By Jonathan Imberi
RF energy uses a feed line to get from your transmitter to your antenna. A feed line is a special cable or arrangement of wires that feed power to the antenna. They also can feed received signals from the antenna to the receiver.
All feed lines share one electrical property: their characteristic impedance. Impedance is another form of opposition to electric current. It includes factors related to the capacitance and inductance in an AC circuit.
The characteristic impedance is determined by the spacing between line conductors and the type of insulating material used. Because we want the feed line to take all the transmitter power and feed it to the antenna, characteristic impedance is very important. In order for this to occur, the transmitter or source must have the same impedance as its load or feed line. On the same token, the feed line must have the same impedance as its load, the antenna.
If there is a difference in impedances, special circuits called matching devices or matching networks may be used. This refers to a combination of capacitors and inductors that form a special circuit. These circuits can be used to "match" one impedance to another. Careful selection of a feed line, however, can minimize such matching problems.
For more information on impedance and matching, see Matching Impedances With Your Equipment.
There are several types of feed line that amateur radio operators use. The most common is the coaxial cable. This is simply called "coax" for short and consists of one conductor inside the other. The center conductor is surrounded by insulation, and the insulation is surrounded by a wire braid called the shield. A tough vinyl outer coating encases the whole cable and makes it weatherproof. Coax can come in many different sizes with many different electrical properties. The drawing below shows several types of coax cables used by amateurs.
Common types of coax have either a 50 ohm or 72 ohm characteristic impedance. RG-58, RG-8, and RG-213 are all designations for 50 ohm coax cable. These designations may also include a suffix such as /U, A/U or B/U, or bear the label "polyfoam". These types of feed lines may be used with most antennas. Cables designated by RG-59 or RG-11 are 72 ohm lines. These cables can be used to feed dipole antennas since the impedance of a half-wave dipole is about 73 ohms, as long as it is located far from any other physical objects. This type of set up is very impractical though, since it is nearly impossible to find a space where an antenna can be placed far from any other objects. When an antenna is installed near buildings, trees, or even the ground its impedance becomes closer to 50 ohms. However, whether you use a 50 or 72 ohm cable the impedance mismatch to the antenna is unimportant.
Electrical characteristics and physical properties are two considerations when choosing the right feed line for your installation. As far as physical properties, RG-58 and RG-59 cables are relatively light-weight and reasonably flexible and have a diameter of about 1/4 of an inch, whereas RG-8, RG-213, and RG-11 are nearly three times heavier, much less flexible, and are about 1/2 of an inch in diameter. As you can see, the physical properties could make a big difference depending on your station's setup. As we mentioned before, there are also the electrical characteristics to consider. For example, RG-58 and RG-59 handle much less power than RG-8, RG-213, and RG-11 will because their size limits their power capabilities.
Any line that is used to feed an antenna will absorb a small amount of the transmitter power. When this happens, the power is lost, because it serves no useful purpose. It does, however, warm the feed line slightly. The loss occurs because neither the wires nor the insulating material are perfect conductors and insulators. This lost is also increased slightly as the SWR values increase, so amateurs need to try to keep their SWR below 2:1.
As with everything in life, there are poor-quality cables and better-quality cables. The poor-quality cables will allow more transmitter power to be lost as heat, whereas better-quality cables will minimize this lost. The easiest way to find a good quality coax is to stick with the name brands. You can also examine the shield braid on the cable, if you can see through the holes in the braid you should probably find another cable because that one does not provide complete coverage of the center insulator.
The larger coax types have less signal loss than the smaller types. Amateurs probably won't notice the small signal loss if their feed line is less than 100 feet long, at least on the HF bands anyway. Many HF operators find the smaller coax better suited to their needs because of its light weight and flexibility, not to mention it costs about half as much per foot as the larger cables.
The losses caused by RG-58 and RG-59 are much more noticeable on the VHF and UHF bands especially if your feed line is longer than 50 feet. Amateurs normally use higher-quality RG-8 coax or even special lower loss coaxial cables. It is also important to remember that good quality connectors are also very important at the VHF and UHF frequencies.
Coax cable is the most common because of its advantages as a feed line:
1) It is readily available.
2) It is resistant to weather.
3) It matches impedances of 50 ohms with most amateur antennas.
4) It can be buried in the ground if necessary.
5) It can be bent, coiled, and run next to metal, all with little effect.
Its only major drawback is its cost.
The most common coax cables used by amateurs are RG-8, RG-58, RG-174, and RG-213. RG-8 and RG-213 are very similar and have the least lost out of the cables listed. RG-174 has the highest loss of the cables listed and is only 1/8 of an inch in diameter. It is normally used for cables that connect sections of a transmitter or receiver or for short inter-connecting cables of a low power system. It could also be used as feed line for a portable low power HF station.
The longer the length of cable, the most attenuation. Amateurs should always try to use a feed line and antenna that have matched impedances, for they should be able to change the length of the feed line without significantly affecting the antenna system. Remember your feed line only has to be long enough to reach your antenna, and if you can obtain a low SWR on the line it means that the impedance recognized by the transmitter will be about the same regardless of how long the feed line is. You can also cut off or shorten excess cable length to reduce attenuation of a signal caused by an antenna-system loss.
Coax connectors are just as important as the feed line of your antenna. The biggest factor when choosing connectors is matching the existing connectors on your radios and antennas. The most common connector used on HF and many VHF radios is the SO-239 connector. The matching connector for this type is called a PL-259. The PL-259 is also sometimes called a UHF connector, even though they are not always the best choice for the UHF bands.
Many VHF and UHF portable radios used BNC connectors. These connectors are designed to work with RG-58 coax and produce a low loss connection that is also weatherproof. BNC connectors are well suited for use with portables and their antennas because they require only a quarter turn to install or remove, yet they lock securely in place.
It is always a good idea to check on your coaxial connectors on a regular basis. It is important to make sure that they are clean and tight to minimize their resistance. If you think there may be a bad solder connection you should always resolder the joints just in case. You can also use a coaxial sealer on outdoor connections to mold around them and make them even more weatherproof.