Here’s chapter 3 of the next edition of my No Nonsense Technician Class License Study Guide. As always, comments welcome!
Radio waves are what amateur radio is all about. Amateur radio operators generate them and send them off into space. And, on the other side, we capture them and demodulate them.
Radio waves are also called electromagnetic waves because they consist of both an electric wave and a magnetic wave. The two waves are at right angles to one another, and as the wave travels through space, energy gets swapped between the electric and magnetic waves. This is what propels them through space.
QUESTION: What type of wave carries radio signals between transmitting and receiving stations? (T3A07)
ANSWER: Electromagnetic
QUESTION: A radio wave is made up of what type of energy? (T5C07)
ANSWER: Electromagnetic
QUESTION: What are the two components of a radio wave? (T3B03)
ANSWER: Electric and magnetic fields
One important characteristic of a radio wave is its frequency, or the number of cycles that it goes through per second. As mentioned earlier, the unit of frequency is the Hertz. We abbreviate Hertz as Hz. One Hz is one cycle per second.
Another important characteristic of a radio wave is the speed at which it travels through space. All electromagnetic waves, no matter the frequency, travel at the speed of light, or 300 million meters per second.
QUESTION: How fast does a radio wave travel through free space? (T3B04)
ANSWER: At the speed of light
QUESTION: What is the approximate velocity of a radio wave as it travels through free space? (T3B11)
ANSWER: 300,000,000 meters per second
Another important parameter of a radio wave is its wavelength, or the distance that a radio wave travels during one complete cycle.
QUESTION: What is the name for the distance a radio wave travels during one complete cycle? (T3B01)
ANSWER: Wavelength
Because radio waves travel at the speed of light, no matter what their frequency happens to be, the lower the frequency, the longer the wavelength, and vice versa, the higher the frequency, the shorter the wavelength. This is a fixed relationship, and there’s a formula for this: wavelength in meters equals 300 divided by the frequency in MHz or frequency in MHz equals 300 divided by the wavelength in meters.
QUESTION: What is the formula for converting frequency to approximate wavelength in meters? (T3B06)
ANSWER: Wavelength in meters equals 300 divided by frequency in megahertz
QUESTION: How does the wavelength of a radio wave relate to its frequency? (T3B05)
ANSWER: The wavelength gets shorter as the frequency increases
In amateur radio, we sometimes use the frequency of a signal and sometimes the wavelength when talking about a radio signal. We use wavelength, for example, when we refer to the amateur radio bands.
QUESTION: What property of radio waves is often used to identify the different frequency bands? (T3B07)
ANSWER: The approximate wavelength
The 2 m amateur radio band, for example, spans 144 MHz to 148 MHz. A radio wave with a frequency of 148 MHz, would have a wavelength of 2.03 meters.
For convenience, we split the entire range of radio frequencies into sub-ranges, including high frequency (HF), very high frequency (VHF), and ultra high frequency (UHF).
QUESTION: What frequency range is referred to as HF? (T3B10)
ANSWER: 3 to 30 MHz
QUESTION: What are the frequency limits of the VHF spectrum? (T3B08)
ANSWER: 30 to 300 MHz
QUESTION: What are the frequency limits of the UHF spectrum? (T3B09)
ANSWER: 300 to 3000 MHz
A radio signal of any frequency is called a radio frequency, or RF, signal.
QUESTION: What does the abbreviation “RF” refer to? (T5C06)
ANSWER: Radio frequency signals of all types
Radio wave characteristics: properties of radio waves; propagation modes
As amateur radio operators, we should always try to use the right frequency and the right mode when communicating. To do this, we need to know how radio signals travel from one point to another and what effect frequency, our antennas and even our location have on signal propagation.
Communications at VHF and UHF frequencies are generally “line-of-sight” communications. This means they normally travel in a straight line from the transmitter to the receiver. For this reason, they are normally used for local communications.
QUESTION: Why are direct (not via a repeater) UHF signals rarely heard from stations outside your local coverage area? (T3C01)
ANSWER: UHF signals are usually not reflected by the ionosphere
We’ll talk more about the ionosphere below.
Because VHF and UHF signals are line-of-sight, at some distance, the signals will be blocked by the curvature of the Earth. The maximum distance for line-of-sight communications is called the radio horizon. The radio horizon extends somewhat farther than the visual horizon.
QUESTION: Why do VHF and UHF radio signals usually travel somewhat farther than the visual line of sight distance between two stations? (T3C11)
ANSWER: The Earth seems less curved to radio waves than to light
One problem often encountered when using VHF and UHF frequencies is multi-path distortion. Multi-path distortion occurs when your signals arrive at a receiving station via two or more paths. Since the signal paths may be different lengths, they may arrive out of phase and cancel one another.
QUESTION: What should you do if another operator reports that your station’s 2 meter signals were strong just a moment ago, but now they are weak or distorted? (T3A01)
ANSWER: Try moving a few feet or changing the direction of your antenna, if possible, as reflections may be causing multi-path distortion
Moving a few feet might eliminate or reduce the effects of the random reflections that are causing multi-path distortion.
Multi-path distortion affects both voice and digital transmissions.
QUESTION: What may occur if data signals arrive via multiple paths? (T3A10)
ANSWER: Error rates are likely to increase
Knowing how VHF and UHF signals propagate can help you communicate even in adverse conditions. When trying to use a repeater, for example, you may find yourself in a place where a direct path to the repeater is not possible. If you find yourself in this situation, you could try using a directional antenna and bounce your signal off buildings or other obstructions.
QUESTION: When using a directional antenna, how might your station be able to access a distant repeater if buildings or obstructions are blocking the direct line-of-sight path? (T3A05)
ANSWER: Try to find a path that reflects signals to the repeater
Another phenomenon you might use when a direct path to a repeater is not possible is “knife-edge” diffraction. You might be able to use this phenomenon to get your signal around a building in an urban setting.
QUESTION: Which of the following effects might cause radio signals to be heard despite obstructions between the transmitting and receiving stations? (T3C05)
ANSWER: Knife-edge diffraction
Mobile operation has its own unique challenges as your transmitter location is constantly changing. This means that the signal at the receiving station constantly changes as well.
QUESTION: What term is commonly used to describe the rapid fluttering sound sometimes heard from mobile stations that are moving while transmitting? (T3A06)
ANSWER: Picket fencing
Another condition that could impede the transmission of VHF and UHF signals is vegetation. So, keep your antennas out of trees or above trees.
QUESTION: Why might the range of VHF and UHF signals be greater in the winter? (T3A02)
ANSWER: Less absorption by vegetation
Antenna polarization is also important at VHF and UHF frequencies.
QUESTION: What property of a radio wave is used to describe its polarization? (T3B02)
ANSWER: The orientation of the electric field
QUESTION: What can happen if the antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization? (T3A04)
ANSWER: Signals could be significantly weaker
When using a repeater, vertical polarization is most often used. So, when using a handheld transceiver, make sure to hold it so that your antenna is vertically oriented.
Different activities use different antenna polarizations.
QUESTION: What antenna polarization is normally used for long-distance weak-signal CW and SSB contacts using the VHF and UHF bands? (T3A03)
ANSWER: Horizontal
The reason for this is that weak signal operators are often using what are called beam antennas, and it’s much easier to mount and operate beam antennas horizontally than it is to mount them vertically.
Even though VHF communications are most often line-of-sight, there are times when it’s possible to communicate over long distances. Sometimes, VHF signals will bounce off the E layer of the ionosphere. This phenomenon is called “sporadic E” because it happens only sporadically.
QUESTION: Which of the following propagation types is most commonly associated with occasional strong over-the-horizon signals on the 10, 6, and 2 meter bands? (T3C04)
ANSWER: Sporadic E
Other interesting propagation phenomena at VHF frequencies include auroral reflection, meteor scatter, tropospheric scatter, and tropospheric ducting. Bouncing signals off the earth’s aurora is very interesting.
QUESTION: What is a characteristic of VHF signals received via auroral reflection? (T3C03)
ANSWER: The signals exhibit rapid fluctuations of strength and often sound distorted
Some hams also bounce signals off meteor showers. This propagation mode is called meteor scatter.
QUESTION: What band is best suited to communicating via meteor scatter? (T3C07)
ANSWER: 6 meter band
One question that I get from people not knowledgeable about amateur radio is whether or not the weather affects radio wave propagation. The short answer is no. The exception to the rule is tropospheric ducting or tropospheric scatter. The troposphere is the lowest region of the atmosphere, extending from the earth’s surface to a height of about 6–10 km.
QUESTION: What mode is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis? (T3C06)
ANSWER: Tropospheric scatter
QUESTION: What causes tropospheric ducting? (T3C08)
ANSWER: Temperature inversions in the atmosphere
Tropospheric ducting can also propagate VHF signals for many hundreds of miles.
Another exception to the rule occurs at microwave frequencies. Precipitation, including rain, snow, or ice can absorb microwave signals, especially at frequencies above 11 GHz. This phenomenon is called rain fade.
QUESTION: What weather condition would decrease range at microwave frequencies? (T3A13)
ANSWER: Precipitation
At lower frequencies, precipitation has little or no effect.
QUESTION: How might fog and light rain affect radio range on the 10 meter and 6 meter bands? (T3A12)
ANSWER: Fog and light rain will have little effect on these bands
HF Propagation
For more reliable long-distance communications, amateurs use the HF frequencies. The reason for this is that HF signals bounce off the ionosphere. This phenomenon allows amateur radio operators to contact other amateur radio stations around the world.
The ionosphere is created by solar radiation, which creates a high concentration of ions and free electrons that reflect radio waves. It extends from about 50 to 600 miles above the earth’s surface. There are three ionospheric layers—the D, E, and F layers—with the D layer being closest to the Earth, and the F layer being the layer farthest from the surface of the Earth.
QUESTION: Which part of the atmosphere enables the propagation of radio signals around the world? (T3A11)
ANSWER: The ionosphere
QUESTION: Which of the following is an advantage of HF vs VHF and higher frequencies? (T3C02)
ANSWER: Long distance ionospheric propagation is far more common on HF
One interesting phenomenon that is related to HF propagation is the sunspot cycle. Generally, the number of sunspots increases and decreases over an 11-year cycle, and HF propagation, especially on the higher frequency HF bands, is best at times when there are many sunspots.
QUESTION: Which of the following bands may provide long distance communications during the peak of the sunspot cycle? (T3C10)
ANSWER: 6 or 10 meter bands
Because of the way that the ionosphere changes throughout the day, propagation is best on the higher frequency bands (10m, 15m and 20m) during the day, while propagation is best on the lower frequency bands (160m, 80m, 40m) at night.
QUESTION: What is generally the best time for long-distance 10 meter band propagation via the F layer? (T3C09)
ANSWER: From dawn to shortly after sunset during periods of high sunspot activity
A common phenomenon of HF signal propagation is fading.
QUESTION: Which of the following is a likely cause of irregular fading of signals received by ionospheric reflection? (T3A08)
ANSWER: Random combining of signals arriving via different paths
This is similar to multi-path distortion of VHF and UHF signals, but in this case, the signals are bouncing off the ionosphere, and because the ionosphere is constantly changing, signals fade in and out.
Antenna polarization is not as important when operating on the HF bands as it is when operating on the VHF/UHF bands.. This is because signals “skip” off the ionosphere and become neither horizontally polarized, nor vertically polarized, but elliptically polarized.
QUESTION: Which of the following results from the fact that skip signals refracted from the ionosphere are elliptically polarized? (T3A09)
ANSWER: Either vertically or horizontally polarized antennas may be used for transmission or reception
Dave New, N8SBE says
frequency in MHz 300 divided by the wavelength in meters.
frequency in MHz equals 300 divided by the wavelength in meters.
(word ‘equals’ left out).
Dan KB6NU says
Yipes! I even paid a freelance copyeditor to find typos like that!