There is no way that impedance matching can be accomplished with changing the length of the transmission line in a 50 ohm system when the transmission line used is also of the same characteristic impedance without a compensating shunt XC or XL stub, at a location determined on a Smith Chart plot. See "Electronic Applications of the Smith Chart in Waveguide, Circuit, and Component Analysis" by Phillip H. Smith, Chapter 9, Waveguide and Impedance Matching, P. 97. Allan Crites wa9zzu
--- On Wed, 8/18/10, n5qs <[email protected]> wrote: From: n5qs <[email protected]> Subject: [Repeater-Builder] Re: Coax length, etc. To: [email protected] Date: Wednesday, August 18, 2010, 9:04 PM This is taken from a Wacom tuning guide please excuse any typos Roger CABLE LENGTH BETWEEN TRANSMITTER AND DUPLEXER The length of the coaxial cable between the transmitter and the duplexer might be a critical length with some transmitters because of an impedance mismatch. A mismatch condition can exist between a transmitter, cavity filter/duplexer, and/or antenna since all of these components can vary from an absolute impedance of 50 Ohms. As an example, a transmitter, or duplexer, or antenna will probably be listed as having a "nominal" impedance of 50 Ohms and a VSWR (referenced to 50 Ohms) of 1.3 to 1 or 1.5 to 1. At a VSWR of 1.5 to 1, the antenna , or duplexer, or antenna could have an impedance of anything from 33 to 75 Ohms and still be within specifications. (At 1.3 to 1 VSWR, the impedance can be anything from 38 to 65 Ohms). In a worst-case-scenario, the transmitter might have an actual impedance of 33 Ohms, the duplexer 75 Ohms and the antenna 33 Ohms. As such, system performance will be degraded and yet, all components will individually meet manufactures specifications. This impedance mismatch problem can usually be resolved by "optimizing" the length of coax cable between transmitter and duplexer and/or by installing one of the impedance-matcher PI Networks available through Wacom and other sources. If a transmitter is overly sensitive to a mismatched load impedance, a variety of symptoms might appear, including one or more of the following: 1) The transmitter might generate numerous spurious radiations. 2) The transmitter output power might become erratic, either too high or too low as measured on a wattmeter. 3) The insertion loss of the duplexer might measure normal with sweep equipment but measure too high or too low on a wattmeter when connected to the transmitter. 4) The reflected power might change when the length of cable between transmitter and first cavity is changed. 5) Use of a ferrite isolator at output of transmitter solves the forward power and reflected power problem, and eliminates change in reflected power when length of cable between the isolator and the first cavity is changed. (In this instance, the ferrite isolator can be used to reveal the symptoms or as a solution to the problem.) The above problems might be reduced or minimized by optimizing the length of cable between the transmitter and first cavity of the duplexer. The optimum length of cable can be found by following procedure: 1) Tune the transmitter into a 50 Ohm dummy load according to the manufacturers instruction. 2) Connect the duplexer to the transmitter. The transmitter output signal should feed through the wattmeter, then through the duplexer then into a dummy load. If there is an impedance mismatch the duplexer will de-tune the transmitter and the cable length should be optimized. 3) Using short lengths of cable (no longer than 1" @ 900 MHz, 2" @ 460 MHz, 6" @ 160 MHz, 24" at 40 MHz) or right angle elbow connectors, gradually increase the length of the cable between the transmitter and duplexer until the optimum length (no de-tuning effect) is found. Up to four of these short cables should be tried. 4) When the proper cable length is determined, i.e., when the system is operating as it should, replace all the short and long lengths of cables, right angle elbow connectors and wattmeter with one continuous length of cable of equivalent electrical length. Do not overlook the fact that the wattmeter was part of the cable length between the transmitter and duplexer during this process. If the wattmeter is removed from the circuit, the cable length without the wattmeter must be lengthened by some amount in order to be the same equivalent electrical length as the cable with the wattmeter in the circuit.
