SWR - The Truth!

What is SWR?

SWR, or more correctly VSWR, is essentially a measurement of how well matched your antenna system is to your radio.  Ask this question on an amateur newsgroup and you'll never get a straight answer, so my best advice is to use SWR as a rough indication of how well your antenna is set up.  

Much myth and rumour exists surrounding this subject - take everyones advice (including this if you must!) with a pinch of salt, and draw your own conclusions based on personal experimentation :)

The Basics

The lower your SWR, the better match between your radio and antenna.  The better the match, the more efficient the system becomes, and hence the more of your 4 watts that actually makes its way out of your antenna.   SWR is actually a ratio (SWR = Standing Wave Ratio), so is expressed as such.   An SWR of 1:1 is a perfect match.   Most meters range from 1:1 to infinity:1.

People may tell you that anything above 1.3:1 is poor.  Ignore them.   In my opinion, anything up to 2:1 is acceptable.   Above that and you start seeing some serious losses. Above 3:1 and you may start doing damage to the radio.   It is however essential to have an SWR of around 1.3:1 if using a linear amplifier - as these tend to overheat very quickly resulting in serious damage.

Power, Watts and more Myths

Legal power output in the UK is 4 watts.   The nature of radio, means that you need to view power/distance as a logarithmic scale.   Huh?   Well ... to double the distance you need to multiply the output by 10.   To halve the distance, you need to divide your power by 10.   This is a very rough figure, and many more things effect distance.  However - a 'poor' SWR of  say 3:1, means you'll be wasting 1 watt of power.   The difference between 3 and 4 watt TX on the receiving end would not be noticeable.

A chart showing the effective radiated power at different SWR's can be found here

A modified version can be found below:

As can be seen above ... SWR is not as critical as many like to make out.   Obviously the lower the better, and an SWR of 1.3:1 should be achievable on most antenna designs.  However, if the lowest you can achieve is 2.0:1, then don't fret.  Does it work?   That's all you need to worry about.

Do try for the lowest you can achieve though - as a high SWR (>2) can indicate a problem elsewhere.  Check your cabling, connectors, etc.  However, if all checks out ok, and the SWR is still 2 or 3:1 then I wouldn't lose sleep over it.