Does a low SWR mean your antenna is a great performer?


George N2APB
 

FYI … I was recently in a roundtable on 75M and a perennial topic was discussed at some length ... "If my antenna has an SWR of 1:1 is it a great performer?"

 

The answer is simple ("not necessarily") but the reasoning is summarized well in the following post from the ham.stackexchange list.

 

I thought this would be of interest to all, but mostly for those on the HF bands where the science of homebrewing antennas is more often practiced.

 

72, George N2APB

 

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What is the relationship between SWR and receive performance?

 

If an antenna analyzer shows 1:1, does that mean it's an ideal receiver as well?

 

No.

 

Assuming we're talking about a characteristic impedance of 50 ohms, a 50 ohm resistor (otherwise known as a dummy load) will show a SWR of 1:1, although it will almost certainly perform very poorly as either a receive or transmit antenna.

 

The low SWR simply tells you that there are no impedance mismatches along the path from the transmitter (antenna analyzer in the case of your question) to the antenna feedpoint, at the current operating frequency.

 

And what about the converse, will a well performing receive antenna show a 1:1 SWR?

 

Yes and no.

 

Yes, a SWR of 1:1 means that you aren't losing signal to impedance mismatch reflections.

 

No, another issue is how efficient the antenna is at picking up the (desired) signal, preferably (especially in the case of directional antennas) while rejecting undesired signals as well. An antenna that is 5% of a full half-length dipole isn't going to pick up as much RF as the full-length dipole, let alone a full-sized directional antenna pointed in the proper direction, simply due to the much smaller physical (antenna aperture) size.

 

Generally speaking … if an antenna analyzer or (other) transmitter shows that the antenna output presents a SWR of 1:1, then what you have is probably about as good as it gets. That does not necessarily mean that what you have is a good antenna setup as exemplified by the extreme example of a dummy load.

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