https://forums.qrz.com/index.php?thread ... ce.227442/
Technical topics on QRZ seem to cycle through periodically. The topic of the week seems to be impedance matching of an antenna system to a receiver. Here's a little primer on the matter. When I worked in ionospheric research, we had to get this right, since we were looking at VERY weak signals.
1) Gain is cheap!
Some of us who have been around a while remember when some H.F. receivers were "deaf" on the higher bands. QST and Handbook preamplifier and preselector projects abounded, to take care of the sometimes weak performance of receivers. Those days are LONG gone. ANY modern receiver has far more gain than you will ever need (in many cases, far TOO much gain for good performance!
2) Modern receiver input impedances only VAGUELY resemble 50 ohms! It is a RARE receiver indeed (and probably non-existent!) that exhibits a 50 ohm input impedance across the H.F. spectrum. At HIPAS Observatory, we measured the input impedance of many high end amateur (and mil-spec) recievers with a $45,000 HP network analyzer. The BEST of these receivers exhibited input impedances ranging from 10 ohms to 150 ohms! (ignoring reactance, which at some frequencies was quite significant!)
3) It is easy to misinterpret the importance of point #2 above. One might conclude that since receiver inputs are so widely varying, you will gain a lot by using a matching network on your receiver. (Again, in the olden days, there were several published "receiver tuner" articles). However, Point #2 is GREATLY overridden by Point #1. The fact is, receiver input matching doesn't matter, because ANY modern receiver has tons of gain you can throw away. In fact, receiver input matching only begins to be important at UHF frequencies.
4) Although this point has been elaborated COUNTLESS times, both on QRZ and elsewhere, nobody seems to get it. I will repeat it once more, at the risk of sounding tedious, in capital letters. IF YOU CAN HEAR A DIFFERENCE IN THE NOISE LEVEL OF YOUR RECEIVER BETWEEN AN ANTENNA BEING CONNECTED AND ONE NOT BEING CONNECTED, YOU HAVE ALL THE RECEIVER GAIN YOU CAN USE! This is an indisputable fact. There is no advantage whatsoever to having receiver gain beyond this level.
5) A receiving antenna is a signal generator. The question comes up often as to how to model a receiving antenna. The correct model for a receiving antenna is a voltage source with a fixed series resistance. This resistance value is the radiation resistance....remember the reciprocity theorem.
With this in mind, it is important to note that the standing wave ratio on an antenna system may be different in receive than in transmit. Remember SWR is determined strictly by the load impedance. In receive, the load impedance is the receiver input impedance, and your antenna is the transmitter! Ideally, the antenna impedance should be the complex conjugate of the reciever/transmission line input impedance....but again, at H.F. frequencies it doesn't really matter.
KL7AJ, Nov 30, 2009