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Advice Pages   How To Use An SWR Meter
12v / 24v Autoswitching   |    Advice on Accessories   |    Advice on Buying   |    Advice on Choosing   |    Advice on Installing   |    Base Station CB   |    CB Features and Facilities explained   |    CB For Car Enthusiast Clubs & Groups   |    CB For Caravanners   |    CB For Farms and Businesses   |    CB For Motorhomes & Campers   |    CB Lingo and slanguage   |    Choosing an EC-990 Echo Chamber   |    Groundplane / Earth connection / Artificial Ground   |    Handheld CB's   |    Handsfree Legislation   |    History of CB   |    Interference   |    PMR446 Licence Free   |    UK and EU bands compared   |   

We now have a revised and updated SWR information page here :
SWR Meters & Readings Explained

How to use an SWR meter

This is an important yet simple and easy to use bit of kit.

Getting the SWR right is crucial not just for getting decent range but also because a high SWR reading can damage your CB - all our CB's have a years guarantee but this does not cover the output transistor (the bit that can be damaged) as it's down to you to install it right, and get a low SWR.

Using an SWR meter allows you to check the readings and make any adjustments necessary, making sure you don't have any problems like this!

If anything is unclear or you have any questions, please feel free to get in touch by email or phone.

What is an SWR meter

When setting up your aerial installation, you must use a special test meter called an SWR meter to make sure that the ground connection is good, and the aerial is the right length.

What the SWR meter readings tell you is how well matched the length of the aerial is compared to the wavelength you are transmitting on, and whether the aerial has a good ground or not

All SWR measurements should be taken with the vehicle in an open area, at least 20 feet from buildings and never inside a garage or carport. The vehicles' doors should also be closed.

There are meters that only measure SWR, and ones that measure SWR and output power too, but we will deal with the SWR functions here and all the meters measure SWR in the same way. On the meter you will have a calibration control knob, and a switch with two settings. One may be marked FWD, DIR, or CAL and the other marked REF. These stand for forward power (to the aerial) and reflected power (back from the aerial).

The reading actually shows you how much power is being reflected back from the aerial - obviously the less reflected back, the better.

How to take SWR readings

Firstly, go to channel 20 on the UK band.

Put the switch to FWD and while transmitting, turn the knob so that the needle swings over to the far right of the scale. You will probably see the word SET or a red mark at the end of the scale - make the needle line up with this.

Once this is done, and while still transmitting, switch over to REF and the needle should drop down to give you a nice low reading.

If the needle hardly moves when you switch over or does not come out of the red portion of the scale - STOP transmitting - you have a problem, probably with the groundplane connection.

A reading of below 1.5 is ideal, a reading of under 2 is ok, but a reading in the red part of the SWR meters' scale means that the SWR is high enough to possibly damage the set.

What you need to do next is to take readings on channels 1 and 40 and make a note of them. (You will need to switch back over to FWD and fine tune the calibration on each of these channels.)

If your readings are LOWER on channel 1 than they are on 40, then your aerial needs to be shortened. (In most cases, this is what you are likely to find.)

If your readings are LOWER on channel 40 than they are on 1, then your aerial needs to be lengthened (by loosening the grub screw and pulling some of the whip out of the coil).

A meter with a built in antenna matcher may help you reduce the readings if you can get it out of the red to start with - but you'll need to make sure you don't accidentally adjust the controls after you have set it up. Mark the settings and lock them by sticking tape over them (or similar) to prevent them moving.

Tuning for 80 channel sets

As the majority of CB use in the UK still happens on the UK band, the procedure above will allow you to tune the aerial for best coverage on that band, however if you want to check the aerials' SWR across both EU and UK bands, the principle is the same as for tuning to one band, but - you should use channel 20 on the EU band instead of channel 1 UK band, and channel 20 of the UK band instead of channel 40. If possible try to get these to read the same if you will be using both bands equally.

The EU band is lower in frequency than the UK band - the lower the frequency, the longer its wavelength (which translates to length of aerial). This means that a better reading on the EU band means the aerial is too long. The same is true if the SWR is better on the UK band than on the EU band - the aerial is too short. However, if the readings are below 2.0 on all channels, there shouldn't be much need for finer adjustment.

Incidentally, there is an unused gap the size of 20 channels between the two bands so if you do get the readings on 20EU and 20UK the same, the "resonant frequency" (the channel where the SWR would be perfect) will be between the bands.

    IN SUMMARY :
  • If the reading is lower on EU band , or channel 01, shorten the aerial, (slide the whip in or cut it)
  • If the reading is lower on UK band, or channel 40, lengthen the aerial (slide the whip out)

Should you keep the SWR meter inline?

When there was a CB licence (abolished in 2006), they reccomended that SWR meters should only be used to check your antenna system, and then removed to minimise any chance of interference. I feel that this is more relevant to base stations causing TV interference than it is to people in vehicles, so I recommend having one inline permanently - if you have the space for it.

The advantage with this is that if anything in the aerial system becomes damaged, or your ground connection goes bad, you will know about it much quicker than you would if you had to wait until you got home to check your SWR. This could prevent you damaging your set.

The metal case of the basic SWR meter we sell can be easily removed by undoing 2 screws. This can then be drilled through and mounted flush on a surface, and the meter put back together again - an ideal way of saving space, and securing it inside or outside a cubby box for example.

More about Groundplanes and earth connections...

We have a separate advice section regarding groundplanes, and the various ways of curing problems resulting from the lack of a good earth...

Click here to read more about ground connections, including our Electronic Ground Plane

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