ANTENNA TUNERS ON COAX FED ANTENNAS.

Place the SWR bridge between the antenna tuner and transmitter and the output of the tuner to the coax going to the antenna.

Many hams seem to believe that if they use an antenna tuner and keep the SWR at 1 to 1 they are radiating the maximum signal.  The truth is that antenna tuners can be deceiving.

A more accurate term for what is usually called an antenna tuner is a transmatch.  It matches the output of the transmitter, which is usually 50 ohm, to the feed line input and not to the antenna.

 

If the feed line, the antenna, and the transmitter output all have the same

Johnson’s Viking Matchbox antenna tuner.

characteristic impedance then everything is matched and maximum energy transfer will be accomplished.

 

The main reason for a transmatch is to protect the transmitter from reflected signal and to reduce stray RF in the shack when the antenna is reactive or poor energy transfer to the load.

 

The only way the antenna can be tuned is to have the tuner at the antenna end of the coax instead of at the transmitter end.  If a random wire or a long wire antenna system, they are different, is being used then the Antenna Tuner is truly an antenna tuner because it can adjust the electrical length of the antenna thus adjusting the resonant frequency and impedance of the antenna.  The

Inside a Johnson’s Matchbox

tuner becomes part of the antenna.

 

There are some considerations when using a transmatch.  One is the feed line can not be used near its operating limits.  If RG58 coax is being used it can be used up to 500 watts if the system is properly matched but if a transmatch is being used to adjust the SWR to the transmitter there are dangers of very high voltage peaks on the coax between the transmatch ant the antenna.  This could result in a voltage breakdown of the coax insulator.  Also the reflected energy can cause excessive heat which could melt the insulator.

 

It should also be remembered that maximum efficiency will not be obtained

Kenwood AT-100 antenna tuner.

especially if there is a large SWR that needs to be tuned out.

 

Some hams will brag about the fact they have a wire antenna and have never had to use a transmatch and that is fine but most wire antennas will not operate over the whole 80 meter band with a safe SWR.If the desired is to operate the whole band using a transmatch is far better then burning out finals.

 

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4 Responses to ANTENNA TUNERS ON COAX FED ANTENNAS.

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