What is the challenge of VHF radio comms?
Range is one of the most important factors in radio communications. VHF range is limited to line-of-sight. This means that there should be an unobstructed line of sight between the transmitter and receiver for contact to be made. The radio link becomes weaker or even impossible if one of the participants is behind an obstacle or behind a mountain or large building. Yet it nevertheless works even for over-the-horizon contacts. But how?
On this page we want to show you the possibilities of VHF radio communications, how you can make contact with other radio operators beyond the (visual) horizon after all, and the importance of choosing the right VHF antenna.
Frequency ranges
The classic VHF bands 2 m and 70 cm carry the largest share of radio operations. The 6- and 4-m-bands and especially the 23-cm-band are rather something for specialists. The frequency range generally referred to as VHF" is divided into the VHF range from 30 MHz to 300 MHz and the UHF range from 300 MHz to 3 GHz. Within these limits are a total of four amateur radio bands:
The most important bands on VHF are found in the lower range (from 110 to about 450 MHz). Specifically, these are the bands with approximately 2 m and 70 cm wavelengths. This concerns amateur radio as well as maritime and aeronautical radio and also the public services like police and fire brigade. Of course, there are other frequency bands which are generally referred to as "VHF" and which play a very important role in other areas (cell phones, Wifi, radar, etc.). However, we will not consider these bands here.
Fields of application for VHF antennas
With regard to the possible applications, a distinction is made between mobile radio, i.e. the stationary radio station, either in the car on a boat or plane, or simply the handheld radio; and stationary use, i.e. operation from home with a stationary station.
Distinguishing characteristics of different VHF antennas
Most VHF antenna types can be classified as follows: Omnidirectional antennas are antennas that radiate the transmitter energy equally in all directions, and directional antennas are antennas that have a more or less strong directivity, i.e., they concentrate the radiated energy in one direction only.
Main characteristics of VHF omnidirectional antennas
Antennas with an approximately circular polar pattern in the horizontal plane are called omnidirectional antennas. Vertically polarized omnidirectional antennas are widely used in the commercial sector, such as in the public service sector and for stationary and mobile radio stations. Vertical polarization is also common in amateur radio in the FM range. All FM repeater stations in amateur radio use vertically polarized, mostly omnidirectional antennas. Omnidirectional antennas are offered not only as monoband versions, but also as two- or three-band antennas, for example 2 m, 70 cm and 23 cm.
There are differences in the feed of omnidirectional antennas
- In its simplest form, a vertically polarized omnidirectional antenna is a ʎ/4 antenna or groundplane antenna for stationary operation. As a so-called "ʎ/4-rod", quarter-wave radiators are often used in mobile operation. The body of the vehicle provides the necessary counterweight.
- The group of half-wave radiators includes the Sleeve antenna, the J antenna, and the ʎ-5/8 radiator extended beyond the half-wave. Vertical half-wave radiators are mostly end-fed and produce a lower radiation angle and already a slight gain.
- Only the coaxial dipole is electrically center-fed, although not obvious at first glance.
- - Last but not least, the family of short helical antennas on handheld radios also includes vertical omnidirectional antennas.
If you arrange two or more vertical omnidirectional antennas stacked on top of each other, you get a collinear antenna. "Collinear" means "arranged in a straight line" on the same axis. Each single radiator is excited in phase via phase lines and must be arranged at a certain distance from each other. As the number of stacked radiators increases, the gain increases and the vertical elevation angle decreases, i.e., the radiation pattern becomes flatter. This design requires a long protecting tube and is also found in amateur radio, but more often in the commercial sector. The popular Diamond "X-nn" antennas are designed this way.
Main characteristics of VHF directional antennas
A directional antenna consists of at least two elements, the radiator and a passive, radiation-coupled element assigned in the same plane, the reflector. On the antenna boom further elements, the so-called directors, can be arranged in the beam direction in front of the radiator. With the increase of their number, the forward gain of the antenna increases and the aperture angle becomes smaller.
Depending on the horizontal or vertical mounting, directional antennas are horizontally or vertically polarized. The radiation pattern of all directional antennas consists of a large forward lobe and the smallest possible backward lobe. Their characteristics are the (forward) gain and the return loss. When evaluating antennas, a distinction must be made between the isotropic radiator (dBi) (theoretical omnidirectional radiator) and the gain over dipole (dBd). The gain specifications in the technical data for VHF antennas most often refer to the isotropic radiator. The gain specification of the same antenna in dBd, i.e. over the dipole as a comparison antenna, is always 2.15 dB lower. This must be taken into account when comparing antenna data! Another criterion is the horizontal and the vertical beam angle. Logarithmic-periodic array antennas are very suitable when very large frequency ranges without coverage gaps are required. The antenna characteristics are almost constant over the entire operating range. However, in favor of broadband performance, they do not achieve the high gains of Yagi antennas with a smaller bandwith.
Yagi antennas
Not without reason, the Yagi antenna in short or long version is the most frequently used directional antenna in the VHF and UHF range of amateur radio. It is easy and inexpensive to manufacture with a small amount of material. This makes it well suited for do-it-yourself construction. Further advantages are its low wind load and the favorable position of the center of gravity with rotatable arrangement on a rotor. Depending on the concept and the number of elements, a single Yagi antenna achieves a forward gain of about 5 dBd to a maximum of 16 dBd. Beyond that, further extension of the antenna support and the addition of further parasitic elements is no longer useful, because the gain does not increase linearly with increasing antenna length and number of elements, but only in an increasingly flat curve. At about 5 to 6 m antenna length, the limit of mechanical feasibility and stability is reached. A further increase of the gain is then in practice only possible by grouping (stacking) multiple identical antennas. Special forms are cross yagis, directional antennas with elements in square loop form and circularly polarized directional antennas (Helix antennas).
Characteristics
Which antenna for what?
Vertical omnidirectional antennas are preferably used for FM communications - directly or via repeater. In local rounds with participants at different locations spread out in all directions, a directional antenna is of course not so well suited. In this case, an additional omnidirectional antenna is needed. The larger omnidirectional antennas with a little more gain are also suitable to get an overview of the band occupancy in advance in the SSB range under good conditions, in order to switch to a horizontally polarized directional antenna to establish a connection afterwards. Even if one can achieve success with an omnidirectional antenna in SSB under very good propagation conditions, for regular participation in DX operation a powerful yagi antenna that can be rotated by means of a rotor is indispensable. With cross yagis, the polarization plane can be switched between vertical or horizontal, and even in a circular fashion. In FM communications the vertical polarization is common, for SSB, CW and other modes the horizontal polarization. In addition, there are so-called helix antennas, which are directional antennas with circular (i.e. rotating) polarization used in satellite radio.
Mobile radio
Of course, there are also special use cases, such as operation over the moon as a passive reflector (EME, earth-moon-earth), or meteor scatter or satellite operation. All these use cases take place almost exclusively on VHF, in most cases with directional antennas. In contrast to terrestrial radio, circular polarization is often used here. On higher frequencies (from 13cm, 2.4 GHz) often parabolic dish antennas are used as directional antennas, because on the higher frequencies (shorter wavelengths) the size of this design becomes manageable..
Troposcatter
Satellite Radio
Meteorscatter
Earth-Moon-Earth
Aviation radio
Marine Radio
How important is the choice of coaxial cable for VHF antennas??
A good antenna system in the VHF range ultimately requires a higher-quality coaxial cable with low attenuation values. A few meters of RG-58 with PL connectors, will be sufficient for casual operation via the local FM repeater. Longer cable runs and cheap connectors would cancel out the gain of a good antenna. In this case low attenuation cables, as well as high quality plugs ( N or BNC), are definitely the better choice. The value of the cable attenuation varies with the frequency and is usually given in dB standardized for a length of 100 m. The attenuation of the individually used cable length can be easily determined by dividing the dB value specified for the cable and the frequency by 100 and multiplying it by the individual cable length. The connection of the coaxial cable to the antenna must be strain-relieved and weather-protected, with a plug that matches the cable diameter and the standard associated with the connection socket.
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Is a rotor absolutely necessary for VHF operation?
Maybe a 4el-Yagi will be mounted in a fixed direction to the next FM-relay. Longer Yagis and other directional antennas need a rotor to use their directivity effectively. Depending on antenna size, wind load, and weight, the market has suitable examples, ranging from small rotors for TV antenna-sized yagis to heavy equipment for rotating large array antennas. Elevation/azimuth systems are required for operation over earth-orbiting satellites and for EME. They consist of a combination of two rotors and can point an antenna both horizontally 360° in azimuth and vertically 90° in elevation.
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What do I need for VHF reception?
Reception only, what do I need? It depends: If you only want to listen to the closest FM repeater, a window quad mounted in the house, a short mobile antenna on the windowsill or a HB9CV in the attic is usually already sufficient.
For reception of distant FM or SSB stations up to DX in good conditions, at least a good omnidirectional antenna on the roof of the house or free-standing on a tubular mast is recommended. Some antennas have horizontal polarization and an omnidirectional patternm, for example the Big Wheel or the Halo antenna.
If you want to receive all bands in the VHF and UHF range, you should choose a Discone antenna. With its wide bandwidth and vertical polarization, it is the ideal antenna for omnidirectional reception.
VHF mobile radio communications is mainly handled by the numerous FM repeaters in the 2 m and 70 cm bands as well as on direct frequencies. Increasingly, the new operating modes of digital voice radio are also gaining in importance here. The so-called monoband "ʎ/4-rod" has somewhat "gone out of fashion". Since it requires the body as a counterweight, only direct mounting in the center of the roof is possible. From the physics point of view, this is the optimal mounting location, but not everyone will want to drill a hole into the roof of the car. For a VHF/UHF mobile radio antenna, there are currently reversible mounting options with the ʎ/2 and ʎ-5/8 antenna shapes, such as the magnetic base, the window clamp, the roof rack, reeling or trunk clamp, and the adhesive antenna for sticking on the inside of the front or rear window.
Reflection at and in the troposphere is the usual and most common propagation path for long-distance VHF contacts. The longer lasting reflections allow complete connections in all operating modes, often even with small transmitting powers and small antennas if the propagation conditions are suitable. Often contacts of up to 700 km and more are possible with tropo-scatter!
Amateur radio operation via satellites, especially currently via the geostationary QO-100, is possible with relatively little effort. Because the satellite is geostationary, i.e. in a fixed position in the sky, tracking of the antenna is not necessary. This is in contrast to earlier amateur satellites. orbiting the the earth. This means that the QO-100 satellite can be reached continuously in its footprint.
The satellite's transponder transmits a 250 kHz wide frequency window, within which many stations can communicate simultaneously in different modes such as SSB, CW or narrowband digimodes. The uplink is at 2.4 GHz, the downlink is in the 10 GHz band. A few watts of transmit power and a satellite dish with a diameter of one meter are sufficient. There are numerous modules available for do-it-yourself construction - and even if there is complex technology behind it, the financial effort is surprisingly low to become QRV via QO-100.
Along the trace of meteorites entering the earth's atmosphere, a narrow zone of ionized gases is formed on which VHF signals are reflected. This process takes place at an altitude of 80 to 120 kilometers and enables communication ranges of over 2000 kilometers. These reflections, known as meteor scatters, are used in amateur radio in the VHF and UHF ranges on the amateur radio bands 50, 145 and 435 MHz, mainly on the fixed dates of the annual meteor showers, for sporadic connections. Since the ionization is usually very short (sub-second to a few seconds), high-speed telegraphy and operating technique is used as a special mode. A development of modern computer application usedstate-of-the-art digital modulation to convey as much data in a short time and with limited bandwidth. A directional antenna and larger transmitting power are required for successful contacts over meteorscattre.
Even the moon can be used as a passive reflector for radio links between two distant points on earth. The high free space attenuation of well over 200 dB and the low reflectivity due to the rough, rugged surface of the Moon, as well as the average 770,000 kilometers the signal must travel, require a very large antenna effort. Large groups of high-gain yagi antennas and antenna tracking (see under Rotors: Elevation/Azimuthal Systems) are therefore required. A group of four long yagi antennas and 400 W transmit power are considered as minimum equipment in the 70 cm band.
Therefore, only frequencies in the VHF range can be considered in order to realize the required antenna gain. The radio signal requires a propagation time of about 2.5 seconds for the approximately 770,000 kilometers to the moon and back. EME operation takes place in the amateur radio bands at 144, 432, 1296 MHz and 10 GHz. In the 2-m and 70-cm bands, groups of long Yagi antennas are mostly used. With a transmitting power of about 750 W one can receive the own echoes from the moon with four long Yagis of 5 m boom length each in a 2 x 2 stacked configuration. Most often mode is CW (telegraphy) and digital modes. On the higher frequencies, starting from 1296 MHz, parabolic dish antennas are used, which enable more reliable EME connections with gains of 35 dB and more.
Aircraft also communicate mostly on VHF. Only over the oceans and very sparsely populated regions of the world is shortwave also used. Classic AM modulation is still the global standard for control and navigation by air traffic controllers. In addition, automated modes such as ACARS (message exchange) and ADS-B (position data) also take place on VHF and UHF.
Although mobile telephones have widely found their place in the maritime world, the VHF radio is still the most important means of communication on board a ship. In turn the most important element of the overall radio system is probably the antenna.
In commercial shipping VHF is used for on-board radio traffic as well as radio traffic with district radio centers, traffic control centers, the ship steering radio service, pilots, ports, and locks, as well as radio traffic with other ships in densely populated traffic areas. This is because of the short range of VHF maritime radio.
VHF also finds use in recreational shipping as a distress transmitter, a source of meteorological and nautical warning messages, a means of communication with district radio centers, ports, and locks, as well as for social interaction.
In addition, martime radios over additonal functions, like integrated GPS or AIS. In some cases an additional antenna must be installed. In others the radio antenna will support the service.
It is important to note that the performance of the radio system onboard is significantly reduced by a suboptimal antenna connection or cable from the transmitter to the antenna.
Mobile radio
VHF mobile radio on 2 m and 70 cm takes place almost exclusively via numerous repeater stations due to the propagation of the radio waves by line-of-sight only. Modes like Echolink and the various repeater stations linked in the meantime increase the coverage area considerably. As a result, VHF radio has long since ceased to be local or limited to one's own country, and even enables worldwide contact to be established beyond Europe. Furthermore, the new digital voice modes, such as C4FM, DMR and D-STAR, are increasingly supplementing the communication possibilities. In mobile radio, vertical antenna polarization is used almost exclusively.
Fixed station operation
The operation of a fixed station with sufficiently high and unobstructed directional antenna makes it possible to participate in the DX action on the VHF bands.
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FM communications
Local radio via FM repeater and on simplex frequencies requires less effort and is already possible with indoor or balcony antennas. Repeater communicatins and most FM radio traffic is handled with vertically polarized antennas.
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Terrestrial DX operation
Reaching distant stations requires the use of directional antennas. Radio operations in VHF contests is done exclusively on the direct frequencies, usually in SSB and CW. Here the antennas are almost always horizontally polarized.
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Special applications such as EME, satellite radio, etc.
In addition to FM radio and DX operation in SSB, there are several other extremely interesting playgrounds on VHF. Examples are the operation over the moon as reflector, or the operation over low flying satellites. Here almost exclusively directional antennas are used, in some cases very large ones. The polarization depends on the situation, often circular polarization is used.
Here are the most important, practical antenna characteristics, which are common for VHF antennas in the listing of technical data:
The gain compared to an isotropic omnidirectional radiator
The gain compared to a dipole
The gain compared to an isotropic omnidirectional radiator
Impedance at the base or feed point of the antenna
The gain compared to an isotropic omnidirectional radiator
Usually specified as the frequency range where the SWR is at or below 2:1.
Not every value can be optimized at the same time. A high gain requires a large form factor (length for yagis), an optimization of the gain is usually to the disadvantage of the FB ratio and the bandwidth, and so on. So you have to decide which value is most important to you when designing your own antenna.
