Radio Frequency (RF) generally refers to electrical signals at frequencies between 3 kHz and 300 GHz. Within the electromagnetic spectrum, this comprises a broad range of oscillating energy, ranging from just little higher in frequency than the audible band up to where the infrared heat band begins.
It takes more than Ohm’s law to predict how RF ought to behave. One reason is that when RF voltage is applied to an electrical conductor, that body can behave as an antenna, a proportion of the energy radiating into the surrounding space with a corresponding reduction in current through the conductor. This is why an antenna exhibits impedance. The antenna’s impedance must match the characteristic impedance of the connected transmission line to prevent reflections that would adversely impact system operation.
Another way in which RF current flowing through a conductor differs from dc or lower frequency ac is that it exhibits the aptly-named skin effect. The current does not penetrate uniformly throughout the conductor; a large percentage of it flows along the surface. This odd phenomenon has a simple explanation. Due to the higher frequency, the greater inductive reactance at and near the center of the conductor creates a stronger opposing electric field and consequent counter-electromotive force, known as back EMF. Thus there is an impedance gradient, the greatest amount at the center of the conductor. Because electrical current always seeks the path of least impedance, a larger proportion of the current flows at the outer surface of a cylindrical conductor.
The skin effect is costly in terms of valuable copper and aluminum resources, so methods have been developed to minimize its effect. It is possible to convey high-frequency electromagnetic energy through a waveguide with essentially no loss. This energy is also in the form of electrical current and can conduct through hollow metal tubing in a way that gives less loss with greater surface area.
Similarly, high-voltage, long-distance transmission lines often make use of bundled conductors, three per phase, to realize greater surface area and thereby diminish the skin effect. The skin effect of high-frequency electrical current also accounts for the fact that a human can receive external radiation burns when exposed to RF radiation. This is because the body acts as a receiving antenna and the resulting electrical current is concentrated at the surface. This type of injury sometimes happens when a technician works on an internet satellite dish that goes into the transmit mode. There is no corresponding hazard regarding a TV satellite dish because it does not transmit.
Unlike dc or low-frequency electrical energy, RF current touching the human body produces surface burns but not the painful effect of electric shock. This is because the rapidly changing direction of the current flow is not capable of depolarizing nerve endings.
Besides exhibiting skin effect and having different biological effects, RF differs from 50- and 60-Hz utility current in that it can more easily ionize air. The rapidly rising current in a bolt of lightning resembles the rising edge of an RF waveform. That behavior accounts in part for its ability to create and briefly maintain an ionized pathway across a large air gap.
Another atypical property of RF energy is that it appears to have the ability to flow through an insulator. That is because insulating material actually can function as the dielectric layer of a capacitor. It constitutes a low capacitive reactance to high-frequency current.
RF as a form of electrical current does not do well with conventional wire conductors. Due to termination discontinuities, the RF current will reflect back toward the source rather than transmit to the load, setting up standing waves so power is not transferred. Transmission lines capable of carrying RF current must have a uniform characteristic impedance that matches source and load. Also, the terminations must be designed and installed so as to be free of discontinuities.
An important application for RF is in wireless communication. Long and short pulses of electromagnetic radiation can be used to convey information using Morse Code, and audio and video information can carry over wire or through a vacuum by modulating the amplitude, frequency or phase of an RF signal, which then becomes known as a carrier wave.
Amplitude modulation is suitable for voice transmission, where excellent fidelity (faithfulness to the original) is not an issue. For music, frequency modulation is better. Amplitude modulation consists of varying the amplitude of the carrier prior to transmission in proportion to the audio or video signal to be conveyed.
Within the transmitter various types of modulation circuits are used depending upon whether high-level (post-amplification) or low-level (pre-amplification) modulation is desired.
The first attempt at carrier wave amplitude modulation consisted of placing a huge water-cooled microphone in series with the antenna. This arrangement worked, but just barely, with poor audio quality. This was a consequence of the nature of the carrier wave, which was essentially a broad-band noise signal created by a spark transmitter. High-quality communication demands a carrier consisting of a sine wave with no energy outside the fundamental. This became possible with the development of the Alexanderson alternator, a continuous sine-wave generator. The first viable AM public entertainment broadcast was on Christmas Eve, 1906.
There are several oscilloscope applications in the area of judging modulated signal qualities. For example, scopes can look at a single-sideband (SSB) RF output for gauging factors such as microphone gain and compression. The ARRL (American Radio Relay League) Handbook lists several ways to use a scope for adjusting SSB transmitters via a two-tone method. One is to sample the output and apply the sample signal to the vertical plates of the scope. Another uses an RF probe and a detector to apply a signal to the scope vertical deflection amplifier input. The antenna on the SSB transmitter is replaced with a dummy load for these tests. For reasons, outlined above, the dummy load must consist of nonreactive resistors.
The point of the two-tone method that ARRL outlines is to confirm the transmitter is putting out a clean signal before adjusting the microphone gain.
Most modern scopes have bandwidths high enough to directly display the HF band transmissions of amateur SSB transmitters. The usual procedure is to make a T sampling network that goes in line with the antenna transmission line. One side of the coaxial T goes to the transmitter output port; another side of the T goes to the antenna feedline; the third port of the T can go to a voltage divider consisting of two carbon resistors. The divider cuts the RF voltage down to some usable level for the scope. (The divider might be unnecessary if 10x scope probes rated to 1 kV or so are available.)
For example, a common divider employs a 1k Ω resistor in series with a 10k Ω resistor (to ground). The scope monitors the RF voltage across the 1k Ω resistor. The setup measures one-tenth of the RF voltage as applied to the antenna. When using a 50-Ω cable feeding a matched 50-Ω antenna, the voltage will never be higher than 21 Vrms (30 Vpk), safe for any scope.
A scope having triggered sweep can likely lock onto the SSB modulation and provide a fairly jitter-free envelope. Alternatively, it’s possible to view the modulation envelope by manually adjust the sweep frequency as someone speaks into the microphone. The transmitter modulation, of course, will constantly change with the voice modulating frequency.
Where quality (noise-free with spectrally uniform rendition) audio and video are a concern, frequency modulation (FM) is superior to AM, although its range is less. AM and FM modulation work in similar ways. The goal is the same – to convey audio or video information by means of modifying a carrier wave. The difference is that in FM, frequency rather than amplitude is altered.
As opposed to amplitude modulation, where the audio or other signal affects carrier amplitude, frequency modulation involves subtler concepts. Frequency modulation may be direct or indirect. In direct FM, the signal is applied to the input of a voltage-controlled oscillator (VCO).
In indirect modulations, the desired signal generates a phase-modulated voltage. First, the output of a crystal-controlled oscillator is conveyed to a frequency multiplier. The end product is the frequency-modulated carrier wave, which after further amplification, is applied the transmitter antenna.
FM demodulation is accomplished through various means. One common circuit is a Foster-Seeley discriminator. This was invented in 1936 by Dudley Foster and William Seeley, originally to serve as an automatic frequency controller in receivers. It quickly found much wider application in frequency modulation. Simply, a tuned RF transformer converts frequency to amplitude variations, which is the definition of frequency demodulation.
Another circuit that is capable of frequency demodulation is the phase-locked loop. In this process, a signal is generated, the phase of which is dependent upon the phase of the input signal. Thus, there is a variable frequency oscillator and a phase detector.
Besides AM and FM, there is phase modulation. This is much used in digital transmission including WiFi and satellite TV. The way it works is that the phase angle in a carrier wave is altered to reflect changes in audio or video amplitude. Phase modulation and frequency modulation are closely related because frequency in actuality is the rate of change of phase. Both are subsets of what is generally known as angle modulation.
When the phase of a carrier wave is modulated, it is altered with respect to its state if there were no modulation. It lends itself to high-low schemes. For this reason, it forms the basis for a large proportion of the newer digital transmission networks that are used worldwide.