ASK ELMER 

Dear Elmer, I always seem to be mystified by the magic of radio transitions. To me it all seems to be like magic. Whenever something seems mysterious it brings questions to my mind. Here is a subject that I have wondered about for quite a while. It is this 50 ohm thing. It seems to me that every single amateur transmitter wants and expects to have it's output matched to a 50 ohm impedance. It does not seem to matter for which band the transmitter is built. This brings a few questions to mind. I am pretty sure that most of them are easily answered. 

1. Why have all manufacturers settled on 50 ohms, why not 75 ohms, 100 ohms, 300 ohms or any other number? 

2. Is 50 ohms some magic number for which transmitters are very easily built? 

3. Would it be very difficult to build transceivers that would easily match some other impedance such as 300 ohms which is closer to the impedance of some antennas? 

4. Is this a standard that will always stay with us? 

Always Mystified Dear Mysti, I imagine that this is a question that is on the minds of many ham operators. Would you believe, Mysti, that there is a reason for the choice of 50 Ohms as well as for special purposes 75 and 90 Ohm cables.

As you well know, if you take an ohmmeter and measure the center conductor resistance of a piece of cable, you will get a very low resistance reading. Conversely, measuring the resistance between the center conductor and shield will indicate nearly an infinite resistance. The 50 Ohm, etc. has nothing to do with the DC resistance of the cable. The 50 Ohms in question is known as the “Surge Impedance” of the cable. In order to measure it you would require a very fast responding ammeter (a High Frequency Oscilloscope will do quite well for this measurement). Were you to connect a battery through a switch and the ammeter between the center conductor and shield of the coaxial cable, and then close the switch. The piece of cable should be moderately long for a fair test. If you measure the maximum current, it would be the battery voltage divided by the surge impedance of the cable. Now that we got that out of the way, why 50 Ohms?

You will have to take this on faith, because I won't go into the math using Maxwell's Equations, but if you calculate a cable's loss as a function of its impedance for a given dielectric material (air, polyethylene, Teflon, etc.), you will find that for air, a 77 Ohm cable is ideal. For Polyethylene, which was the material available at the time (around 1940), 52 Ohms was the ideal value. So for a minimum loss cable, 50 Ohms was adopted. Actually this is quite convenient. A dipole antenna is 75 Ohms (one of the reasons 75 Ohm coax is used in TV systems) so they have 75 Ohm cable. At one time in the past, transmitters used tuned output circuits (as do most KW linear amplifiers). These tuned output circuits were able to match a wide variety of impedances.

Manufacturers used to specify that their transmitters will match anything from 40 to 600 Ohms. The primary reason that modern transmitters are set to a constant impedance output, is that the power stages are broad banded. They do not require tuning. It would be impractical to do in transmitters what they do in the power industry (like CL&P or Con Ed), have the output stage as a constant voltage line (Zero Impedance), so that you can connect anything to it. The amount of excess power you would have to generate would be horrendous. It is infinitely more practical to have a constant impedance transmitter and use a Transmatch to transform its impedance to match your feed line and antenna. It is also very convenient to interconnect equipment using the same types of cable and connectors. By the way, the original UHF and BNC connectors were not impedance matched to their cable. This has been corrected in the modern connector, primarily the BNC connector, not necessarily the UHF connector. For HF thru VHF, this is not too critical, but be careful at UHF and very high power systems. Will this 50 Ohm standard always be with us? My crystal ball is cloudy in this respect. I would imagine that it will, at least for amateur radio use, but anything is possible. You never can tell, the music industry may claim they have the copy write on 50 Ohms and start taking everyone to court under the Digital Millennium Copy write Act. With that in mind, I hope you have been demystified.

73, Elmer