Embedded Files
SWR Meters & Readings Explained
SWR - How to use SWR meter and SWR settings
SWR - How to use SWR meter and SWR settings
An SWR meter is an important yet simple and easy to use bit of kit.
This section of the site will show you how to use one correctly and tell you what to do to improve your readings if necessary.
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!
In most cases, if an aerial and mount has been installed correctly and in a suitable place, it should just be a matter of connecting the meter, taking a couple of readings, seeing that the SWR is within limits, and the job is done.
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
An SWR meter will show you whether your aerial is correctly tuned (the right length) and if you have a good enough ground plane/earth connection on the mount.
It is connected inline between the CB and the aerial with a short connecting lead (often called a patch lead).
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.
Looking at the meter face, the top line is the SWR reading - you can see that any reading above 3 is "in the red" and considered a risk to the CB - you should be able to get it under this without too many problems.
Under the actual SWR reading is the percentage of reflected power - i.e. 25% of your power being reflected back is just in the red.
Below this is the FS reading - you don't have to worry about this - it stands for field strength and is not a facility used when setting up a standard legal CB.
How to take SWR readings
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 and bonnet should 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 SWR meters measure SWR in the same way :
Firstly, go to channel 20 on the UK band.
Put the switch to FWD and transmit. 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.
What the readings tell you
- A reading of below 1.5 is ideal
- A reading between 1.5 and 2 is ok, and nothing to be worried about
- A reading between 2 and 2.5 will usually be reduceable with adjustment to the aerials length
- A reading between 2.5 and 3 usually indicates that the aerial is being affected by something around it, or that the groundplane is present but maybe not big enough
- A reading in the red part of the SWR meters' scale means that the SWR is high enough to possibly damage the set and usually indicates an insuffcient ground plane/earth connection, or a short or open circuit in either the cable, plugs or mount.
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.
SWR tuning to cover all channels
Generally, the smaller an aerial is, the fewer channels it will give a good SWR over so if you have a small aerial and can't get the reading low across all 40 channels, you may have to decide which band you will use the most, and fine tune it for that band.
Should you keep the SWR meter inline?
It is recommended 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, and especially if your cable is outside the vehicle and could potentially get snagged on branches etc.
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.
How to avoid a high SWR in the first place
There are a few important things you need to know when installing the aerial and mount which should prevent you from having an SWR problem :
- All mounts apart from magnetic mounts will usually need a metal-to-metal ground/earth connection. This doesn't need to be a "back to battery" earth, just a connection to a large enough piece of metal to act as a ground for the aerial. The roof or roof rack, bonnet, chassis, etc will normally do it, but the larger the better.
- Aerials shouldn't have metal too close to them (apart from whatever they're mounted on) - ideally they should be sited well away from anything that might interfere. You cannot mount an aerial on a Defender's back bumper, and run it up the back of the vehicle, for example. If the aerial is being affected by things around it, the usual effect is that the readings will indicate that the aerial is too long, so you may still be able to get a good SWR by shortening it.
- In particular it's the coil of an aerial (they'll all have one of one kind or another) that needs to be kept away from metal objects.
- Plugs and connectors are crucial - if you're buying new pre soldered connectors from us this shouldn't be a problem, but if you are reading this because you are trying to cure a high swr in an already installed system, then you should make sure there are no shorts in the cable or plugs. The outer braid of the cable makes connection with the outer of the plug, which in turn makes connection with the mount, and there gets routed to ground, under the aerial. The inner core of the cable goes to the inside of the plug, which feeds the actual aerial itself - this should be electrically isolated from the braid/earth.
SWR Meters are available for sale with us. please contact us
Google Sites
Report abuse