HF and VHF Propagation Fundamentals
Ground Wave Propagation: involves the transmission of a radio signal along or near the surface of the Earth. The groundwave signal is divided into three parts: direct wave, reflected wave, and surface wave.
- Direct Wave - travels through the atmosphere from one antenna to the other via a line-of-sight (LOS) mode. Maximum LOS distance is dependent on the height of an antenna above the ground; the higher the antenna, the further the LOS distance. Because the radio signal travels in air, any obstructions between two antennas can block or reduce the signal and prevent communications.
- Reflected Wave - like the direct wave travels through the atmosphere but reflects off the Earth in going from transmitting antenna to a receiving antenna. Together, the reflected wave and the direct wave are called the space wave.
- Surface Wave - the third part of a groundwave, the surface wave travels along the surface of the Earth and is the usual means of groundwave communications. The surface wave is very dependent on the type of surface terrain between two antennas. With a good conducting surface, such as sea water, long groundwave distances are possible. If there is poor surface between the antennas, such as sand or frozen ground, the distance expected for the surface wave is small. The surface wave distance can also be reduced by heavy vegetation or mountainous terrain.
Skywave Propagation: Beyond the distance covered by the groundwave signal, High Frequency (HF) communications are possible through sky wave propagation. Skywave propagation is possible because of the bending (refracting) of the radio signal by a region of the atmosphere called the ionosphere. The ionosphere is electrically charged ( ionized) region of the atmosphere that extends from about 37-620 miles above the Earth's surface. The ionization results from energy from the sun and can cause radio signals to return to Earth. Although the ionosphere exists up to 620 miles, the area for HF communication is below about 310 feet (500 km). This region is divided into four regions: D, E, F1, and F2.
- D Region - This is the closest region to the Earth and only exists during daylight hours. It does not have the capability to bend an HF radio signal back to Earth, but it does play an important role in HF communications. The D region absorbs energy from the radio signal passing through it, thereby, reducing the strength of received signals.
- E Region - the E region is present 24 hours a day, although during night hours it is much weaker than during the day. The E region is the first region with enough charge to bend HF radio signals. At times, parts of the E region become highly charged and can either help or block out HF communications. These highly charged areas are called Sporadic E and occur most often during the summer.
- F1/F2 - The most important regions for HF communications are the F1 and F2 regions. The majority of HF Skywave communications depend on these regions with the F2 region being used the most for long-range daytime communications. During the night, these regions combine to form a single F region.
The bending of a radio signal by the ionosphere depends on the frequency of the radio signal, the degree of ionization in the ionosphere, and the angle at which the radio signal strikes the ionosphere. At a vertical (straight up) angle, the highest frequency that will be bent back to Earth is called the critical frequency. Each region of the ionosphere (E, F1, F2, F) will have a separate critical frequency. For a vertical angle, signals above the highest critical frequency will pass through all ionospheric regions and on into outer space. Frequencies below the critical frequency of a region will be bent back to the Earth by that region; however, if the frequency is too low, the signal will be absorbed by the D region. In order to have HF Skywave communications, a radio signal must be a high enough frequency to pass through the D region but not too high a frequency so that it does not pass through the reflecting region.
The angle at which a radio signal strikes the ionosphere plays an important part in Skywave communications. As stated earlier, any frequency above the critical frequency launched at a vertical angle will pass through the reflecting region. If the radio signal having a frequency above the critical frequency was launched at a lower angle, instead of straight up, the signal could be bent back to Earth instead of passing through the region. This can be compared to skipping stones across a pond. If the stone was thrown straight down at the water, it would penetrate the water's surface. But if the angle at which the stone is thrown is lowered, an angle will be reached where, instead of going into the water, the stone will skip across the pond. For every link, there is an optimum angle above the horizon, called take-off angle, that will produce the strongest signal at the receiving station. This optimum take-off angle is used to select the appropriate antenna for a specific link.
Although a radio signal is actually bent (refracted) by the ionosphere, the term reflection is commonly used to describe the turning back of a radio signal by the ionosphere. Reflection will be used in this handbook, even though refraction is what actually occurs.
Because many antennas radiate energy at several angles, more than one wave from the transmitter may reach the receiver. Depending on the antennas used, signals can be received from any or all of the different paths. Because each path covers a different distance, the signals arrive at the receiver at different times. When two or more signals arrive at the receiver from different paths, they can interfere with each other and cause what is called multi path interference. This type of interference will produce echoes or "motor boating" on circuits even through a receiver's S-meter shows a strong received signal.
Depending on the frequency, antennas, and other factors, an area may exist between the longest groundwave distance and shortest Skywave distance where no signal exists. This is called the skip zone.
Very High Frequency Communications (30 to 88 MHz)
VHF communications are possible through what is called VHF line-of-sight (VHF-LOS) propagation. VHF-LOS propagation is influenced by four separate components that results in the received signal: the direct ray, the reflected ray, and the diffracted ray.
Direct Ray - travels the straight line distance from the transmitting antenna to the receiving antenna. Because of the curvature of the Earth, the maximum distance between two antennas for a direct ray is determined by the height of the antenna above the Earth. The higher the antennas, the longer the effective distance.
Reflected Ray - like the direct ray, travels through the atmosphere but reflects off the Earth's surface in going from one antenna to the other. The reflected ray may cause a troublesome type of interference. The path traveled by the reflected ray is longer than that of the direct ray; therefore, the reflected ray arrives at the receiving antenna after the direct ray. If the two rays are "in phase", they will reinforce each other producing a stronger signal. If they arrive "out of phase", one signal will cancel the other resulting in poor, or loss of communications. It is a canceling effect that explains why, at times, no signal is received even though the transmitting antenna is within LOS. Under this condition, moving the antennas either closer or further from each other, or changing the height of one of the antennas should result in a usable signal.
Refracted Ray - causes the radio LOS distance to be greater than the visual light-of-sight. The differences in the electrical characteristics of the lower atmosphere as a function of height cause the transmitted signal to bend slightly back to Earth. This bending permits the refracted ray to travel further than the direct ray.
Diffracted Ray - scatters around obstacles and permits communications in the shadow region behind obstacles. Low frequencies scatter (diffract) more than higher frequencies, so it is not uncommon for a lower-frequency signal to diffract across a hill top and result in reliable communications at a receiver antenna located not far below radio LOS, while at the same time a signal of higher frequency will not be heard.
Antenna Fundamentals
Overview - to be able to properly select antennas for a radio link, certain antenna concepts need to be understood.
Wavelength/Frequency - in radio frequency (RF) communications, there is a definite relationship between antenna length and the wavelength corresponding to the transmitter frequency. This relationship is important when constructing antennas for a specific frequency or range of frequencies. The wavelength of a radio signal is the distance traveled in the time it takes to complete one cycle.
Wavelength is usually represented by the Greek letter lambda (λ). All radio signals travel at the speed of light. The wavelength of a frequency is equal to the speed of light (300,000,000 meters/second) divided by the frequency in Hertz. To find the wavelength of 3 MHz:
Wavelength (λ) = 300,000,000 m/s / 3,000,000 Hz = 100 meters or 328 feet
This means that in the time it takes to complete one cycle at 3 MHz, the signal travels 100 meters or 328 feet. This is the distance the signal will travel through air; the distance in a wire is slightly less and will be discussed later.
Resonance - Antennas can be classified as either resonant or non- resonant depending on their design. In a resonant antenna, almost all of the radio signal fed to the antenna is radiated. If the antenna is fed with a frequency other than the one for which it is resonant, much of the fed signal will be lost and will not be radiated. Such resonant antennas are referred to as "narrowband". A resonant antenna will effectively radiate a radio signal for frequencies close to its design frequency, usually only 2% above or below the design frequency. In practice, this means that if a resonant antenna is used for a radio link, a separate antenna must be built for each frequency to be used for the radio circuit. A non-resonant antenna will effectively radiate a broad range of frequencies with a lower efficiency. Non-resonant antennas are frequently called "broadband". Both resonant and non-resonant antennas are commonly used on tactical links. If a resonant antenna is fed with a frequency outside of its bandwidth, losses of radiated power occur. Signal energy from the antenna feed line is "turned back" from the antenna and causes standing waves on the feed line. A measure of these standing waves, called standing wave ratio (SWR), is used to determine if an antenna is resonant at a particular frequency. A SWR of 1 to 1 (1:1) is the ideal situation but in the real world 1.1 to 1 is about the best that can be done. When constructing wire antennas, the length of the antenna should be adjusted until the lowest SWR is measured. Suppose the situation exists where the only antenna that can be erected is one with a large SWR, one that is too large for the transmitter to work. In this situation, a coupler or "antenna tuner" must be used. A coupler is a device that is inserted between a transmitter and it's antenna to make a transmitter think that it is connected to a low SWR antenna.
Polarization - is the relationship of the electric field of the radio energy radiated by an antenna to the Earth. The most common polarizations are horizontal (parallel to the Earth's surface) and vertical (perpendicular to the Earth's surface); however, others such as circular and elliptical are also used. A vertical antenna normally radiates a vertically polarized signal and a horizontal antenna normally radiates a horizontal signal. In HF groundwave and VHF-LOS propagation, both the transmit and receive antennas should have the same polarization for best communication. In the case of HF groundwave propagation, vertical polarization should be used. Either vertical or horizontal polarization can be used in VHF-LOS. For HF Skywave propagation, the polarization of the transmitting and receiving antennas does not have to be the same because of the random changing of the signal as it is bent by the ionosphere. This random changing allows the use of either vertical or horizontal polarization at the transmitting or receiving antenna.
Reflections - a quarterwave vertical antenna requires a good ground connection in order to be resonant. When a quarterwave vertical antenna has its base on the ground, the earth below the antenna acts like a large reflector and supplies another quarter wavelength as an image antenna due to reflection. In effect, the quarterwave vertical antenna acts like a half wave antenna. If the electrical characteristics below the antenna are poor, there will be large losses in the ground resulting in poor radiation by the antenna. It is important to remember that a quarterwave vertical antenna needs a good ground below it to work properly. Ground screens and ground planes are used with vertical antennas to improve their efficiency. Efficiency of an antenna is a measure of how well an antenna radiates the radio energy delivered to it.
In HF communications, the ground screen is placed on the ground with the center of the screen directly under the antenna. This configuration would cause problems in VHF communications where the antenna should be as high as possible to obtain maximum VHF-LOS range. However, the short length of a quarter-wavelength at VHF (2.5 to 1 meter) allows the use of tubing to form a ground-plane antenna. The lower elements of this antenna provide the ground required for the quarter-wave vertical antenna to work properly. With its artificial ground, the ground-plane antenna can be placed at any height and still function properly. Tactical antennas have their ground-plane elements dropped down at an angle. This dropping of the ground plane causes the antenna to radiate its radio signal at a low take-off angle best for VHF-LOS propagation.
Gain - describes how well an antenna radiates. It is necessary to know what the gain of an antenna is being compared to before two antennas can be compared. In some cases, an antenna is said to have gain compared to an isotopic antenna and the gain is expressed in dBi. An isotopic antenna is a theoretical mathematical antenna with a gain of 0 dB in all directions. Other times, gain is referenced to a horizontal half wave dipole in free space whose gain over an isotopic antenna is 2.14 dB. To determine the isotopic gain of an antenna whose gain is given compared to a dipole, add 2.14 dB. For example, if an antenna has a given gain of 2 dB compared to a dipole, it's gain compared to an isotopic antenna is 4.14 dBi.
