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) found in many radio textbooks. Instead, it aims to explore practical issues from a more intuitive, real-world perspective.

VSWR is one of the most commonly used parameters in RF technology to assess how well components are matched. When amateur radio operators communicate, the first thing they often think about is measuring whether the VSWR of their antenna system is close to 1:1. If it is, that’s considered good. But I often hear questions like: What if you can’t reach 1:1? If the standing wave ratio is just a few, is the antenna acceptable? Why didn’t old military radios like the 81-sized ones have a standing wave meter?

VSWR and Nominal Impedance

The condition for a transmitter and an antenna to be matched is that their impedance resistive components are equal, and any reactive parts cancel each other out. In the vacuum tube era, the output impedance of tubes was high, and low-impedance coaxial cables weren’t common. Instead, high-impedance parallel feeders were widely used, typically around several hundred ohms. So transmitters were designed with similar output impedances. Today, modern solid-state radios usually have a nominal impedance of 50 ohms, which is why VSWR meters are calibrated for that value.

If you have an older radio with a 600-ohm output, there's no need to use a 50-ohm VSWR meter to adjust your antenna. Just focus on maximizing the antenna current. The VSWR value alone doesn’t tell the whole story.

When VSWR is not exactly 1, comparing values becomes less meaningful. Most VSWR meters aren’t as precisely calibrated as voltmeters or ohmmeters, and many don’t even specify their error margins. The accuracy of these meters can vary significantly due to phase frequency characteristics of RF coupling components and diode nonlinearities at different frequencies and power levels.

VSWR = 1 Doesn’t Mean a Good Antenna

The most important factor affecting an antenna’s performance is resonance. Let’s take the example of string instruments—whether it’s a violin or guzheng, each string has a natural frequency based on its length and tension. When it vibrates at this frequency, the ends are fixed, and the middle swings the most. This is similar to a half-wave dipole antenna, where the voltage is highest at the ends and current is highest in the middle.

To get the strongest sound, the string must vibrate at its natural frequency, and the driving point must match the string’s impedance. Similarly, for an antenna, the signal frequency must match the antenna’s resonant frequency, and the driving point must be properly placed. Even if the VSWR isn’t perfect, a well-resonated antenna will perform better than one with a lower VSWR but poor resonance.

In early transmitters, like the Type 71 radios, resonance was achieved using series inductors and capacitors, and impedance matching was done through coil coupling. Exact impedance matching wasn’t always necessary, but proper resonance ensured good performance.

Antenna VSWR vs. System VSWR

VSWR should be measured at the antenna’s feed point, but this is often difficult because it’s high up. Usually, we measure at the cable’s lower end, giving us the VSWR of the entire system, including the cable. If the antenna is 50 ohms and the cable is also 50 ohms, the reading is accurate. However, if the antenna isn’t 50 ohms, the cable length and loss will affect the measurement. At UHF frequencies, cable impact can’t be ignored.

Asymmetric Antennas

A dipole antenna requires both arms to be 1/4 wavelength. If they’re unequal, does that create two resonant points? No. The total length still determines the resonant frequency. Even with asymmetry, as long as the total electrical length is correct, the antenna will resonate at one frequency. The VSWR might change slightly, but the radiation efficiency remains largely unaffected.

QRPer and VSWR

High VSWR can cause issues, especially when the antenna isn’t resonant. It may lead to overvoltage on the final stage of the transmitter. In the past, this could damage components, so VSWR was kept below 3. Modern equipment has better protection, reducing the risk, but it’s still wise to keep VSWR low when possible.

For QRP operators, with low power, VSWR is less of a concern. I once used a short curtain wire as an antenna with VSWR=∞, yet still made contacts. Later, after adjusting, VSWR improved, but signal strength didn’t change much. It turned out that losses in the sky had a bigger impact than VSWR.

In summary, VSWR is a complex topic. As an amateur radio operator, you’ll always deal with it. Observe, experiment, and share your experiences. A VSWR of 1 means the antenna system matches the transmitter, but it doesn’t guarantee good radiation. An antenna may have VSWR=1 but still perform poorly if it’s too small or inefficient. So, don’t rely solely on VSWR numbers—focus on resonance and overall performance.