Coax Basics.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.
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 (*) Velocity Factor Description.
RG-8 1.15 db .66 50 Ohm
RG-59 2.0 db .66 75 Ohm
RG-8/U 0.85 db .80 50 Ohm
RG-59/U 1.5 db .79 75 Ohm
Belden 9913 0.7 db .84 50 Ohm
1/2" Hardline 0.35 db .81 Special coax
(*) Attenuation in db per 100 feet on 28 Mhz.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! Also, below you will see a practical reason for cutting coax to a 1/2 wavelength.
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! Also, below you will see a practical reason for cutting coax to a 1/2 wavelength.
Assemble Your Coax Correctly
Bad connection cause loss. If you are going to solder connectors on the ends of your coax, be sure to do it right. You must have the right tools. Most Cbers and Ham radio operators think that they can solder on connectors to coax with their 25 Watt pencil tip soldering iron. You can't. You should use a high wattage iron, preferably over 100 watts. You must heat the connector up quick, so you do not damage the coax and connector, and the only way to do this is with a high wattage soldering iron. I am not going to go into detail of soldering on PL-259 connectors to coax but let you look at figure 2. Trim the coax carefully, do not nick the inside when cutting. And I have one big tip you do not want to forget, before soldering the PL-259 plug to the coax, do not forget to slide the PL-259 collar over the coax! I have done this so many times! Soon as you start working just slide that collar on, push it back far down the coax out of the way so it does not slide back off..you will thank me for this! If you solder the connector on without sliding the collar on, you will have to start over (the collar will not fit over the connector once it is soldered on)! After you are done (or to check coax you suspect is shorted or bad) take an Ohm meter and check you coax as shown in figure 3.
How to check to make sure your coax is ok. This does not check for cable loss, it just makes sure the cable is not shorted or an open circuit.
When you are done, be sure to waterproof the ends of the cable. Wrap it in quality electrical tape (I like 3M electrical tape) or use a special wrap you can get at radio shack. Water will easily find its way into coax ends. Remember I said old coax has high loss? This is probably the number one way coax is ruined.
Why does my coax length affect the SWR of my antenna?
How many of you change the length of your coax to tune your antenna? One of my good friends said to me, "I think changing the length of the coax is the same as moving the gamma rod adjustment on my Moonraker 4". Sorry to say, this is not true. As most people will find, varying the length of coax to the antenna will vary the SWR that the SWR meter is reporting. Actually, SWR should remain constant no matter how long the coax is or where it is placed on the line (if its 5 feet down the coax from the radio or 50 feet down the coax from the radio). In most cases, the cause of inconsistant SWR meter readings is from poor SWR meter design or component failure. For the SWR meter to read consistant SWR readings on the coax, the meter has to have an impedance itself of 50 Ohms. Any deviation of the SWR meter's self impedance (from 50 Ohms) from poor design or componenet failure will cause inconsistant SWR readings when the SWR meters position on the coax or length of the coax is varied. I have found this to be the case of every SWR meterI have ever owned! I don't have equipment to test the self impedance of my SWR meters, but emprically found this to be the case.
Varying coax line length presents different imdepance load components to the radio - meaning coax lengths changes how you radio couples with your antenna. I have found that using a 1/2 wave multiple run of coax from the radio to the antenna makes the radio couple to the antenna better (better power transfer). This is only the case if you are certain your antenna is resonant on the same frequency as your coax (ex. antenna resonates on 27.405 Mhz and the coax is a 1/2 wavelength multiple of 27.405Mhz). The coax doesn't add any "extra" impedance components (reactance) in this case - it merely presents the true load of the antenna feedpoint back to the radio. I (still) recommend using 1/2 wavelength multiples of coax. Keep in mind, this does not make SWR readings "more true" (like I previously thought), I think it provides consistantly better loading. Similiarly, you could use an antenna tuner to achieve a similiar effect. If you remember nothing else, just remember there isn't a "magic" length of coax that will turn solve your problems and turn your weak signal into a strong one.