- The frequency of CB signal is in the 27.4 MHz range.
- The wavelength (in feet) of our CB signal is 36', a Half wavelength is 18', a Quarter wavelength is 9' (or 108").
- A CB Radio is a “system” of parts that work together, they are:
- Radio, typically mounted in a vehicle or room
- Antenna, typically mounted in a convenient location
- Cable, from radio to antenna
- Vehicle, yes it is a part of how well the CB operates
- Tuning, of the antenna to optimize performance
All CBs work about the same, some have more power (FCC 4W max), but to be legal they are about the same.
- You get what you pay for. Some of the more expensive CBs have features you may never use. Most have a connector for an external speaker, which could be a handy option.
- The size and shape will need to fit in your vehicle. With the cramped space of a Jeep, look at other Jeeps and see what they did.
- Be sure to GROUND the Radio well.
A quarter wavelength is 9’. You may recognize 9’ or 108", is the length of a long steel whip antennas made popular during the CB craze of the 70's. Some smart guy figured out that if you take 108” of wire and wrap it around a stick it will work just fine and not whack your garage door. There are many antenna options out there, but a taller antenna is better than a short one.
Considerations for antenna, mounting:
- Because you must tune the CB system, get an antenna that is easily tunable. This means the top of the antenna has an adjusting screw under the end cap. Many are available such as from Fire Stick.
- An antenna is an odd thing, it works when all of the “physics” for radio waves are setup correctly. Normally the antenna will need a ground plane. This means the metal base under the antenna will equal the height of your antenna. Yea like how can you make that work? Well if you mount it in the middle of a steel roof it will work equally well in all directions. If it is mounted on the side of you hood, it will work better in the direction that the hood metal is on. Generally we mount them the best we can and hope they will work OK.
- They do make a “No-Ground Plane” antenna; they are expensive but work great in all directions. Truckers use twin antennas with a special setup to get them better performance, primarily in the forward and left/right directions.
- Tuning can only be accomplished in a completely installed system. Any changes to the radio or vehicle after tuning will alter the tuning or reflection.
- Be sure to GROUND the Antenna well. This means a good ground connection from the base of the antenna to the ground on the Radio. A painted tire carrier is not good. Run an extra ground wire if you need to.
- RG-58 (50ohm) cable is typically what people buy.
- RG-8X (50ohm) is a better cable and more expensive.
- The length of a cable is a debate that will never end. Most will say 18’ (half wavelength), but people use longer and shorter all the time. The most important thing is tuning, if you can tune your system it will be fine.
- Because our vehicles are shorter that 18’ the question of what to do with that extra cable.
- It needs to be located where it will not get smashed, bent or any physical damage.
- It cannot be coiled. A coil is an inductor and causes resistance and for radio waves reflectance. The solution is to make a figure 8 with a minimum 6” diameter on each end. For some reason this cancels the inductor problem.
Not much we can do to the vehicle, but remember the best CB in one Jeep will still need to be tuned if you move the antenna or move it to another vehicle.
This is very important to check. There are two measurements that need to be made after your antenna system is installed. Unplug the coax cable from the back of the radio leaving the coax plugged into the antenna. First using an Ohm meter, measure resistance from the bottom half of the antenna (meter lead on coax body) to the antenna somewhere above the mounting bracket and you should measure an open circuit. Second measure from the bottom half of the antenna so somewhere on the vehicle (non-painted metal) and you should have less than 1 ohm of resistance. If either of these measurements is not correct you need to correct the issue before using the radio and proceeding to tune the antenna.
This is the most important and most misunderstood part of a CB. Every CB must be tuned. A CB radio when transmitting, sends power down a coax cable to the antenna. The signal is trying to escape all along the cable, but is contained in the coalial shielding of the cable. However, when the signal gets to the unshielded antenna, the power leaves into the air. If the system is not tuned properly, some of that signal energy cannot leave the antenna, and is called reflectance. Reflected power is returned to the CB, where it can heat up and burn up a CB. The solution is to tune the antenna to get the reflectance number as low as possible.
NOTE: Never key a CB without an antenna attached.
Tuning a CB is easy if you have a tunable antenna. Non-tunable antenna will require you to add or remove antenna wire from the top of the antenna (you will find out it is worth the extra cost of a tunable antenna).
- The test equipment is known as a Standing Wave Ratio (SWR) meter. Available at Radio Shack (the only place I have ever found them), or borrow one from a friend.
- NOTE: the meter will need a short 12” cable that is not a part of the meter.
- To install the meter, disconnect the antenna cable from the radio and install the short 12” cable. The other end of the 12” will go into the meter in the connector labeled radio. The antenna cable will also go into the meter in the connector labeled antenna.
- Park the vehicle in an area that is free of metal, such as power lines, houses and other vehicles. If you have a 10-20 yard free zone that is best, if not do what you can.
- Now set the CB to the channel you use the most, for us it is Ch4. For general use Ch20. Once you have the best tune, you will notice the farther away you go from the channel the higher the SWR will be.
- Now follow the instructions for the SWR meter to (CAL) calibrate it.
- Now set the meter to SWR and Key the mike. Look at the SWR meter, the reading should be as low as possible. Adjust the antenna in small amounts until a low number is achieved.
- Testing actual operation.
- Leave your vehicle parked, get a friend to go down the road while talking with you and noting distance. The distance will vary but you need a mile or two for it to be a use full CB.
- If you real curious, you can turn your vehicle in different directions relative to your friend and see if your setup works best in one direction.