Antennes VHF/UHF/SHF

VHF

VHF antennas in radio technology

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:

* Special approval, limited until 31.12.2022

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.

Mobile radio, i.e. the stationary radio station, either in the car on a boat or plane, or simply the handheld radio

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.

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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).

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Characteristics

Which are most important?
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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.

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FAQ

Puis-je encore utiliser un câble coaxial RG-58 bon marché dans la gamme VHF ?
Oui, mais il est préférable de n'utiliser que de courtes longueurs de câble pour la réception des stations de répéteurs FM les plus proches et du trafic radio FM local.
Une antenne intérieure est-elle suffisante pour les QSO au-dessus des stations de répéteurs FM les plus proches ?
Si les répéteurs les plus proches ne sont pas trop éloignés, une antenne d'intérieur ou de fenêtre fera l'affaire.
Les connexions DX sont-elles également possibles avec une antenne FM omnidirectionnelle verticale ?
Dans des conditions de propagation exceptionnellement bonnes, des connexions sur des centaines de kilomètres ou plus sont possibles même avec une antenne omnidirectionnelle haute et autoportante. Cependant, la plupart des connexions radio DX en SSB et CW se font en polarisation horizontale, votre taux de réussite sera beaucoup plus élevé.
Une antenne directionnelle permet-elle de réaliser des connexions DX régulières ?
Bien sûr, une antenne directionnelle augmentera la portée par rapport à une antenne omnidirectionnelle, même dans des conditions de propagation normales ou mauvaises. Cependant, le véritable DX dépend de bonnes conditions de propagation même avec son utilisation. Selon l'emplacement, des portées de 200 à 400 km sont possibles même sans conditions de propagation particulières.
J'ai un TOS pour les ondes courtes, puis-je l'utiliser pour mesurer le TOS dans la gamme VHF ?
Non, ce n'est malheureusement pas possible. Il existe des TOS principalement conçus pour les ondes courtes, qui peuvent également mesurer la bande 2 m. Mais pour des mesures précises sur l'ensemble de la bande VHF, il n'est pas nécessaire d'utiliser un TOS. Mais pour des mesures précises dans toute la gamme VHF, il faut un instrument distinct conçu pour ces fréquences.
Est-il plus difficile ou plus facile de respecter les limites de rayonnement radio sur les ondes métriques que sur les ondes courtes ?
En raison des courtes longueurs d'onde dans la gamme VHF, le champ lointain commence beaucoup plus tôt. De plus, la hauteur des antennes au-dessus du sol par rapport à la longueur d'onde est beaucoup plus grande que sur les ondes courtes. Ainsi, en dehors des réflexions possibles, la situation dans la gamme VHF est plus facile à gérer. Pour les mesures >144 MHz, il n'est également presque jamais nécessaire de mesurer la composante magnétique due aux conditions de champ lointain, un appareil de mesure avec une sonde de champ électrique est donc suffisant.