What is SWR?
SWR is short for STANDING WAVE RATIO. It is a measure of how well the antenna is performing. Most problems with the antenna or antenna cable will show up during an SWR check. Please note that the SWR is a measure of antenna performance. A poor SWR is caused by the antenna, the antenna cable, or a poor antenna installation. Poor SWR is not caused by the radio.
Since the SWR is a function of the antenna system, changing radios will not affect the SWR.
Why is SWR important?
SWR readings typically have a range of about 1 to 5. The lower the number, the better the antenna is working. As the SWR climbs, both transmit and receive range will drop. An SWR of 1 is perfect, but it is not possible to get a perfect reading on all channels. When an antenna is properly adjusted, the SWR is will be highest on channels 1 and 88, and lowest on channel 74 (channel 74 is at the middle of the marine frequency band). Optimum performance is attained by adjusting the antenna so that the SWR is the same on channels 1 and 88. This gives the best performance throughout all channels. The SWR reading must never exceed 3, otherwise the transmitter in the radio can overheat and eventually fail. Excessively high SWR can also cause other problems such as a squealing noise, or can cause interference with other electronic devices in the boat. An SWR below 2 is very good, and anything below 1.5 is excellent.
How can I measure my SWR?
SWR is measured using an SWR meter and a short "patch cable". The SWR meter will have 2 connectors on the back. The antenna cable will screw onto the connector marked ANT. The patch cable will screw onto the other connector. The other end of the patch cable screws onto the radio. The patch cable needs a "PL-259" connector on each end. The patch cable should be kept as short as possible, no longer than 3 feet in length.
Make sure that the SWR meter covers the marine frequency band. SWR meters for CB radios will not work. The marine band is at 156 megahertz.
Where can I get an SWR meter?
SWR meters can be found at places that sell boating accessories. You might also find a suitable meter at places that sell amateur radio equipment. Try www.aesham.com.
What is the procedure to check SWR?
- SWR must be checked in an open area. Park the boat away from buildings, trees, and other structures, which can affect your readings. It is recommended to check the SWR only when the boat is in the water.
- Select the channel to check. It is recommended to check channels 1, 74, and 88.
- Set the switch to the CAL or FORWARD position (depending what is says on the meter).
- Push the transmit button on the microphone, then adjust the calibrate knob on the SWR meter so that the needle goes to the cal marker on the display. The cal marker is always located on the far right-hand side of the display (maximum deflection).
- While still holding the transmit button, set the switch to the SWR or REFLECTED position.
- Read the SWR on the meter.
When you switch to a different channel, you should always re-check the CAL setting (repeat steps 3 through 6).
How is the SWR adjusted?
The object is to get the SWR the same on channels 1 and 88, and also below 3. Whatever you do, make sure the SWR is below 3. Once the SWR is about the same on channels 1 and 88 and is also less than 3, the antenna is tuned for optimum performance.
Begin by checking the SWR on channels 1 and 88, then refer to the following:
- If the SWR is higher on channel 1, the antenna is too short.
- If the SWR is higher on channel 88, the antenna is too long.
The antenna is tuned by adjusting its length. After an adjustment the SWR on channels 1 and 88 is re-checked. This process is repeated until the readings are about the same. Remember that the readings must also be below 3. When you are done, check the SWR on channel 74. It should be even less than what you got on channels 1 and 88.
Most antennas are adjustable. Refer to the instructions that came with your antenna or call the manufacturer. Here are a few methods, which vary with manufacturer:
- Antennas with a steel whip are usually adjusted by loosening a set screw and raising or lowering the whip. If you have a center-loaded antenna (fat section in the middle of the whip), adjust the top whip up and down. Even if the instructions do not refer to raising and lowering the whip, virtually all whip antennas can be adjusted in this manner.
- Some fiberglass antennas can be adjusted as follows:
- Remove the plastic cap from the tip of the antenna.
- Slit the plastic cover to expose the top coils of wire.
- To shorten the antenna, cut off the top coil.
- To lengthen the antenna, pull up the top coil.
- Replace the plastic cap.
It is best if you refer to the specific instructions for your antenna or call the antenna manufacturer.