On Sat, Oct 12, 2019 at 11:06 AM, Bill Hemmings wrote:
You know, after all the talk about matching impedances, I'm really
wondering .... we talk about POWER in watts but that's pretty much for
transmitters. But we talk about receivers in? VOLTS. Well, okay, microvolts,
usually. Somehow, I don't see a good reason to CARE about matching
receiver input impedance. Maybe it lowers the antenna bandwidth or Q?
Impedances have PHASE, but voltages do not. Care to comment?
Bill, you get maximum power transfer when the impedance is matched. Wiki "Maximum Power Transfer Theorem" for details. This applies to transmitter to coax to antenna to air (the ether). Or the ether to antenna to coax to receiver. Some energy is lost in the resistance of the coax wire, and in the inductance and capacitance that we measure as Xl and Xc. When there is a change in impedance, a discontinuity, some of that energy is absorbed at the discontinuity and some is reflected, causing a standing wave. The reflection moving back down the coax also loses the same proportion of energy as the first wave. Where it hits the transmitter matching unit or final, is reflecfted forward to the antenna and loses energy again in the coax . Etc. Signal strength (in the ether) is measured in micro volts (uV) per meter because it is a field, not just a current in a wire. If you have a mismatched receiving antenna system, you lose some of the signal, (and atmospheric noise, too) but retain the noise inside the receiver, so your signal to noise ratio goes down.
also see:
Jim, KA6TPR
Starsekr