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SWR, Description & How To.
animated SWR gague
By Brian MW0GKX.

(V)SWR described

(Non - technical description) Skip non - technical description

(V)SWR stands for "(Voltage) Standing Wave Ratio". All it really means is the ratio of power that is reflected back down the antenna lead from the antenna. A perfect antenna would have an SWR reading of 1:1 meaning that all the power transmitted up the antenna lead would be radiated by the antenna, however no antenna is perfect and there is always a small amount of power reflected back down the antenna lead. Basically speaking the higher the SWR ratio the less power being radiated by the antenna, and the more power coming back down the antenna lead. Where does the reflected power go? It is dissipated as heat, too much power coming back = too much heat at the transmitters output stage and finally a rather expensive radio receiver! Therefore we aim to get the lowest SWR reading possible, well below 2:1 is fine.

(V)SWR described

(Technical description) Skip technical description

To get maximum power into an antenna requires that the load impedance match the transmitter impedance. Any difference, or mismatching, of these impedances would not produce maximum power transfer.

Except for handheld radios most antennas are not connected directly to a transmitter. The antenna is usually located some distance from the transmitter and requires a feedline to transfer power between the two. If the feedline has no loss, and matches BOTH the transmitter output impedance AND the antenna input impedance, then - and only then - will maximum power be delivered to the antenna. In this case the VSWR will be 1:1 and the voltage and current will be constant over the whole length of the feedline. Any deviation from this situation will cause "standing waves" of voltage and current to exist on the line.

There are a number of ways VSWR or its effects can be described and measured. Different terms such as reflection coefficient, return loss, reflected power, and transmitted power loss are but a few. They are not difficult concepts to understand, since in most instances they are different ways of saying the same thing. The proportion of incident (or forward) power which is reflected back toward the transmitter by a mismatched antenna is called reflected power and is determined by the reflection coefficient at the antenna.

The forward power and the reflected power cause there to be two travelling waves in the feeder. As the two travelling waves pass each other in opposite directions, they set up an interference pattern called a "standing wave". At certain places on the feedline the voltages will add producing a voltage maximum, and at others their relative phase difference will cause a voltage minimum to exist on the feedline. These maximum and minimum points occur 1/4 wavelength apart. In the days when open-wire feedlines were used these points could easily be measured with simple indicators. Coax cable however presents another problem since the "inside" of the cable is not readily available for measurements. Consequently, VSWR measurements on coax are usually made at the transmitter end of the feedline. Therefore you are presented with the VSWR of the entire system which includes all losses associated with the entire system.

How can I check my SWR?

The basics.

In the examples I use the 2 meter band (144 - 146 MHz here in the UK) but the method is the same for the whole spectrum. It is best to use a constant carrier mode such as FM or CW for the tests as varying power levels, such as SSB, will make the needle swing in time to your speech making the readings, at best, difficult or, at worst, impossible. Much has been written by many people about exactly how to do it. This is just a simple guide:

To check the SWR reading you must use an SWR meter. There are various meters on the market and you should get one that, at least, covers the bands and power rating that you are using. To connect the meter you will also need a patch lead, this is a short piece of coax with a suitable plug at either end. The meter simply connects between the antenna and the radio. Decide where you are going to connect the meter. Connect at the antenna and you will be tuning the antenna for resonance. If you connect the radio then you will measure any inconsistencies in the feedline as well (not really a problem unless you have a fault in the feeder). Obviously if your antenna is mounted on top of a tower or mast connecting to the antenna may well be dangerous or impractical!

There are two connections on the meter, one for the antenna to plug into, and the other to connect to the radio (using the patch lead). When you have successfully connected up your meter you are ready to check your SWR!

On any type of meter, if it has a built in ATU, make sure it is switched to "off" or "thru" before starting.

switched meterIf there is a switch on the meter labelled "FWD/REF" or "CAL/SWR" (Forward and Reflected or Calibrate and SWR) and a "CAL" knob:

  1. Set your radio to a frequency at one end of the band (i.e.145.900 for 2 Mtrs).
  2. Make sure the switch is set to "FWD" (Forward, CAL).
  3. Key the microphone and use the control knob (usually labelled CAL) on the meter to adjust the needle so that is at the "Set" position on the meter scale, usually labelled on the far right of the meter scale, whilst giving your callsign and stating that you are making a test transmission (you did check the frequency was not in use didn't you?).
  4. Let go of the microphone and now switch the meter to "REF" (Reflected, SWR).
  5. Key the microphone again and the meter will now show the SWR reading of the antenna. Make a note of the reading.
  6. Select a frequency at the lower end of the band (i.e. 144.100 for 2 Mtrs).
  7. Repeat steps 2 - 5, again noting the reading.

Now go to the What the readins mean section to see what to do next.

2 meters

If your meter has 2 meters (or 2 pointers in one meter) one labelled "Forward" (Power), the other labelled "Reflected" (SWR) and a "CAL" knob:

  1. Set your radio to a frequency at one end of the band (i.e.145.900 for 2 Mtrs).
  2. Key the microphone and use the control knob (usually labelled CAL) on the meter to adjust the "Forward" (Power) needle so that is at the "Set" position on the meter scale, usually labelled on the far right of the meter scale, whilst giving your callsign and stating that you are making a test transmission (you did check the frequency was not in use didn't you?).
  3. The "Reflected" (SWR) meter will now show the SWR reading of the antenna. Make a note of the reading.
  4. Select a frequency at the lower end of the band (i.e. 144.100 for 2 Mtrs).
  5. Repeat steps 2 & 3, again noting the reading.

Now go to the What the readins mean section to see what to do next.

cross needle meter

If Your meter is a cross needle design:

  1. Set your radio to a frequency at one end of the band (i.e.145.900 for 2 Mtrs).
  2. Key the microphone and, whilst giving your callsign and stating that you are making a test transmission (you did check the frequency was not in use didn't you?), look at where the needles cross. Make a note of the reading.
  3. Select a frequency at the lower end of the band (i.e. 144.100 for 2 Mtrs).
  4. Repeat step 2, again noting the reading.

Now go to the What the readins mean section to see what to do next.

If you have a different type of meter, check the instructions for it's use.

What the readings mean

If the readings are 2:1 or below then all is well, job done, you can disconnect the meter (don't forget to reconnect the antenna to the radio!).

If you have a reading that is higher (above 2:1) at both ends of the band, select a frequency in the middle of the band (i.e. 145.000 for 2 Mtrs) and repeat steps (2 - 5 for FWD/REF type, 2 & 3 for twin meter type, or 2 for twin needle type), again noting the reading.
If this 3rd reading is also above 2:1 then check all your connections and try again. If still above 2:1 at all 3 points of the band seek assistance as you are beyond the scope of this article.

If the mid band reading is below 2:1 then the antenna doesn't have a sufficiently wide bandwidth to cover the whole band. The best thing to do in this instance (besides changing the antenna) is to decide which part of the band you are going to use most and tune for that (i.e. between 145.200 - 145.800 for 2 Mtr FM simplex and repeaters). see table below for tuning details.

Top of BandBottom of BandRemedy
Above 2:1Below 2:1Antenna should be a bit shorter
Below 2:1Above 2:1Antenna should be a bit longer

How much is a "bit"? Well that depends on the frequency in use, the type of antenna and the difference between the readings, so it's trial and error for the adjustment.

The above advice is for monopole type antennas, i.e. 1/4 wave groundplane whips etc.

For Dipoles: the same method is used but adjust both ends equally.

For "Gamma" matching: adjust the shorting bracket one way and check your readings. If the readings for the section of the band you want to use are increasing, adjust the other way until the lowest reading is achieved. If the readings improved, try a little more in the same direction and check again. Repeat until you're happy.

What readings should I get?

The best SWR reading would be 1:1 but as I said earlier this could only be achieved with a perfect antenna and no antenna is perfect! So basically the lower the SWR reading the better. Over 3:1 is unacceptable and can damage your radio or amplifier, 2:1 or below will give you a good range and is quite acceptable, 1.5 or below is fantastic and will give you optimal performance.

What if my SWR is too high?

You can usually tune your antenna quite easily by altering the length of the radiator, most mobile whip antennas have a grub screw or similar in the side that you can undo to adjust it. Remember though only a very small adjustment may needed, a few mm (depending on band) can dramatically change the SWR! Sometimes the antenna is not to blame for a high SWR though. If you have a short circuit in your cable or mount for instance then you will have a high SWR, this should be easily spotted though because the SWR will read infinity on the meter (needle full over).

Why don't I just use an ATU?

The use of an ATU is fine, especially for HF multi band antennas, however if you tune the antenna for optimum performance first then less of your output power is being "lost". In simple terms an ATU tunes capacitance and inductance so that the standing waves are, in effect, canceled out. An ATU does not improve the performance of a mis-matched antenna, it only "fools" the radio (or amplifier) into thinking a good match has been achieved. Remember, an SWR reading of 3:1 at the antenna is still 25% of your output power not actually making it out of the antenna, even if an ATU shows the radio a 1:1 (perfect) match.

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