An old mate of mine is thinking of erecting an inverted L aerial. This is a great idea, but he wants to feed it with coax. A single band inverted L fed with 50 ohm coax? No problem. You adjust the length of the horizontal section of the aerial until you have a 50 ohm feed impedance, which will match the 50 ohm coax. What some people do to use the same aerial on another band is stick a matching unit at the shack end of the coax. Let's assume that our inverted L was cut for 160 metres, matching perfectly to the 50 ohm coax. That's great, but what happens when we use the same aerial on 80 metres? The feed impedance of the aerial at this frequency is no longer 50 ohms. In fact, it might be as high as 1000 ohms or more. 50 ohm coax looking into a feed impedance of 1000 ohms? Er... I don't think this is right. So, our knowledgeable radio amateur stuffs the shack end of the coax into his matching unit and tunes out the SRW. One to one. Brilliant! The output of the transmitter is seeing exactly what it wants, 50 ohms. Hang on a minute. The 50 ohm coax at the aerial end is still looking at a feed impedance of 1000 ohms. The matching unit doesn't change the impedance of the coax, and neither does it change the aerial's feed impedance. The result? The massive mismatch at the aerial feed point still exists.
you can buy a matching unit for use on 2 metres. Coax in from the aerial,
coax out to the rig. This is going against all I've said, but... If your
SWR is slightly high, the protection circuit in the rig will shut down
the PA. By using a matching unit to tune out the SWR, the rig will 'see'
50 ohms and run at full power. This is acceptable where the SWR is slightly
high. But not where there are massive mismatches at the aerial end of