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Aviation Glossary :: Feed Line  Aviation Glossary :: Feed Line FAA Written Test Preparation
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Feed Line
Feed Line
The wires or cable used to connect a transmitter, receiver or transceiver to an antenna. See Transmission line.
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source: ARRL Ham Radio Glossary

Feed Lines:

Regardless of whether you are operating at HF, VHF or UHF, the quality of your feed line is critical to your station. The feed line (also called the transmission line) is the RF power conduit between your radio and your antenna. All the energy you generate travels to the antenna through the feed line. By the same token, all the signals picked up by your antenna must reach your radio through the same feed line.

The problem with any feed line is that it isn’t perfect—it always loses a certain amount of the energy. To complicate matters, all feed lines are not created equal. The amount of loss at any frequency will vary considerably from one type of feed line to another.

The most common type of feed line is coaxial cable, or simply coax. It is called coaxial because there are two circular conductors positioned “co-axially” (on the same axis), one inside the other. The inner conductor is usually called the “center conductor.” It is surrounded by a solid or multistranded outer conductor commonly called a “shield.” The shield is usually surrounded by an insulating plastic jacket. There is also insulating material between the center conductor and the shield. This material can be hard plastic, foam plastic or even air.

A popular type of feed line for HF use is ladder line. In fact, at HF frequencies it is the most common feed line for random-length dipoles and other antenna designs. Ladder line consists of nothing more than two wires in parallel separated by insulating material.

When rating feed lines for loss, we use “decibels (dB) per 100 feet.” If you’re not familiar with the decibel, don’t worry. Just remember that the higher the decibel number, the greater the loss.

Feed lines also have a characteristic impedance value measured in ohms. Coaxial cable commonly used for Amateur Radio has an impedance of 50 ohms while ladder line impedances can vary from 300 to 600 ohms. Amateur Radio transceivers are designed to work with an impedance of 50 ohms, so you must use 50 ohm coax, or find a way to convert the 300 to 600 ohm impedance of ladder line to 50 ohms. If your radio “sees” anything other than 50 ohms, it will reduce its output to protect itself from the possible damage that can result in a high SWR condition.

If you are using an antenna that is designed to deliver a 50-ohm impedance, it is best to use a coaxial feed line to provide a 50-ohm antenna system impedance for your transceiver. Even these 50 ohm antennas can be a little “off” at times, so you may need to tune them by physically cutting or adjusting the antenna to the correct length, as we discussed earlier, or by adjusting a matching section at the antenna.

The other approach is to use a device called an antenna tuner to transform the impedance of the antenna system to 50 ohms for your radio without physically adjusting the antenna at all. An antenna tuner is a kind of adjustable impedance transformer. Some tuners operate manually; you twist the knobs until the SWR meter shows a 1:1 SWR, or something reasonably close to it. Other tuners are automatic and do all the adjustments for you.

Taking the antenna tuner approach is not a good idea when you are using coaxial cable under high (greater than 3:1) SWR conditions. The tuner may provide the 50 ohm match to your radio, but the mismatch and high SWR still exists between the antenna tuner and the antenna! This translates to high losses in the coaxial cable.

On the other hand, using an antenna tuner with ladder line is a good way to go ­– at least for HF work. At HF frequencies, the loss in ladder line is so low, you can still see good results even when the SWR is horrendous. The antenna tuner provides the 50 ohm match to your radio and you really don’t care what the SWR is between the tuner and the antenna.

So which type of feed line should you use at your station? Fortunately, the answer is simple: You want the feed line that has the lowest loss at the highest frequency you want to operate.

As you probably guessed, low-loss feed lines are more expensive. Some of the low-loss feed lines are also rigid and hard to work with (they don’t bend easily). A little planning and common sense goes a long way when it comes to selecting feed line.

In a mobile installation, you can use an inexpensive feed line such as RG-58 because you’re only using a short length. As long as the SWR is low, the loss will be acceptable.

However, if you have an antenna that is 100 feet from your radio and you are operating at, say, 440 MHz, RG-58 would be an extraordinarily bad choice! For this installation you’ll need to invest in something much better—probably LMR-400 or Belden 9913.

For base stations in particular, always buy the lowest-loss coax you can afford. Since you’ll probably be using your feed line for several years or longer, you want something that can support your changing interests. For instance, 100 feet of LMR-400 is overkill quality for a station that only operates on the 40-meter band. But if you someday want to switch to 440 MHz, you’ll be glad that you already have a low-loss feed line in place!

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source: ARRL Ham Radio Glossary

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Disclaimer: While this glossary in most cases is likely to be highly accurate and useful, sometimes, for any number of editorial, transcription, technical, and other reasons, it might not be. Additionally, as somtimes you may have found yourself brought to this page through an automated term matching system, you may find definitions here that do not match the cotext or application in which you saw the original term. Please use your good judgement when using this resource.

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