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Coaxial Cable

Most CB and HAM radio operators use coaxial (coax) cable to feed their antenna. Another name for the cable you use to hook your radio to your antenna is "feed line". Feed line is a generic term for all types of cable including coax. Coax has been around for a long time and became very popular with Radio Amateurs after World War II, when army surplus stores were filled with miles of coax cable. This is one of them main reasons why we use coax today, it became such a trend of sorts. Coax cable consists of two concentric wires, as shown in figure 1. It is important to note that coax cable is unbalanced, no current flows on the outside shield of the cable. This is in comparison other types of feed line that are balanced such as twin-lead, which you may be familiar with from your old TV antenna. Figure 1 shows a close up of the end of coax, and its makeup.

coaxview.jpeg

Figure 1 - Close up view of coax.

Coax has several advantages. You do not have to be careful what type of metal objects you run your coax over like you do twin-lead. It is even possible to bury some types of Coax, if the outer jacket is suitable. Its major disadvantage is that some types of coax have high loss at CB frequencies and get even worse as SWR increases. Usually companies rate their coax in decibels (db) of attenuation per 100 foot lengths. So at a given frequency, if you are using exactly a 100 foot length, you would incur a loss of however many db's the manufacture states. Here is a chart of the losses for the most common types of coax used for CB service:

Coax

Attenuation in db per 100 feet (On 27MHz)

Velocity Factor

Description
RG-8
1.15 dB
.66
50 Ohm coax
RG-59
2.0 dB
.66
75 Ohm coax
RG-8/U Foam
0.85 dB
.80
50 Ohm coax, Foam Dielectric
RG-59/U Foam
1.5 dB
.79
75 Ohm coax, Foam Dielectric
Belden 9913
0.7 dB
.84
Premium 50 Ohm coax
1/2" Hardline
0.35 dB
.81
Special coax, expensive!
RG-8X (Mini-8)
1.2 dB
.78
Small 50 Ohm coax. Preferred over RG-58. Recommended for mobile installations.
RG-58
2.35 dB
.66
50 Ohm coax
RG-213
1.35 dB
.66
50 Ohm coax.
RG-11/U Foam
0.85 dB
.80
75 Ohm coax.
RG-11/U
1.35 dB
.66
75 Ohm coax.


As you can see, some coax has high loss. Loss is RF energy that the coax turns into heat or "leaks" instead of passing on to the antenna (or to the receiver from the antenna). The lower the db of attenuation the better the cable is. Think of cable loss as negative gain! The higher the attenuation, the less efficient our antenna system is.

Loss is primary dependent on the coax's shield and dielectric. The shield is the outer wire braid that surrounds the inside of the cable. A thick, tight braid results in less loss. Also, the dielectric (usually white), the plastic type material that separates the inside wire from the outside braid has an effect on cable loss. Cables that use foam dielectric, that is where the insulation is mixed with an inert gas, have very low loss. It is important to use quality low loss cable! As you can see from the chart, the losses can be quite high. You must make perfect connections at the coax ends or, even higher losses will occur. It is also important to note that old coax has high loss also. The cables properties break down over time, resulting in very inefficient cable. If you are still using that coax from the 1970s, its time to replace it! New coax is manufactured better than the coax was in the 1970s also, so this newer cable should last a lot longer.

Two special cables are listed. One is Belden 9913. Belden is the name of the company that makes the cable and they call it "9913". It is a special coax that has two outside shields! The first is a foil material that is on the outside of the dielectric, then over that is the regular copper braid. As a result, the cable is very efficient (low loss) and also STIFF (though they now make a 9913F that is supposed to be flexible)! With low loss comes cost, this cable is expensive. The other special cable listed is hardline. This cable has a solid aluminum cover on the outside for the shield. It is thick, and very efficient---stiff (can't really bend it) and costly. It is used by cable TV companies. Since they run miles and miles of cable, they need low loss cable. Cable loss is still so bad, they still need to have amplifiers along the cables every few miles or so. You can see hardline on telephone poles if your area has cable. It is usually a silver cable about 3/4 inches in diameter.

I said before that loss becomes even worse as SWR increases. These attenuation numbers in the chart are assuming a perfect 1:1 match. If your SWR is over 3:1, cable loss is horrendous no matter what kind of coax it is!

Coax Impedance

Again, the term impedance in "Coax Impedance" has different meaning...you can not measure it with your trusty Ohm meter. It is determined by the spacing (ratio) of the inner wire and outer braid. In CB service, the two impedance's mainly used are 50 Ohm and 75 Ohms.

Velocity Factor

Wow, doesn't that sound like a serious high tech term! You can be king nerd of your CB group if you know things like "velocity factor". Ok, ok we said before that waves travel different speed through different materials, if you missed it, its under the "1/2 Wavelength Dipole" Section of "Antenna Basics". Velocity factor is simply a number we use to determine how fast or slow a wave travels through coax. Different coax models have different velocity factors. Lets look at some numbers. Say we want to make a coax that is exactly 1/2 wavelength long (this means when the wave travels through the coax, it makes exactly 1/2 of a cycle while it travels from one end of the coax to the other). If this sounds confusing, better check out the "Antenna Basics" section. We will take our formula for figuring out 1 wavelength and modify it.

One Wavelength in coax, in feet = 984 * (Velocity Factor) / Frequency in Megahertz (MHz)

Ok, say we want a 1/2 wavelength RG-8/U Foam on channel 40 (27.405)

984 is for a 1 wavelength, so we want a 1/2 wavelength or half of 984, 984 / 2 = 492. Get the Velocity Factor from the table above for RG-8/U Foam, which is .80. Put these numbers into the formula:

1/2 Wavelength, RG-8UFoam, Ch.40 = (492 * .80) / 27.405

1/2 Wavelength, RG-8UFoam, Ch.40 = (393.6) / 27.405

1/2 Wavelength, RG-8UFoam, Ch.40 = 14.362343 feet

The length of coax is 14 feet 4 inches! Practice and see if you can get lengths for other coax types with different velocity factors. This will become important if you ever "stack" or co-phase antennas. You must cut certain length coax lines for co-phased antennas to work!

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