The feed line is the cable that goes from the transmitter to the antenna. Feed lines can also be called transmission lines. There are many different kinds of feed lines, but the most common feed line is made from coaxial cable.
Not all feed lines are created equal. When buying a feed line, it is the type of thing where you are probably going to get what you have paid for. No feed line is perfect though, you will always have a little bit of energy loss from your feed line unfortunately.
SWR stands for standing wave ratio and is important to keep impedance in mind when purchasing a feed line. Ideally, you want your SWR to be 1:1 which would indicate perfect impedance matching between your antenna and your feed line.
So how do you find what SWR is? Your transmitter is connected to the feed line which is connected to the antenna. When you transmit a signal you are sending a voltage down the feed and eventually to the antenna.
If the impedance of the feed line and the antenna are way off, then some of that voltage is going to bounce back and go right back down the line. SWR is the ratio of the voltage that gets bounced back and the voltage that makes it through.
So you want that SWR to be 1:1 ideally which would result in the least amount of voltage being sent back down the line and the better your signal quality is going to be. This is why impedance matching is so important and needs to be kept in mind when shopping for a feed line. A high SWR ratio translates to more feed line loss.
If your antenna and your feed line don’t have matching impedances, there is always the option to use an antenna tuner. Antenna tuners are like adjustable impedance transformers and have the capability to match impedances between antennas and feed lines.
If you are using coaxial cable and you have a high SWR, an antenna tuner might not help your situation. It would help a little bit but not enough to solve the problem. It would be a better idea to try to find a feed line or an antenna that works better for your rig.
Coaxial vs Ladder Line
The most common type of feed line is coaxial cable. It is super easy to use and extremely durable. Coax cable is able to withstand interference and can be run alongside metal objects like a tower without any issues.
Coax cable usually has an impedance of about 50 to 75 ohms. It is an unbalanced line which means that one of the conductors inside is grounded and the conductors don’t carry the same signal. The grounded conductor is also called shield and is what protects the cable from outside interference.
Ladder line is another popular type of feed line that is used in HAM radio. Ladder line gets its name because of two parallel conducting wires running alongside each other connected by insulating material that is spaced out so it looks like a ladder.
The ladder line is generally lighter and has lower signal loss due to weather conditions and dirt and water getting into the line. The ladder line is balanced and usually has an impedance somewhere between 300 and 600 ohms.
Ladder line’s main advantage is having such low loss for HF frequency ranges. You could run a long distance of the ladder line and still not have very much loss of signal.
The disadvantages of ladder line is that it’s a little bit more fragile and cannot be installed near metal objects. Because the ladder line is not shielded, it will pick up a lot of extra noise and you will have to use a balanced to unbalanced converter.
How do you know which feed line is right for you? It all depends on what you are planning on using it for. How long of a run do you need? What kinds of conditions will the feed line be subjected to? Will it have to be near metal? What frequencies are you wanting to use?
All of these things are things to consider when purchasing a feed line. Unfortunately with the feed line, you get what you pay for. The goal is to find the feed line that works for you with the lowest loss at the highest frequency you want to transmit on.
Feed line loss is rated at decibels per 100ft, so the higher the number, the higher the loss of signal. For shorter lines, you can probably go a little cheaper, but longer lines are most likely going to be more expensive.