Section 4.3: Feed Lines #

Introduction to Feed Lines #

Ever wondered how the radio waves actually get from your transceiver to your antenna? That’s where feed lines come in. Think of them as the highways for your radio signals, connecting your radio to your antenna.

Coaxial Cable: The Ham Radio Standard #

The most common type of feed line you’ll encounter in amateur radio is coaxial cable, often just called “coax.” Picture a copper wire running down the center of a tube, surrounded by insulation, then wrapped in a shield of braided wire or foil, and finally covered in a protective outer jacket. That’s coax in a nutshell.

Impedance: Why It Matters #

The great thing about coax is that it is really easy to use. There isn’t a lot you need to remember when you install it and for the most part it will Just Work™. That said, you need to know what kind of coax to get! Coax comes in different “flavors,” and one of the key differences is impedance. In the ham radio world, we mostly use 50-ohm coax. It’s like the standard gauge for our hobby’s railroads.

But wait, you might say, “I’ve got 75-ohm coax running to my TV antenna. Can’t I use that?” Well, you can, but it’s not ideal. Many homes indeed have 75-ohm coax installed for TV and cable systems. While it’ll work in a pinch for ham radio, it’s not the best choice.

Most radio transceivers have an impedance of 50 ohms, because 50 ohm coax tends to be ideal for higher voltage and power, which makes it a good choice for a transmitter. The 75-ohm coax used by your cable TV or internet system is optimized more for weaker signals, which is what you have when receiving. That said, maximum power transfer (minimum loss) occurs when the impedances of two connecting systems match, though, so if you use 75-ohm coax with a 50-ohm transmitter there will be some loss due to the impedance mismatch regardless of whether it would otherwise be a good choice.

Types of Coaxial Cable #

Let’s talk about some common types of coax you might encounter:

1. RG-58: This is thin, flexible 50-ohm coax. It’s great for short runs or portable setups, but it has higher loss over longer distances.

2. RG-8X: A step up from RG-58, it’s a good middle-ground option. More flexible than RG-8, but with lower loss than RG-58.

3. RG-8 (and variants like RG-213, LMR-400, and many others): This is the heavyweight champion of commonly used ham radio coax. It’s thicker, less flexible, but has much lower loss. Perfect for longer runs or higher frequencies.

4. RG-59 or RG-6: These are the 75-ohm cables you might find in your house for TV. They’ll work, but they’re not ideal for ham radio use.

5. Hardline or Heliax: You probably won’t run into this unless you set up your own repeater, but “hard-line” or Heliax coaxial cables are more rigid cables which have better shielding and lower less, however they tend to be much more expensive, less flexible, and harder to work with. Most repeater systems use Heliax to maximize the performance of the repeater antenna system, since it affects all users of the system. If you ever find yourself at a radio or television station you may see what look like large pipes going between the transmitter building and the antennas. These are most likely air-insulated hardline, which has extremely low loss at VHF and UHF frequencies.

Understanding Loss in Feed Lines #

You may notice that the term “loss” comes up a lot. Loss is the inverse of gain or in other words, loss is negative gain. If 3dB gain doubles the power then 3dB of loss will half it. As a rule, the higher the frequency on any given coax cable the more loss it will have. Because of this it’s common at really high frequencies – like wireless access points – it’s not uncommon to have the transceiver mounted directly to the antenna and built to withstand the elements! Connectors also have an impact, but we’ll discuss that in the next section! Remember that power can neither be created from nor sent into nothing. If your antenna has gain then it is getting that by focusing the signal so that more of it goes in the direction you want. If there is loss in your feedline the lost power is being converted to heat.

Potential Problems with Feed Lines #

Now, while coax is great at keeping your signal contained, it’s not perfect. There’s always some loss along the way - think of it like a leaky hose. But here’s the kicker: it’s not just the cable itself that can cause problems. There are a few sneaky culprits that can add to your signal loss:

1. Water intrusion: Coax and water mix about as well as oil and water. If moisture gets into your connectors or cable, it can really mess with your signal. That’s why weatherproofing your outdoor connections is super important.

2. Poor connections: A loose or corroded connector is like a roadblock for your signal. Always make sure your connections are tight and clean.

3. High SWR: Remember when we talked about SWR? Well, high SWR can increase your feed line losses too. It’s like your signal is playing ping-pong in the cable instead of making it to the antenna.

4. Too many connections: Every time you add a connector or an adapter, you’re adding a potential point of loss. It’s like adding extra junctions in a pipe - each one is a chance for a leak.

5. UV Damage: Ultraviolet (UV) light from the sun can damage the outer jacket of coaxial cable over time. This damage can allow water to enter the cable, leading to increased signal loss and potentially rendering the cable useless. That’s why it’s crucial to use coax with UV-resistant jackets for outdoor installations. If you’re using cable that isn’t specifically rated for outdoor use, consider protecting it with UV-resistant tape or conduit.

Alternative Feed Lines: Ladder Line #

All that said, coax isn’t the only game in town. Some hams use ladder line, especially for HF antennas. Ladder line looks like a ribbon with two parallel wires. It has very low loss, which is great, but it’s more susceptible to interference and needs to be kept away from metal objects. It’s like the high-performance sports car of feed lines - fantastic performance, but a bit finicky to work with.

Special Types of Coax #

While most hams use solid or foam dielectric coax, there’s also a high-performance variant called air core (or air-dielectric) coax. As the name suggests, it uses air as the insulator between the center conductor and shield. This design gives it extremely low loss - great for those long runs or high-frequency applications. But there’s a catch: you need to be extra careful to keep moisture out. If water gets in, it can ruin the cable’s performance and potentially create all sorts of issues. That’s why air core coax requires special weatherproofing techniques and installation care. It’s like having a high-performance sports car - amazing capabilities, but needs extra maintenance to keep it running right!

Choosing the Right Feed Line #

When choosing your feed line, consider these factors:

1. Length of the run: Longer runs mean more signal loss, so you might need heavier-duty coax.
2. Frequency: Higher frequencies suffer more loss in the cable.
3. Power: If you’re running high power, you’ll need coax that can handle it without overheating.
4. Installation environment: Will it be indoors, outdoors, buried? For outdoor use, ensure UV-resistant cable or proper protection.

Conclusion #

Remember, your feed line is just as important as your antenna. The best antenna in the world won’t help much if your feed line is losing all your signal along the way. It’s like having a sports car with a leaky fuel line - you’re not going to get the performance you’re hoping for.

So next time you’re setting up your station, give some thought to your feed line. It might not be as exciting as choosing a new radio or putting up a new antenna, but it’s the unsung hero that can make or break your signal. Choose wisely, and those radio waves will be zipping from your radio to your antenna and out into the world before you know it!