On Sun,11/6/2016 11:06 AM, Ron D'Eau Claire wrote:
Sri Jim: You must have misunderstood. You defined the rig as the 'load' so I used your definition. Perhaps that's the problem.
I think you misunderstood me. ) The rig is the load for the line ONLY when the line is feeding the RECEIVER. Loss in the line is almost never a factor in hearing weak signals. That happens only when the signal is so low, and the noise received by the antenna is so low, that the signal is near the level of internal noise. This is usually a factor only for very weak signal work at VHF and UHF.
Avoiding that terminology, if there is no reflected power at the rig, the SWR will be 1:1. Power can be "lost" in the antenna (hopefully radiated) or consumed in the transmission line. But, as long as there is no reflected power arriving back at the rig, the SWR at the rig will be 1:1 no matter what it is at the antenna.
There is a common misconception that the output impedance of a power amplifier is 50 ohms resistive. This is NOT generally true. Rather, 50 ohms is the LOAD that the rig is designed to drive. The output impedance of the power amplifier is generally a lot lower than 50 ohms. The "match" that antenna tuners provide is to transform the complex impedance of the transmission line to that 50 ohm value as seen at the output terminal of the power amp (whether an external amp or the 100W amp of a rig running barefoot). Also, tube power amps include variable matching networks that perform that impedance transformation for moderate values of mismatch.
You are right; the antenna is usually up in the air, and some hams climb towers to make the adjustments at the antenna in order to ensure a good impedance match with the transmission line to minimize transmission line losses.
Hams who have done their homework to learn about modern test equipment use Vector Network or Vector Impedance analyzers to measure the impedance in the shack, use the Time Delay Reflectometer function in those analyzers to find the electrical length of the line, and plug the data into a program like SimSmith to transform the measurement made in the shack to the impedance at the antenna.
But most of us with dipoles, etc., make do with an approximate match based on physical measurements. We measure the SWR at the rig to reassure ourselves that the SWR at the antenna is reasonable.
The fixation on SWR is unreasonable. The real reason to use an SWR indicator is to find the approximate resonant frequency of an antenna, and to determine whether the impedance at the transmitter end of the line is in the range that the transmitter can provide power to it. Solid state rigs and power amps have protection circuits that cause them to shut down if the SWR is higher than about 2:1 -- that's because voltage and/or current in the transistors get high enough to cause destructive failure. Also, DISTORTION rises in output devices (both solid state and hollow state) when they are not driving their intended load. This is something that my neighbor, fellow contester, and Elecraft engineer K6XX made certain that I understood when I moved in five miles down the road from him ten years ago. :)
And, of course, if the rig is not equipped with an ATU, we are concerned that the SWR at the rig is not high enough to be reduce final amplifier efficiency and increase heat dissipation. That was the original concern that started this thread.
See above. 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
