One correction here; the Bird power meter is not just a "voltage measuring" meter. It does in fact measure voltage and current to calculate power. It will give true power even if used in a non 50 ohm circuit. But you must always subtract reflected power from indicated forward power to find true power delivered to the load.
When measuring SWR you must always calculate it (or use the chart) and compare reflected to forward indicated on the meter. It is easy to be fooled as indicated forward power also drops as reflected power drops. 73 Gary K4FMX _____ From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Russ Hines Sent: Saturday, August 14, 2010 4:30 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Coax length, etc.73 Some related comments, if you don't mind. Temperature changes seem to be the biggest "detuner" of largely mechanical devices like cavity duplexers. We often send our repeaters off to live in less-than-ideal environments, then expect cavity input/output impedances to remain as we measured them in the shop? Don't think so. IMHO, we're making the same mistake I made in a post the other day, saying "VSWR" when what we really mean is "reflected power" as indicated on a meter. Jeff is correct, VSWR along a transmission line doesn't change if source, load and line impedances are stable, the ratio remains the same. What does change, and what is affected by line length, are actual impedances along the line under not-so-perfect-or-stable conditions; the actual impedances along the line change but the ratio does not. For example, 100+j0, 25+j0, 40+j30, and 40-j30, are different impedances yet all exhibit a VSWR of 2:1 in a 50-ohm impedance system. Voltage is proportional to impedance. We can't really have a voltage standing wave ratio greater than 1:1 without a voltage differential, and that really can't happen if impedances along the line remain the same. Our friends at Agilent have put together a Java applet demonstrating what happens along a transmission line. Maybe you're aware of it, it's really kind of cool. The applet allows you to change the load impedance of the model and see the changes, so have fun with it. http://education.tm.agilent.com/index.cgi?CONTENT_ID=6 Our in-line power meters, like our trusted Bird 43, do not directly measure power. They're really voltage meters calibrated in watts at a specific impedance. That's why they can be fooled into displaying an erroneous reflected power reading, perhaps lulling us into a sense of security that the VSWR on the line is acceptable when it may not be. With most transmitters I'm familiar with, a "high VSWR" condition is detected from a reflected RF sample from a directional coupler at the transmitter's output, so it's not a "real" VSWR measurement per se, it's a voltage measurement. Worse, these couplers tend not to be very selective, so out-of-channel and even out-of-band energy can cause "high VSWR trips" even when our measurements indicate all is well on our frequency of interest. Great discussion, keep it going. If I repeated what was already mentioned, my apologies. 73, Russ WB8ZCC _._,___
