Section 4.1: Feed Lines and Connectors #

You’ve just worked your first transatlantic contact. The European station gives you a “five by three” report—perfectly readable, but your signal is weak. You’re running 100 watts into what should be a decent antenna. Where did your power go?

As a Technician, you learned that impedance matching matters and that SWR tells you something about your antenna system. Now as a General operator working HF with higher power and longer feed line runs, you need to understand that “antenna system” isn’t just the antenna—it’s the complete chain from your radio through connectors, feed line, and finally to the antenna itself. Each link in this chain affects your signal, and understanding how they work together separates stations that barely get out from those that work the world.

The challenge? Most operators focus exclusively on their antenna while overlooking the feed line and connectors that deliver power to it. You can have the world’s best antenna, but if your feed line wastes half your signal before it arrives, you’re still losing. Let’s start by understanding these critical but often overlooked components. In the next section, we’ll see how everything connects to the antenna itself to form a complete, efficient system.

Understanding Characteristic Impedance #

Every feed line has a characteristic impedance—a fixed property determined by its physical construction:

Key Information: The characteristic impedance of a parallel conductor feed line is determined by the distance between the centers of the conductors and the radius of the conductors.

The physics is beautifully simple:

Spread conductors farther apart? Impedance goes up.
Use fatter wires? Impedance goes down.
What you connect to either end? Doesn’t change the cable’s impedance.

Common impedances you’ll encounter:

50 ohms: The amateur radio standard—your radio expects it, most antennas are designed for it
75 ohms: TV cable—cheaper but wrong for ham gear
450 ohms: Ladder line—wide spacing for low loss

This characteristic impedance is a fundamental property of the feed line itself. When you connect your radio (expecting 50 ohms) to 50-ohm coax and then to an antenna that also presents 50 ohms, power flows smoothly through the entire system. But any mismatch anywhere in this chain—whether at the radio, connectors, feed line, or antenna—creates reflections that waste power. We’ll explore exactly how those antenna mismatches affect your system in the next section, but first you need to understand what determines feed line impedance and how to preserve the signal you’re sending through it.

Key Information: The nominal characteristic impedance of “window line” transmission line is 450 ohms.

Window line (ladder line with rectangular cutouts) achieves remarkable efficiency by using air as its primary dielectric. Those windows aren’t decorative—they remove lossy plastic while maintaining conductor spacing.

The tradeoff? Window line demands respect:

Keep it away from metal objects
Avoid sharp bends
Protect it from ice buildup
Don’t run it parallel to other cables

Coax is the easygoing alternative—its shield contains RF and blocks interference. Window line trades convenience for efficiency—worthwhile when you need every watt to count such as for QRP (weak signal) operation.

Feed Line Loss: Where Your Power Goes #

Here’s a sobering thought: You might be losing more power in your feed line than you’re putting into your antenna. Every foot of cable between your radio and antenna acts like a resistor, converting your carefully generated RF into useless heat.

Key Information: The attenuation of coaxial cable increases with increasing frequency.

Three culprits steal your signal:

Skin effect: At RF, current crowds onto the conductor’s surface. Higher frequency = thinner skin = more resistance.
Dielectric heating: The insulation absorbs energy, especially as frequency climbs.
Radiation: All feed lines leak a little RF. In good coax this is tiny, but damage or poor shielding can make it worse. Open-wire or window line can also radiate more if it’s unbalanced or routed near metal.

Key Information: RF feed line loss is usually expressed in decibels per 100 feet.

Let’s put this in perspective with RG-8X (a popular “compromise” cable):

10 MHz (30 meters): 1.0 dB/100 ft—barely noticeable
50 MHz (6 meters): 2.3 dB/100 ft—starting to hurt
146 MHz (2 meters): 4.5 dB/100 ft—ouch!
440 MHz (70 cm): 8.6 dB/100 ft—yikes!

That 4.5 dB loss on 2 meters? You’re delivering 35 watts to your antenna from a 100-watt radio. The other 65 watts? Warming up your coax. Now you know why that distant station can’t hear you. And remember—this loss affects both transmit AND receive. Your signal weakens going out, and incoming signals weaken coming back in. It’s a double penalty.

Choosing Feed Line for Your Station #

Selecting feed line is like choosing tires for your car—snow tires for winter, all-terrains for off-road, high-performance for the track. Your choice depends on frequency, distance, power level, and installation constraints.

The Distance Factor: Running 10 feet to an attic antenna? Almost any coax will work fine—the losses are minimal. Running 200 feet to that tower? Now feed line choice becomes critical. At HF, even mediocre coax might work for short runs, but those same losses multiply with distance until they dominate your signal budget.

HF Operations (160-10 meters): Both window line and coax have their place. Window line offers extremely low loss—ideal for long runs or when you need maximum efficiency. But it requires careful installation away from metal (including other cables, tower legs, raingutters, etc), needs an antenna tuner, and doesn’t tolerate ice or water well.

Quality coax trades some efficiency for convenience—it’s mostly weatherproof, doesn’t care about nearby metal, and connects directly to your radio. For most HF stations, good coax is the practical choice.

VHF/UHF Operations: Higher frequencies mean higher losses. Coax that works adequately at HF might lose half your power at 2 meters over a long run. At UHF you could see significant losses even with good coax. Short runs? Use what you have. For longer runs? Invest in better quality cable.

Power Considerations: QRP operators can use lighter coax—when you’re running 5 watts, power handling isn’t a concern. But push 1500 watts through undersized coax and you risk damage from heating, especially at impedance mismatch points.

RF Connectors: Moving Beyond Handheld Adapters #

As a Technician, you learned about SMA and BNC connectors on your handheld, maybe even added that BNC adapter everyone’s talking about. Now as a General operator working HF with higher power and demanding applications, connector choice becomes even more critical. The best radio and antenna in the world become expensive decorations if a poorly installed connector blocks your signal. A damaged PL-259, corroded connection, or wrong connector type can waste just as much power as having the wrong antenna.

Quick Review: The Connector Lineup #

You already know the basics—SMA for handhelds, UHF connectors (PL-259/SO-239) for mobile and base gear, BNC for quick connections. As a General operator, you’ll use these same connectors but in more demanding situations where details really matter.

BNC at HF Frequencies #

Key Information: A typical upper frequency limit for low SWR operation of 50-ohm BNC connectors is 4 GHz.

BNC handles 100 watts and works fine through HF frequencies. You’ll see it on test equipment like antenna analyzers and oscilloscopes where the quick twist-lock connection is handy. But HF transceivers and antennas come with UHF connectors, and installing BNC on thick coax is challenging, so BNC remains rare in HF stations despite being technically capable.

Type N: An Underappreciated Option #

Key Information: A type N connector is a moisture-resistant RF connector useful to 10 GHz.

Type N is the connector that does everything right—highly weather-resistant, handles legal limit power, maintains constant impedance, works into microwave frequencies. It’s technically superior to UHF connectors for almost everything we do.

So why don’t we all use Type N? Simple: most amateur radio transceivers come with UHF connectors, so that’s what we use. Switching to Type N means adapters or replacing connectors, which adds hassle and potentially negates some benefits. Still, for permanent outdoor installations or VHF/UHF weak signal work, Type N is worth considering.

SMA: Small but Capable #

Key Information: An SMA connector is a small threaded connector suitable for signals up to several GHz.

Beyond handhelds, you’ll find these tiny threaded connectors on SDR equipment and compact test gear. Their main advantage is size—they pack impressive frequency handling into a connector barely larger than a pencil eraser.

Audio and Control Connections #

Key Information: RCA Phono connectors are commonly used for low frequency or DC signal connections to a transceiver.

Those RCA jacks behind your transceiver handle audio and control signals for digital modes, PTT keying, and external speakers—never RF.

Building Your Complete Antenna System #

Here’s the bottom line: That exotic antenna you built won’t compensate for lossy feed line. That expensive amplifier won’t overcome bad connectors. Your feed line system is where the rubber meets the road—or more accurately, where your RF meets the real world.

Choose your feed line based on physics, not price tags. Install connectors like your QSOs depend on it (they do). Route cables with respect for RF’s quirks. Get these fundamentals right, and you’ve built the foundation for a station that performs.

Getting power to your antenna efficiently is only part of the story. What happens when that power arrives at the antenna? Does your antenna accept it and radiate it effectively, or does it reflect power back down the feed line, creating the standing waves you learned about as a Technician? That’s where impedance matching and SWR come into play—the critical final link in your antenna system that we’ll explore in the next section.