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Don Morehouse Sent: Friday, June 29, 2007 8:25 AM To: [email protected] Subject: [Repeater-Builder] Cable formula Does anyone know the formula for the cable length between a repeater and the duplexer? Thanks Don VE7EDA
