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The CB Shack

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SWR

WHAT IS SWR?

SWR is the abbreviation for Standing Wave Ratio. It is simply a way of describing how much of your radio`s output power is sent to the aerial and transmitted, and how much is reflected back down the Co-Ax cable and wasted due to a bad match.

 

Too much reflected power will give poor performance and may damage your CB.

The ideal SWR is 1:1 (one to one) which means that all of your CB`s power is being radiated by the aerial, and that none is being wasted. This is quite difficult to achieve in the real world, especially in truck installations, and we normally see readings of 1:1.5 or 1:2 - This is fine, and will give you good results if the aerial is good quality and is mounted properly.

Now an important point that many people get wrong - A low SWR is NOT a measure of antenna performance! - It simply means that the aerial is properly tuned. To put it another way, A 5 foot aerial with a bad SWR will still outperform a 2 foot aerial with a perfect SWR - To get maximum performance, always use the biggest and best CB aerial that you can possibly fit on your vehicle - THEN tune it!

The secret to a low SWR is to have all the components of the transmitter system (The CB, the Co-Ax cable & the aerial,) matched perfectly. Your CB is already matched at the factory to 50 ohms, good quality Co-Ax cable and plugs are also 50 ohms, so this just leaves the aerial itself that we need to try to tune to 50 ohms to achieve a perfect match, or 1:1 SWR reading.

 

USING YOUR SWR METER

You need two items to read the SWR: 
1) An SWR meter 
2) A Patch lead - (a short length of Co-Ax with a PL-259 Plug on each end)

 

Your SWR meter has two sockets usually marked "ANT" and "RTX" or similar - connect your patch lead to the socket marked RTX, and the other end of the lead to the CB`s aerial socket. Connect the plug coming from the aerial to the socket marked ANT. (Antenna) The SWR meter is now connected in-line between the CB and the aerial.

Turn on your CB and set it in the middle of the band (Channel 20, or 21 etc) Make sure that the channel is free to avoid interference to other users.

 

Your SWR meter has two main controls:

1) A switch marked FWD / REF (Forward and Reflected power) 
2) A rotary or slider control marked CAL or SET (To calibrate the meter)

 

First you need to calibrate the meter. Put switch 1 to FWD, press the mic button and slowly turn the CAL control clockwise until the needle on the meter lines up with the CAL (or SET) mark on the right hand side of the meter scale. The meter is now set, and you should leave the CAL knob where it is.

Now set switch 1 to REF and press the mic button again - look at the meter, this is your SWR reading, if it is near the left side of the scale (less than 2 then the aerial is fine - If the reading is between 2 and 3 then you should tune the aerial, or sort out your aerial problem. If the SWR is over 3 in the red part of the meter`s scale, you should not transmit for long periods until the problem is solved.

If the reading is a little high, say just over 2 then check the SWR readings on channels 1 and 40, if the reading is higher on channel 40 then the aerial is too long and needs to be shortened slightly. If the reading is higher on channel 1 the aerial is too short and needs to be extended.

 

Aerials are tuned in different ways, some have a small tip that you need to adjust, but most have a whip which you need to move up or down inside a coil, usually there is a small grub screw that holds the whip in place - If you are not sure how to adjust your aerial, bring it into the shop and we will tell you what to do. Never cut the whip to shorten it - All good maufacturers leave plenty of adjustment available.

 

When you have finished tuning you should have an equal reading on channels 1 and 40 and a slightly lower reading on channel 20. This is called middle band dip tuning.

 

POINTS TO CHECK IF THE READING IS HIGH

If the SWR reading is very high, the problem is usually one of the following: 
1) A Co-Ax plug not soldered, and/or badly fitted. 
2) A bad earth from the Co-Ax braid to the vehicle chassis. 
3) A short between the outer and inner wires of the Co-Ax cable, - (Water etc.) 
4) Have you coiled up any of your Co-Ax cable? - This can cause a problem. 
5) The aerial, especially the coil, is too close to the bodywork etc. 
When you have finished tuning, the SWR meter should be taken out of line, this will avoid wasting power, and can prevent some types of interference.