Standing Wave Ratio SWR N.A.R.C. - M0NAR

Northwest Amateur Radio Club. NARC - M0NAR

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Standing Wave Ratio (SWR)(VSWR)

To anyone who has an interest in any form of radio the term Standing Wave Ratio (SWR) should be familiar, but what exactly is it? The true term is Voltage Standing Wave Ratio, or VSWR but for simplicity we'll stick to SWR.

Since SWR is most often used in radio circles as an indication of how well your antenna system is working probably the biggest misconception is that your SWR should be as close to 1:1 as possible, otherwise " you won't get out very well." A 1:1 SWR implies a perfect match between all elements of the antenna system. When the load impedance matches the line and antenna impedance there will be no Standing Waves. The only problem is that it is possible to have a low SWR and still have some very serious things wrong with your antenna system. Other misconceptions such as a high SWR causing television interference, or other unwanted problems are often heard and can cause unnecessary worry.

So what is it?

SWR is a measure of a transceiverís output power verses the portion of that power reflected back to the transceiver by the antenna system. If the antenna system is working well, most of the transmitted power will be radiated by the antenna with very little power reflected back to the transceiver.

If an antenna system is not working well, the power reflected by the antenna will travel back through the transmission line and arrive at the output of the transceiver. Modern day semiconductor transceivers do not handle this reflected power well. If the reflected power is sufficiently high, it can severely damage the transceiverís power output transistors. To avoid damage, manufacturers now design protective circuits into the power output stage. The protective circuit reduces the transceiverís output until the magnitude of the reflected power is below that which would cause damage to the transceiver. In terms of SWR ratios, transceiverís typically operate at their full nominal output power at SWR values of less than 1.5:1  For SWR values greater than 1.5:1 the transceiverís protective circuitry reduces the power to avoid damage.

To get maximum power into an antenna the transmitter and antenna has to have the same impedance or 'match'. As there is usually some distance between the transmitter and antenna we require feed line to transfer the power between them. If this feed line has no loss and is of the same impedance as the transmitter and antenna then we should have a perfect match or 1:1 SWR and all power will be radiated from the antenna. In practice however, this case is rarely achieved as the feed line, be it coax or ladder line etc...  will have losses and therefore the antenna system will always reflect some power back to the transceiver.

Many proprietary SWR meters are calibrated to read FORWARD power as well as REFLECTED power. They may actually be measuring voltage, and therefore have the scales calibrated in power.

If we assume that the SWR meter contributes no errors, the FORWARD reading is the SUM of the forward power and the reflected power. As a result, it is greater than your actual power output. The REFLECTED power reading is that amount of power which was not initially absorbed by the antenna and has been sent back down the feed line.

The first number in the SWR ratio is greater than 1 in most cases. From a practical point of view, SWR numbers in the range from 1:1 to 1.5:1 are considered to be very good, meaning that the antenna is radiating most of the power sent to it.

The conclusion that can be drawn from this is that the threshold SWR, as seen by the transceiver, is about 2:1. An SWR above 2:1 will result in a power loss that will be noticeable at the receiving end. An SWR of less than 2:1 will not create a noticeable drop in power. In terms of perfection, tuning your antenna system for an SWR of less than 1.5:1 will result in full output power from your transceiver and negligible reflection losses at the antenna. Spending a lot of time trying to reduce your SWR from 1.5:1 to a perfect match of 1:1 is generally not time well spent. From a practical stand point, an SWR of 1.5:1 is indistinguishable from a perfect match of 1:1.

 

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