Since the late 1800s, when Nikola Tesla first demonstrated wireless transmission, there has been a need to measure the output of RF circuits. A major focus of Telsa’s work was wireless transmission of electrical power; so he was often working in the megawatt range, and a relative indication of power was the discharge length of the “RF lightning” he produced. For obvious reasons, there was little incentive to attempt any sort of “contact” measurement!

Around 1888, an Austrian physicist named Ernst Lecher developed his “wires” technique as a method for measuring the frequency of an RF or microwave oscillator. The apparatus, often known as Lecher Wires, consisted of two parallel rods or wires, held a constant distance apart, with a sliding short circuit between them. The wires formed an RF transmission line, and by moving the shorting bar, Lecher could create standing waves in the line, resulting in a series of the peaks and nulls. By measuring the physical distance between two peaks or two nulls, the signal’s wavelength in the transmission line, and thus its frequency could be calculated.

Initially, Lecher used a simple incandescent light bulb across the lines as power detector to locate the peaks and nulls. The apparent brightness of the bulb at the peaks also gave him a rough indication of the oscillator’s output amplitude. One of the problems with using a bulb, however, was that the low (and variable) impedance of its filament changed the line’s characteristics, and could affect the resonant frequency and output amplitude of the oscillator.

This was addressed by substituting a high-impedance, gas-discharge glow tube for the incandescent bulb. The glass tube was laid directly across the wires, and the field from a medium-voltage RF signal was adequate to excite a glow discharge in the gas tube. This didn’t change the tank impedance as much, while keeping it easy to visually determine the peak and null locations as the tube was slid up and down the wires. Later, a neon bulb was used, but the higher striking voltage of neon made the nulls difficult to locate precisely.

Figure 1.3.1 Historical photo of Tesla lightning

In 1933, H.V. Noble, a Westinghouse engineer, refined some of Tesla’s research, and was able to transmit several hundred watts at 100 MHz a distance of ten meters or so. This wireless RF power transmission was demonstrated at the Chicago World’s Fair at the Westinghouse exhibit. His frequencies were low enough that the transmitted and received signal voltages could be directly measured by conventional electronic devices of the day – vacuum tube and cat’s whisker detectors. At higher frequencies, however, these simple methods did not work as well – the tubes and cat’s whiskers of the day simply lost rectification efficiency and repeatability.

Figure 1.3.2 Earnst Lecher’s apparatus for measuring RF amplitude

The Varian brothers used another indicator technique in the late 1930s during their development of the Klystron. They drilled a small hole in the side of the resonant cavity and put a fluorescent screen next to it. A glow would indicate that the device was oscillating, and the brightness gave a very rough power indication as adjustments were made. In fact some small transmitters manufactured into the 1960s had a small incandescent or neon lamp in the final tank circuit for tuning. The tank was tuned for maximum lamp brightness. These techniques all fall more under the category of RF indicators than actual measurement instruments.

The water-flow calorimeter, a common device for other uses, was adapted for higher power RF measurements to measure the heating effect of RF energy, and found its way into use anywhere you could install a “dummy load.” By monitoring flow of water and temperature rise as it cooled the load, it was simple to measure long-term average power dissipated by the load.

The thermocouple is one of the oldest ways of directly measuring low RF power levels. This is done by measuring its heating effect upon a load, and is still in common use today for the measurement of “true-RMS” power. Thermocouple RF ammeters have been in use since before 1930 but were restricted to the lower frequencies. It was not until the 1970s that thermocouples were developed that allowed their use as sensors in the VHF and Microwave range.

In later years, thermocouples and semiconductor diodes improved both in sensitivity and high-frequency ability. By the mid 1940s, the fragile, galena-based “cat-whisker” detectors were being replaced by stable, durable packaged diodes that could be calibrated against known standards, and used for more general-purpose RF power measurement.

Diode-based power measurement was further improved in the 50s and 60s. By 1958, units could measure from below one millivolt to several volts. With a suitable termination, this yielded a calibrated dynamic range of about -50 dBm to +22 dBm over a frequency range of 200 kHz to 500 MHz.

RF voltmeters and power meters continued to evolve throughout the 70s with the application of digital and microprocessor technology, but these were all “average-only” instruments and few had any ability to quantify peak measurements. When a pulsed signal had to be characterized, the accepted technique was to use an oscilloscope and crystal detector to view the waveform in a qualitative fashion, and perform an average power measurement on the composite signal using either a CW power meter or a higher power measurement such as a calorimeter.

Figure 1.3.3 Early RF Voltmeter

The “slideback wattmeter” used a diode detector, and substituted a DC voltage for the RF pulse while the pulse was off, giving a way to measure the pulse’s amplitude while compensating for duty cycle. However, a more common approach was to simply characterize a diode detector to correct for its pulse response – a technique pioneered by Boonton Radio, a company that provided a great deal of technology to the power measurement industry.

The modern realization of the peak power meter came into being in the early 1990s. Companies like Hewlett Packard and Wavetek introduced instruments that were specifically designed to measure pulsed or modulated signals, and correct for non-linear response of the detector diodes in real time. These instruments have evolved over time with the application of better detectors and high-speed digital signal processing technology.