People like to measure things. So, not surprisingly, radio operators have been measuring radio signals since the early days.
The main challenge in measuring radio signals is the enormous range of signal strength. The signals floating past you right now vary in strength by a scale of 1,000,000:1 Consequently, operators need a simple way to visualize relative and absolute strength over this amazing range.
First of all, let’s do a thought experiment. Begin by considering the weakest understandable signal that you can hear on your shortwave radio. Represent the power in that signal with a single light bulb. Now, what would you use to represent the power of the strongest signal you can hear? The surprising answer is: all of the electric power used by a city of 500,000 people! Our ham radio gear has a dynamic range of around 100 dB – a power ratio of a million to one.
Tools for Measuring Radio Signals
Operators have used many types of gadgets measuring radio signals over the years. You can see a few in the picture above.
At the top left is a simple meter which measures micro amperes of current. Tune for the peak and get the strongest part of the signal. Usually, such meters were connected to a detector or IF amplifier circuit. The “magic eye” is shown upper middle. Consumers loved radios with this feature in the 1930’s. Early cathode ray tubes (the forerunner of television) were used to present a circle of green light that closed as a station was properly tuned.
After World War II, most ham and shortwave radios adopted the S-meter, shown upper right. This was sort of calibrated with a scale of 1 to 9. Hams adopted a signal strength scale in 1935 where a barely perceptible signal was given a “1” and an very good signal was given a “9”. This system continues in use today. The novelty with S-meters is their connection to automatic gain control circuits. AGC adjusts receiver amplification to keep the audio output in a small 2:1 range. The AGC feedback voltage is used to drive the meter, where S-9 is calibrated to 50 μV of signal.
Over the past ten years, though, we have shifted from meters to computer displays. The lower part of the picture shows a whole band of strengths for many signals, not just the one tuned. This new method of measuring radio signals is based on the mathematics of software defined radio.
You can read about the history of radio meters if you are interested. In our next article, we will talk about the decibel (dB) and why it is important.