Saturday, December 31, 2011

SWR – General Information

First, let’s clear up a common misconception.  SWR does not tell you anything about how well your antenna “gets out”, i.e. radiates.  It is a measure of the quality of your antenna system1 or its ability to transfer RF energy from your radio onto your antenna.   It is a very important measurement to make with any new antenna or coax.  It needs to be made periodically on any antenna system, especially with mobiles2.

SWR tells you if your radio will be damaged by the antenna system.  There is another prevalent misconception one hears from time to time.  “The radios these days have protection circuits that cut back the power SO THE RADIO WON’T GET DAMAGED.”  This is half true.  The radios do have protection circuitry.  However, that circuitry will not protect the radio from long term abuse such as transmitting into a seriously mismatched antenna system.  Continuing to operate into a mismatch will “slow cook” your radio.  Just how long that will take is hard to judge.  If there is an emergency situation use the radio, but keep the transmissions as short as possible.  It is even more important that the radio not fail in that case.  The receiver will not be damaged by high SWR.  The receiver’s performance may drop slightly but your ears won’t notice.  However, if you do notice a performance change in your receiver, you can be pretty sure that there is something seriously wrong with the antenna system.  Do not transmit in this case.  There is something so seriously wrong that your signal will likely not to get out anyway.

If you are wondering if an SWR meter is a worthwhile investment let me put it this way.    Do you change the oil in your car regularly so the car will keep running?  Checking the SWR on your antenna system is the same kind of thing and you need the tools to do the job.  You need to invest in a SWR meter.

One thing about buying a SWR meter you need to know.  Be sure it covers the bands where you intend to use it.  Most of the moderately priced ones cover 160 meters through 6 or 2 meters and others cover 1.25 meters through something above 70 cm.  Be wary of ones that cover 160 or 80 meter through 70 cm.  These are either very expensive or are not very accurate especially on the 70 cm.   

Future posts will cover SWR measuring of coax and antennas.

1 - The term antenna system includes the antenna, the feed line (generally coax), and the connectors.
2 - Most FM mobile radios indicate power out and not SWR on their displays.  Mobile antennas take a beating just from wind, bird strikes, trees, garage doors, etc.  I check the SWR around each federal holiday.  That is an easy reminder to make well space tests, ten times a year.

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