Antenna and SWR

Knowledgebase

Radios-uk website > Radios-uk support system > Knowledgebase

Antenna and SWR

Solution SWR - Standing Wave Ratio

The most important accessory that you will own is a SWR Meter. If you haven't got one, get one now! http://www.radios-uk.com/cbantenna.htm

Any aerial should be matched to the wavelength being used for it to be at its most efficient. This is particularly true when the antenna is being used to transmit. For an antenna to be at its most effective and not have the potential of causing damage to the transmitter (the CB radio) it must be matched to the transmitter. This is done by checking the SWR.

The "standing wave ratio" in the feeder (the coaxial cable) is measured with an SWR meter and is an indication of how well the antenna is tuned to the particular frequency in use and matched to the feeder. CB antennas are designed to appear as an impedance of 50 Ohms when the antenna is resonant - i.e. tuned to the transmission frequency.

The resonance of an antenna depends on its physical length - a relationship to the actual wavelength of the frequency being transmitted - and having the correct loading (any coils that may be included in the design of the antenna). If an antenna is not tuned to be resonant at the wavelengths being used then it will not operate at its optimum efficiency because not all of the radio frequency energy being produced by the transmitter (the CB radio) will be radiated by the antenna, so the range of transmission will be reduced. Perhaps more importantly than this, however, is that some of that wasted energy will be reflected from the antenna back towards to transmitter, measured by the resulting standing waves, and this can cause damage to the radio.

Usually the SWR is adjusted by changing the length of the antenna, this may involve a sliding section that is held in place by a grub screw or clamp, or by physically reducing the length of the antenna by cutting it ( just a few millimetres at a time!), or by an electrical adjustment on the aerial such as moveable rings.

How can I check my SWR?

To check the SWR reading you must use an SWR meter. To connect the meter you will also need a patch lead. The meter simply connects between the antenna and the radio. 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! There should be a switch on the meter labelled 'FWD/REF' (Forward and Reflected). Make sure the switch is set to FWD. Now set your radio to a channel that is half way through the band, i.e. if you have a 40 channel radio then set your radio to channel 20, for an 80 channel radio set it to UK channel 1. Now key up your 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. Let go of the mic and now switch the meter to 'REF'. Key up your mic again and the meter will now show the SWR reading of the antenna.

What if my SWR is too high?

You can usually tune your antenna quite easily by altering the length of the whip, most antennas have a grub screw in the side that you can undo to adjust it. Remember though only a very small adjustment is needed, 1/8 of an inch 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 is easily spotted though because the SWR will read infinity on the meter (needle full over). Another cause of high SWR is a bad 'Ground Plane'. If you adjust the length of your antenna and have a good ground plane and still cannot get the SWR reading down then you may need to reposition your antenna on your vehicle. If you have an acceptable SWR reading you are now ready to start using your CB!

Was this article helpful? yes / no

Article details

Article ID:	31
Category:	CB Radio
Date added:	2009-09-13 12:33:46
Views:	137
Rating (Votes):	(2)

<< Go back

Radios-uk Customer Service

Antenna and SWR