Detailed analysis based on practical problems of antenna standing wave ratio

This article isn't about rehashing the standard theory of voltage standing wave ratio (VSWR) that you can find in countless radio engineering textbooks. Instead, it's focused on practical issues and real-world experiences from a hands-on perspective.

VSWR is one of the most commonly used parameters in RF technology to assess how well components are matched. When amateur radio operators talk about their antennas, they often start by checking if the VSWR of the antenna system is close to 1:1. If it is, everything is good. But I often hear questions like, "What if I can't get it down to 1:1? What if the VSWR is just a few—does that mean the antenna is acceptable?" And then there's the question: Why didn’t old military radios like the 81-sized ones have a standing wave meter?

VSWR and Nominal Impedance

For a transmitter and an antenna to be properly matched, their resistive components must match, and any reactive parts should cancel out. In the vacuum tube era, transmitters had high output impedance, and coaxial cables with low impedance weren’t widely used yet. Instead, parallel feeders with hundreds of ohms were common. That’s why older equipment was designed for higher impedance levels. Modern solid-state radios typically use 50 ohms as the standard, which is why most VSWR meters are calibrated for that value.

If you're using an old radio with a 600-ohm output, there's no need to worry about a 50-ohm VSWR meter. Just adjust your antenna to maximize current flow instead.

When VSWR Isn’t 1, It Doesn’t Mean Much

The reason VSWR values other than 1 aren't always precise is because most VSWR meters aren't as accurately calibrated as voltmeters or ohmmeters. Many don't even specify error margins. The inaccuracies come from the phase frequency characteristics of RF coupling components and the non-linear behavior of diodes. As a result, the accuracy of VSWR measurements can vary significantly depending on frequency and power level.

VSWR = 1 ≠ Good Antenna

One of the most important factors affecting an antenna’s performance is resonance. Think of a string instrument: whether it's a violin or a guzheng, each string has a natural frequency based on its length and tension. When it vibrates at that frequency, the ends are fixed, and the middle swings the most. This is similar to a half-wave dipole antenna, where current is minimal at the ends and maximal in the center.

To produce the strongest sound, the string must vibrate at its natural frequency, and the driving point must be positioned correctly. Similarly, for an antenna, the operating frequency must match its resonant frequency, and the feed point must be chosen appropriately. If the antenna isn't resonant, even with a perfect VSWR, the signal will be weak.

Resonance Is Key

In early transmitters, such as the type 71 radios, resonance was achieved through series inductors and capacitors. Impedance matching wasn't always exact, but as long as resonance was sufficient, the system worked well. So, when VSWR isn't perfectly 1, the main goal should be ensuring the antenna resonates with the operating frequency.

Antenna VSWR vs. System VSWR

VSWR should be measured at the antenna feed point, but this is often impractical. Instead, we measure at the lower end of the cable. However, this measures the entire system, including the cable. If the antenna is 50 ohms and the cable is also 50 ohms, the reading is accurate. But if the antenna isn't 50 ohms, the cable length and loss can distort the measurement. Especially in UHF bands, cable effects shouldn’t be ignored.

Asymmetric Antennas

A dipole antenna usually has two equal arms, each a quarter wavelength. But what happens if they’re not equal? The resonant frequency is still determined by the total length. Even if one arm is longer and the other shorter, the antenna will still resonate at one frequency, though the impedance might be higher. This is similar to plucking a string at an odd spot—it still vibrates, but not as efficiently.

QRPer Don’t Need to Worry Too Much About VSWR

High VSWR can cause issues if the antenna isn’t resonant, leading to large reactance. In the past, this could damage transistors. But modern equipment often includes protection mechanisms that reduce power when VSWR is too high. For QRP users, even a high VSWR may not be a problem due to low power. Some experiments show that even with VSWR=∞, signals can still be received, proving that VSWR alone doesn't determine performance.

Conclusion

VSWR is an important parameter, but it’s not the only one. Whether the energy gets radiated effectively depends on many factors. A 50-ohm resistor can have a perfect VSWR but no radiation. On the flip side, a mismatched antenna can still work well if it's resonant. So, while VSWR matters, it's just one piece of the puzzle. As an amateur radio operator, you’ll always deal with it. Observe, experiment, and share your experience. That’s the real spirit of ham radio.