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Cribbed from 4x4CB.com - thank you.
[FONT=Verdana,Helvetica,Arial][SIZE=-1]How to use an SWR meter
[/SIZE][/FONT]
[SIZE=-1]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. [/SIZE]
[FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT][FONT=Verdana,Helvetica,Arial][SIZE=-1]What is an SWR meter[/SIZE][/FONT]
[SIZE=-1]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. [/SIZE]
[FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT][FONT=Verdana,Helvetica,Arial][SIZE=-1]How to take SWR readings[/SIZE][/FONT]
[SIZE=-1]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.
[FONT=Verdana,Helvetica,Arial][SIZE=-1]SWR tuning to cover all 80 UK legal channels[/SIZE][/FONT]
[SIZE=-1]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.
[FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT][FONT=Verdana,Helvetica,Arial][SIZE=-1]Should you keep the SWR meter inline?[/SIZE][/FONT]
[SIZE=-1]When there was a CB licence (abolished in 2006), they 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. [/SIZE]
[FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT][FONT=Verdana,Helvetica,Arial][SIZE=-1]How to avoid a high SWR in the first place[/SIZE][/FONT]
[SIZE=-1]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 :
[SIZE=-1]Some aerials have the ability for you to change the length by moving the whip in and out by a certain amount, and some aerials you simply cut short bits of the whip off.
[/SIZE]
[FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT]
[SIZE=-1]Springer/Stinger/Orbitor
All the aerials with an open coil spring have to be cut of you need to shorten them, but if you need to lengthen (if they are a bit too close to something metal, for example) you can undo the grubscrew and move the whip up by upto 30mm or so.
These aerials are supplied as being the correct length for an "ideal world" installation (what some suppliers rather misleadingly call 'pre-SWRd' or 'pre tuned') but location on the vehicle and of course ground plane can have a effect on SWR readings, but as long as these factors are ok, you shouldn't have to actually do anythig to these aerials to get a reasonable reading.
The smaller Springer aerials can need a little bit of fine tuning, usually shortening - you do this by taking readings to make double sure that it does need to be shorter (i.e. a better reading on channel 1 than on 40) and cut 10mm off the top of the aerial, and take more readings.
You're looking for readings on channel 1 and 40 that are about the same, and a nice low reading in the middle at channel 20.
If you cut too much off and find the readings going the other way (indicating it's too short) undo the grubscrew and pull the whip out by 10mm to get you back to the length you were at before you cut too much off! Generally speaking, the larger an aerial is, the greater number of channels it will give a good reading over - this means that the large Springer aerials will normally give reasonable readings over both bands legal in the uk, whereas the small Springer may have to be set up to favour the band you will be using most. [/SIZE]
[/SIZE]
[FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT][FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT][FONT=Verdana,Helvetica,Arial][SIZE=-1]Groundplanes and earth connections...[/SIZE][/FONT]
[FONT=Verdana,Helvetica,Arial][SIZE=-1]What is a ground connection?[/SIZE][/FONT]
[SIZE=-1]To work properly, an aerial has to be grounded - i.e. the “bottom” of the mount needs to have an earth.
In a fixed aerial used at home, this will be the pole going down into the ground, but on a vehicle, the actual metal of the vehicle has to do the job.
Where your mount makes contact with the vehicles’ body, you will need to scrape a bit of the paint away, so that you make a good metal to metal connection.
On Land & Range Rovers, when the aluminium is exposed to air, a chemical reaction called oxidisation starts to happen. Over a relatively short period of time this can introduce a resistance to the earth connection. Without prevention, the connection is soon lost, and your aerial is trying to work without a ground plane, with a very noticeable deterioration in performance (and the risk of damage to the set).
One method you could use is to run a short “earth strap” (heavy gauge speaker cable will do) from the bottom of the mount to a point on the chassis or elsewhere on the vehicle.
There are a couple of products available that allow you to get a good “ground plane” without having to worry about scraping paint away, or using an earthing strap... [/SIZE]
[FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT][FONT=Verdana,Helvetica,Arial][SIZE=-1]Artificial Ground Planes[/SIZE][/FONT]
[SIZE=-1]An Electronic Ground Plane. This can be fitted to a bracket or mirror mount (although too large for fitting to gutter mounts) and has a standard 3/8” aerial connection on the top, and socket for a standard CB cable plug on the bottom. This unit simulates a connection to ground, so you do not have to worry about your metal to metal earthing. This will cure any SWR problems which are down to poor earthing, but if an aerial is in the wrong place, i.e. too close to metal around it, the EGP will not help. Also a good way of helping to reduce interference as it allows the aerial to be completely isolated from the vehicle. [/SIZE]
[FONT=Verdana,Helvetica,Arial][SIZE=-1]How to use an SWR meter
[/SIZE][/FONT]
[SIZE=-1]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. [/SIZE]
[FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT][FONT=Verdana,Helvetica,Arial][SIZE=-1]What is an SWR meter[/SIZE][/FONT]
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. [/SIZE]
[FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT][FONT=Verdana,Helvetica,Arial][SIZE=-1]How to take SWR readings[/SIZE][/FONT]
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.
[FONT=Verdana,Helvetica,Arial][SIZE=-1]SWR tuning to cover all 80 UK legal channels[/SIZE][/FONT]
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)
[FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT][FONT=Verdana,Helvetica,Arial][SIZE=-1]Should you keep the SWR meter inline?[/SIZE][/FONT]
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. [/SIZE]
[FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT][FONT=Verdana,Helvetica,Arial][SIZE=-1]How to avoid a high SWR in the first place[/SIZE][/FONT]
[SIZE=-1]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 doesnt need to be a "back to battery" earth, just a connection to a large enough piece of metal to act as a groud for the aerial. The roof or roofrack, bonnet, chassis, etc will normally do it, but the larger the better.
- Aerials shouldnt 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 from us this shouldnt 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. [/SIZE]
[SIZE=-1]Some aerials have the ability for you to change the length by moving the whip in and out by a certain amount, and some aerials you simply cut short bits of the whip off.
[/SIZE]
[FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT]
All the aerials with an open coil spring have to be cut of you need to shorten them, but if you need to lengthen (if they are a bit too close to something metal, for example) you can undo the grubscrew and move the whip up by upto 30mm or so.
These aerials are supplied as being the correct length for an "ideal world" installation (what some suppliers rather misleadingly call 'pre-SWRd' or 'pre tuned') but location on the vehicle and of course ground plane can have a effect on SWR readings, but as long as these factors are ok, you shouldn't have to actually do anythig to these aerials to get a reasonable reading.
The smaller Springer aerials can need a little bit of fine tuning, usually shortening - you do this by taking readings to make double sure that it does need to be shorter (i.e. a better reading on channel 1 than on 40) and cut 10mm off the top of the aerial, and take more readings.
You're looking for readings on channel 1 and 40 that are about the same, and a nice low reading in the middle at channel 20.
If you cut too much off and find the readings going the other way (indicating it's too short) undo the grubscrew and pull the whip out by 10mm to get you back to the length you were at before you cut too much off! Generally speaking, the larger an aerial is, the greater number of channels it will give a good reading over - this means that the large Springer aerials will normally give reasonable readings over both bands legal in the uk, whereas the small Springer may have to be set up to favour the band you will be using most. [/SIZE]
[/SIZE]
[FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT][FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT][FONT=Verdana,Helvetica,Arial][SIZE=-1]Groundplanes and earth connections...[/SIZE][/FONT]
[FONT=Verdana,Helvetica,Arial][SIZE=-1]What is a ground connection?[/SIZE][/FONT]
[SIZE=-1]To work properly, an aerial has to be grounded - i.e. the “bottom” of the mount needs to have an earth.
In a fixed aerial used at home, this will be the pole going down into the ground, but on a vehicle, the actual metal of the vehicle has to do the job.
Where your mount makes contact with the vehicles’ body, you will need to scrape a bit of the paint away, so that you make a good metal to metal connection.
On Land & Range Rovers, when the aluminium is exposed to air, a chemical reaction called oxidisation starts to happen. Over a relatively short period of time this can introduce a resistance to the earth connection. Without prevention, the connection is soon lost, and your aerial is trying to work without a ground plane, with a very noticeable deterioration in performance (and the risk of damage to the set).
One method you could use is to run a short “earth strap” (heavy gauge speaker cable will do) from the bottom of the mount to a point on the chassis or elsewhere on the vehicle.
There are a couple of products available that allow you to get a good “ground plane” without having to worry about scraping paint away, or using an earthing strap... [/SIZE]
[FONT=Verdana, Arial][SIZE=-1][/SIZE][/FONT][FONT=Verdana,Helvetica,Arial][SIZE=-1]Artificial Ground Planes[/SIZE][/FONT]
