Mobile CB Antennas 101

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Mobile CB Antennas 101

Post  Cobra on Wed Oct 17, 2007 7:11 pm

You’ve got a CB radio and now you need an antenna. The antenna is the most important part of any radio system. Even the best radios can’t make up for a poor antenna. A cheap radio and a good antenna will get your signal out and talk better than an expensive radio with a poor antenna. With that said, get the best antenna you can for your application. This document is presented for those new to the CB hobby and is not a step-by-step guide to mounting an antenna—there are just too many ways to mount an antenna.

Things to consider:
The environment you plan to use your radio in may dictate what kind of antenna you need. For example, if you’re off road a lot with low hanging obstacles, you probably don’t want a big expensive antenna mounted up high that could get damaged or torn off.
Now consider the vehicle you plan to mount the antenna to. If your vehicle is mostly plastic, aluminum, or fiberglass, you won’t be looking at magnetic mount antennas. Look for places that you can mount the antenna. The best place on most metal vehicles is the center of the roof. This is usually the highest spot and provides a good ground plane. This may or may not be possible or practical for your application.
Look at how to route the coax cable you will need for the antenna. No use mounting an antenna to an area that you can’t get the coax to.
How much power do you want to run through the coax and antenna? Your antenna should be rated to handle twice the power you plan to run through it. Even though the antenna manufacture says it can handle more, it will dissipate much of your signal into heat if you try to run it to it maximum limit.
Choose the right coax that can handle the peak power you plan to put through it. RG58 can handle 500 watts peak, RG8X (Mini can handle 1000 watts peak, Times 240 can handle 2000 watts peak, and RG213 can handle 4000 watts peak. The coax cable is a means for transferring your RF signal to the antenna. The cable is meant to contain the RF energy. Because it does not radiate and serves only to transfer RF between two components of the system, it's performance in terms of efficiency is affected by length. In other words, using a long run of coax will reduce the total amount of signal or energy that gets to the antenna.
The 102-inch whip (with 6-inch spring or spacer) is a ¼ wave antenna that stands about 108 inches tall over all. It is good antenna and is usually very inexpensive. Don’t overlook this antenna because it does not have big fancy coils. This antenna works very well and can handle quite a bit of power. The only draw back is the length.
The only reason manufactures put coils on their antennas is to reduce the overall height of the antenna. More and bigger coils don’t do any thing but reduce the overall length of the antenna. Also, the big air coils have less inductive losses than the tightly wound coils you will find on those cheap antennas.
In terms of fiberglass antennas, the antenna part is not actually fiberglass. The fiberglass only holds the antenna wire so that it does not bend or get damaged. Fiberglass antennas work well were the antenna may come in contact with low tree branches or where steel whips are just too springy. Too many hits on a fiberglass antenna with objects will eventually damage the antenna.
The placement of the coil or load on the antenna has an affect on how the antenna performs. You cannot say that a base loaded antenna works better than a middle loaded antenna, which works better that a top loaded antenna. They each have their use, depending on the application. It can be said that the coil or load needs to be above all the sheet metal of the vehicle. The placement of the coil or load does affect the angle that the signal takes off from the antenna, but that gets into antenna theory and we won’t be discussing that here.
The amount of surface area on the antenna can also affect how it performs. The more surface area, the better the RX and TX will usually be (to a point). Surface area is the largest player in SWR broadband operation. The more surface area, the broader range the antenna will hold a low SWR (generally). Although, it is really not practical to drive around with 108 inch x 2 inch diameter aluminum tube sticking out of the top of your vehicle either.
In terms of mounting, drilling through the vehicle roof and mounting the antenna to the sheet metal (assuming it is steel and not aluminum) is always the best. But if you can’t bring yourself to drill a hole into your vehicle, you are left with magnetic mounts (Assuming your vehicle body is steel) or a clamp type mount on a well-grounded mirror, luggage rack, or bumper. Most manufactures and dealers have a variety of antennas and mounts for you to choose from.
Not all antennas work the same on all vehicles. What works on someone else’s vehicle may not work well on yours. Try to find a person with the same vehicle as you and see what worked for them.


Some Antenna choices:
Some good and simple antennas for the beginner to setup are the Wilson 1000/5000, Lil Wil and the K40. The Wilson is more popular and does not have the whip bottoming capacitor problem that K40 has, but the K40 is fine antenna for low power applications. The Wilson antennas can handle more power than the K40. The Wilson 1000 is good for 500 watts and the 5000 is good for 2500 watts (remember what was said earlier: half the rated value). You will need to upgrade the coax on the Wilson 5000 antennas if you plan to run more 1000 watts to it. Both the Wilson and the K40 antennas come in magnetic, trunk lip, and through sheet metal mounts.
In some circumstances, a Wilson magnetic antenna works on the side of the body or the rear door horizontally with the antenna whip bent 90 degrees to make it vertical. Sounds crazy, but it works.
For a fiberglass antenna, the Firestick and the Francis antennas are very good especially in the longer lengths. The Francis 8 foot is a great performing fiberglass antenna. If running a fiberglass antenna, keep the power to it down to 500 watts or less.
Fiberglass is the most commonly used antenna for mirror mounts. Several of these work well, but they are usually very narrow banded. These antennas are usually 3-4 feet in length. The K40 Superflex and Francis work fairly well in most cases using under 500 watts. If mirror mounted and there is no body/chassis ground to the bracket, the SWR may not be able to be adjusted into an acceptable range. Coax length WILL play a role in SWR on these vehicle mounting locations. Try grounding the antenna bracket with the braid removed from a scrap piece of coax.
If you can’t get the whole antenna above all the vehicle sheet metal, a middle or top loaded antenna will work better. The Wilson Silver load has been proven to work well when only the top 2/3 of the antenna is above the vehicle sheet metal.
As for big air coil antennas like the Predator 10K, the 55, the BB, and others, these are all high performance air coil antennas. These antennas are rated as high as 30k watts, but will work great even with just a stock, four-watt radio. These antennas will give you the best performance from a coil antenna, but they are big and conspicuous, drawing attention—wanted or unwanted—to your vehicle. These antennas must be tuned and good equipment is needed when setting up one of these antennas.


Setting your SWR (Standing Wave Ratio)
Get or find someone with a SWR meter. In many radios, the SWR meter is fairly accurate, but it is a good idea to verify it with an actual SWR meter. You can usually find an inexpensive meter for around $30 and yes, the Radio Shack meter is good for checking SWR. It’s AVG/RMS power accuracy is fair and it’s PEP power abilities are questionable. As you get into the hobby more, you will want a better meter that gives accurate RMS (Root Mean Square, or average power) and PEP (Peak Envelope Power) power reading along with reflected power. Bird meter are considered the best and most accurate by many people in the field. The ultimate in antenna tuning is an antenna analyzer that can check the antenna resonate frequency or impedance. For now, we just need an SWR meter.
Find a place that is clear of trees, buildings, other obstruction and other antennas that could give you false SWR readings. The areas should be clear of objects by 50 feet or more in all directions.
You will need a short jumper or coax with connectors on each end. Place the meter inline between the radio and antenna using your coax jumper.
Hook the jumper from radio to the TX or transmitter side of the meter and antenna to the ANT or antenna side of the meter.
Turn the radio on and set it to channel 1.
Turn the adjustment knob to the minimum or full counter clockwise position. Set the meter to the CAL or calibrate position.
Key the mic and turn the calibration knob so that the needle moves to the CAL mark on the scale.
Set the meter switch to the SWR setting without touching the calibration knob.
Key the mic and record the SWR meter reading.
Now repeat steps 6-9 on channel 40.
If the SWR is lower on channel 1 than channel 40, your antenna/whip is too long. If the SWR is higher on channel 1 than channel 40, your antenna/whip is too short.
Adjust your antenna in ¼ inch increments. Some antennas have a set screw that allows you to adjust the height of the whip and others must have some of the whip cut or ground off to adjust the SWR. Many of the whips are stainless steel and very hard to cut. A hacksaw, cutoff wheel, or bolt cutters will be necessary to cut these antennas.
After you make an adjustment, repeat steps 5 through 12 until your SWR is the same at both channel 1 and channel 40.
The goal is to get the lowest SWR possible. If your SWR is below 2:1, you are ok, but an SWR of 3:1 can damage your radio. If you have or plan on getting an amp, you need an SWR of 1.3:1 or less. See “Amp Install 101” in the Amplifier section of this site for more information.
If you have tuned the antenna but still have a high SWR reading, you may try moving the antenna to another location on the vehicle or you may need to add grounds to the vehicle.


Vehicle Grounding:
Getting the SWR down on a vehicle can be one of the most frustrating things to do when setting up a radio system. If you are or are going to run an amp, this will help eliminate many headaches.
Ground everything! Ground the chassis to the battery with a 4AWG wire. You may have to go bigger if you are going to run a big amp. See “Amp Install 101” in the Amplifier section of this site for more information. Whatever size wire you use, make sure it is very finely stranded, like welding cable. All connections should be crimped and/or soldered with the appropriate terminals on to the wires. Before attaching the wire, clean all connection areas down to a clean bare metal surface and then protect it from corrosion or oxidation after you make the connection. Note: If you solder the terminals on, use a non-corrosive flux when you solder. Ground the body (and bed if in a pickup) to the chassis. Ground the hood (and trunk if in a car), and doors (if using a mirror mount) to the body. Grounding straps are the best, but heavy gage wire will work. Try to keep the grounds straps/wires to 12 inches or less.
If grounding does not work to get your SWR down, then you will need to enlist the help of an expert.
All this grounding may not be necessary for basic installs. Most often a magnetic antenna on the roof, a "stick" antenna on a pickup bed (3 feet or more away from cab), or a bumper mounted LONG antenna will work with no hassles and low power application (250 watts or less). Antennas of any kind on a vehicle trunk will likely require grounding the trunk lid to the inside cowl in several points.


Other Terms Explained:
You may hear some other terms used to describe antennas and their properties. We will quickly describe them here.

SWR, VSWR or Voltage Standing Wave Ratio: This is a mathematical expression of the non-uniformity of an electromagnetic field on a transmission line such as coaxial cable. Usually, SWR or VSWR is defined as the ratio of the maximum radio-frequency (RF) voltage to the minimum RF voltage along the line. Under ideal conditions, the RF voltage on a signal transmission line is the same at all points on the line. The ideal VSWR is therefore 1:1. If this sounds too complicated, just use a meter.
Reflected Power: This refers to the power that gets reflected back down the coax from the antenna. The amount of reflected power is power that does not go out through your antenna and is lost as heat in the radio. Too much reflected power can damage the radio or amp. Most CB radios without an amp inline, can handle 15-20% reflected power. Amps are a different story and it depends on how hard you drive the amps to how much reflected power it can handle. Typically, if more than 5% of your total output power is reflected back to the amp, this is too much. If running an amp, try to keep the reflected power to less than 2%. Even though you may have a low SWR, you may have high reflected power. The only way to check for reflected power is on a meter that is capable of measuring this parameter.
Impedance: This is the opposition to an alternating current presented by a circuit. This applies to the coax and antenna and what they look like when a RF signal is input into them. It is measured in ohms but cannot be measured with a standard volt or ohm meter. A RF signal must be input to measure the Impedance of the coax and antenna. Radios are designed for specific impedance and the coax and the antenna must match this designed impedance or the system will work poorly, if at all. CB radios are designed for a 50 ohm impedance.
Resonate Frequency: The resonate frequency is the specific frequency that an antenna is tuned for or the antenna will resonate at. Without going into devices that broadband or tune the resonate frequency, an antenna’s resonate frequency is set by its length. A ¼ wave dipole has a resonate frequency that its wavelength is 4 times (4 x ¼ = 1) the length of one side of the dipole.
Broad Band: This is simply how wide the useful frequency range of an antenna is before its SWR gets too high to use. Different antenna designs and matching devices make an antenna useful over a range of frequencies. This and the mounting or installation of the antenna dictates the useful band of the antenna.

Cobra
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Location: Yuma, AZ
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Radio Model: Cobra WX75
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