CB radios. Everybody uses them on the trail so here are a few tips and a couple of tricks and a myth or two.
VSWR this is a ratio of the forward to reverse voltage on your transmission line, the coax. It is just that, a ratio, so there is no such thing as SWR's. It is Voltage Standing wave ratio and the closer you can get to 1:1 the better.
RF or radio frequency measured in kilohertz, megahertz or gigahertz.
CB radios operate in the 26-27 MHz band
Police radios operate in the 150MHz, 450 Mhz band and 800 or 900 Mhz band.
Yes, I know there are still a few places on low band 42 to 50 Mhz.
RX stands for receive and TX stands for transmitter.
Watts this is how the RF power your radio puts out is measured.
Wavelength is the actual length of the radio wave in free space.
FRS family radio service governed by part 47 CFR of the FCC rules and regs.
GMRS General Mobile Radio Service you must have an FCC license to operate in this service. FRS and GMRS are in the 450 to 470 Mhz band.
FM frequency modulation.
AM amplitude modulation.
VHF low band 30 to 54 MHZ
VHF high band 140 to 174 Mhz plus some commercial radios in the 220 Mhz band.
UHF 400 to 470 Mhz
800 where the old analog cellular rests, some Nextel and other SMR(specialized mobile radios)and yes some digital cellular.
All the above with the exception of the commercial stuff at 220 Mhz uses FM modulation. CB radios use AM so lets get one of the myths in right here.
Plain and simple if you yell into the mic or handset of an FM unit you are hurting your sound , not helping due to the nature of FM you will cause your signal to distort.
On the other hand, up to a point the louder you talk into a mic of an AM rig the more power you cause the transmitter to put out.
Coax used in mobile CB radio work is usually rg58u or rg58au and another myth is that cutting the coax to a certain length is the best way to help with VSWR. This is totally false. the only thing you do by cutting the coax to a 1/4 wavelngth or half wavelength etc is to fool your wattmeter. So make the coax 17' which is pretty much a standard length.
Antennas: I will not argue with the fact that a 102" whip, 108" with the spring added is the RULER by which all other antennas are judged. this is a standard 1/4 wavelength antenna for CB's and mounted properly with a good ground plane under it will get out super. The trick to getting a good working antenna system is proper mounting. The farther away from large metal obstructions you can get itand the higher you can mount it the better it will perform. an antenna mounted on the mirror frame does not work well simply because there is no metal under it for a ground plane. the ground plane is electrically the other half of the antenna. Another thing to stay away from is an antenna that has most of the windings near or at the bottom of the "stick" because an antenna radiates from the top. Take a look at a good FireStik or K40 antenna, the loading is at the top, there is a reason these antennas work well!
Power cables. Most modern rigs have a good source for powering a CB inside the cab but, take care not to tap into a fuse, wire or cable bundle that has the YELLOW airbag or ECM wires in it or you may get a surprise. Keep the ground wire inside the cab instead of running it all the way into the battery and keep the ground wire short. This helps keep the radio from picking up spark noise and alternator noises. As with any wire job keep the fuse as close to the source as you can for safety reasons. I know most stuff comes with the fuse close to the radio etc but, spend a buck and put a fuse at the power source.
Did you know the little ball at the tip of metal antennas has a function? It is to disperse the static electricity the antenna picks up as it is whipped through the air.
A 5 amp fuse is usually plenty for all modern CB radios.
When attaching the ground wire use a star washer to get a good bite into the metal. Most problems with wire jobs are due to a poor ground.
One last myth the dual antennas have to be at least 1/4 wavelength apart to work as designed to get a doughnut shaped radiation pattern. Most of the trucks that have dual antennas are too close together but they get a figure 8 radiation pattern so talking to the trucks in front of them or behind them it works great bur forget it side to side for very far away.
Setting the VSWR:
Some facts and pointers:
As the frequency of operation increases the radio wave gets shorter so a 1/4 wave antenna at CB frequencies is nominally 108"
As you have noticed the are plenty of 2', 3', 4' and 5' "top loaded" fiberglass antennas on the market. The manufacturer simply winds or loads the top of the antenna with the magnet wire so you still have 108" of antenna it has just been shortened. Now for you fellow electrical engineers out there I know this is oversimplified but the readers here do not want a bunch of technocrap, man we have too much of it here just explaining the basics.
So on to setting the VSWR or tuning the antenna.
1st and this does need to be mentioned once again, CUTTING THE COAX to tune VSWR is not acceptable, you only are fooling the wattmeter by giving it a false reading of the forward to reverse voltage it should be measuring.
Hook up the wattmeter to the antenna and radio and tune the CB to ch. 40 and take a reading then go to ch. 1 and take a reading. If the VSWR is higher on ch. 40 than on ch. 1 then the antenna is too long so loosen the tip and slide it in some, not much go SLOWLY. If you have the type of commercial antenna where you cut some off the bottom then do so at the rate of 1/4" at a time. Repeat the measurement process till you have the best match on both ch. 40 and ch. 1 If the VSWR was higher on ch 1 you will have to slide the tunable tip out.
So what if you have slid the tip out as far as is possible and the VSWR is still very high on ch. 1 and high but not as high on ch. 40? this shows that the antenna is still a little short so try using a spring which will lengthen the antenna and also give you extra flex.
If you have done all the above and VSWR is still way high then you need to see if you have shorted the connector , easy to do, make sure you trimmed the braid neatly no single strand toughing the center conductor, even the tiny single strand can short the coax. If this was not the problem then you need to check the entire run of coax for a nick or see if it has gotten against the exhaust pipe and melted? I have seen some instances of just plain bad coax from a manufacturer. My recommendation is always use Belden or Tram or Andrew heliax brand coax, they are very high quality.
Mark "Old Goat" Hulsey