Mobile radio without limits:

The versatile amateur radio in the car

Mobile amateur radio in the car offers numerous possibilities, from short-range communication on VHF to worldwide radio operation on shortwave. There is something different about making direct vehicle-to-vehicle contact with other OMs, rather than using a mobile phone, without relying on the infrastructure of external mobile networks. Of course, mobile amateur radio is often only used for mundane communication: making an appointment during the journey to the Radio Club evening, flea market, fieldday or amateur radio fair, asking for directions or simply having a chat. And "last but not least", if you are QRV mobile, you can also provide help in an emergency - especially if the mobile networks should fail or even if you are just in a "dead zone".

Car Connect Mobile

Definition and differentiation from mobile phones

Even if the term mobile radio is primarily associated with public cellular mobile radio services (2G, 3G, 4G, etc.), "mobile radio" is also the collective term for the operation of radio devices that are operated in vehicles of all kinds while moving. Thus, the radio operation of a two-wheeler, a watercraft and an aircraft in visual flight also count as mobile radio operation. In amateur radio, the call sign is supplemented by the suffix /mobil or /m in all these applications.

Source: Excerpts from Amateur Radio Lexicon, Pietsch, H. P., DJ6HP

Mobile radio in the VHF range

With increasing motorisation and the advent of the first commercially manufactured small FM mobile radios, around the end of 1960 to the beginning of 1970, and the expansion of relay radio networks, VHF mobile radio began to become popular. A relay network that has been developed almost without gaps since then allows stable radio communications to be established almost everywhere. Although mobile communication in amateur radio is preferably handled via the numerous FM relays, as well as on direct frequencies in the 2-m and 70-cm bands, the new modes of digital voice radio, such as DMR, D-STAR and C4FM, are increasingly gaining importance in mobile radio. The numerous relay stations linked by Echolink in the meantime considerably increase the area of coverage . This means that VHF radio has long since ceased to be local and limited to one's own country, but even makes it possible to establish mobile contact worldwide beyond Europe.

The sporty version of mobile radio offers mobile competitions and awards. Already since 1960, the DARC e.V. has issued the mobile badge, which is awarded to licensed radio amateurs who operate a mobile station and have achieved at least 25 points. By participating in further mobile competitions, the "Silver Wreath" is awarded when 50 points are reached towards the vehicle badge, and the "Mobile Master" is awarded when 100 points are reached.

Source: DARC Mobile Competitions

Frequency modulation has prevailed over amplitude modulation (AM) in mobile radio for several technical reasons: For example, fluctuations in the received signal (QSB) have hardly any influence on the received volume and audio quality. Furthermore, interfering signals on the receive frequency cause significantly less interference.

FM versus SSB

Relay radio stations increase the range

Relay radio stations, are located at high, exposed sites. This results in a considerable increase in range for all participating stations. Originally designed for mobile radio, portable and fixed stations also benefit from the increased radius of action. A relay receives the signals on its input frequency in order to retransmit them in simultaneous receive and transmit mode on the output frequency, offset by the shelf. For this purpose, the transmit frequency of the participating station must correspond to the input frequency - and vice versa the receive frequency to the output frequency of the relay radio station. In idle mode, a relay station is in receive mode. With a tone call of 1750 Hz, the signalling systems Continuous Tone Code Squelch (CTCSS) or Digital Code Squelch (DCS), as well as the multi-frequency dialling method DTMF, a relay station is activated (keyed). This is set up differently from relay to relay.


Mobile radio on short wave

One "variant" of mobile radio has largely been able to retain its initial fascination! Try mobile radio on short wave. Drive to a hill or mountain with a free HF position and make QSOs worldwide from this exposed location. Then you will experience, literally and figuratively, what the former fascination was in the early days of mobile radio. And not only that, for those who cannot be QRV at home, this is a great opportunity to participate adequately in amateur radio.

Here, due to the equipment, all operating and modulation modes of a common shortwave transceiver are possible, but SSB voice radio dominates the action. In the 10 m band, not least because of the many relay stations in this area, FM is also common. Some freaks are even QRV in CW or the new digital modes, which is possible at least from a stationary vehicle.

Radios for mobile use

VHF mobile radios

In the category of VHF mobile radios you have the widest choice: Mono, duo and tri-banders or transceivers with even more ranges between 10 m and 23 cm on board, as well as in different power classes and almost all operating modes. The range extends from monobanders "for FM only" to multimode transceivers, including the new digital modes in voice radio. You will always find something to suit your needs and requirements.

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HF mobile radios

Only the very compact design of the current shortwave mobile transceivers makes it possible to accommodate such a device in the front area of a car. Compared to VHF transceivers, the number of shortwave mobile transceivers is relatively small. They are either permanently installed in the vehicle or used flexibly as portable or stationary transceivers. Increasingly, these small transceivers with 100 W transmitting power, which in addition to shortwave also have 2 m and 70 cm on board, are used exclusively as fixed stations.

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Handheld radios

For occasional use in the vicinity of a relay station (city relay) or for direct contact with one or more other vehicles, handheld mobile radios can also be used. Due to the low RF power, the reduced efficiency of short helical antennas and the shielding by the vehicle body, the range is limited. Connecting the handheld radio to a mobile antenna definitely improves the range. To prevent the handheld radio from "lying around loose in the car", there are also appropriate holders for one or the other model. An optional loudspeaker microphone would halfway complete the "handheld radio" to a small mobile station - an inexpensive solution for the occasional mobile user.

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Where is there room for the mobile radio?

New car models do not offer many possibilities to mount a radio set in the dashboard area. Everything is covered and there is not as much space under the dashboard as there used to be. Even the once popular slide-in slot, where you used to be able to fit a radio using a homemade panel, is a thing of the past. If you drive the latest model, it will be difficult to fit the radio in a sensible place without further intervention. Mounting the control unit on the dashboard and connecting it to the remote radio via a separation cable is the obvious solution if there is a place for the actual radio and the cable can be laid without problems. If this is not possible, a remote control microphone for wireless control of all functions via Bluetooth is the last resort. Then you can banish the mobile transceiver to the boot. From there, you still have the coaxial cable to the antenna and the two-core cable to the car battery.

Mobile antennas

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VHF mobile antennas

Antennas with an approximately circular directional pattern in the horizontal plane are called omnidirectional antennas. Vertically polarised omnidirectional antennas are widely used in the commercial sector, such as in BOS radio, for mobile radio stations. Vertical polarisation is also common in amateur radio in the FM range. All FM relay radio stations in amateur radio work with vertically polarised, omnidirectional antennas. In addition to the monoband versions, omnidirectional antennas are also available as two- or three-band antennas for 2 m, 70 cm and 23 cm.

In the simplest form, as a so-called "ʎ/4-rod", quarter-wave radiators are often used in mobile operation. The body of the vehicle provides the necessary counterpoise. The group of half-wave radiators suitable for mobile radio includes the J-antenna (once known as a window clamp antenna under the name Moba-6K), as well as the ʎ-5/8-radiator extended beyond the half-wave. Besides the simple quarter-wave radiator, half-wave radiator and ʎ- 5/8 radiator are the most commonly used VHF mobile antennas. They have the advantage that they are not quite as demanding on the counterpoise as quarter-wave radiators.

Source: Excerpts from Rothammel Antennenbuch, 13th edition, ch. 28, Amateurfunkantennen für den beweglichen Einsatz, pp. 909-924. To the products

Magnetic base antennas

Magnetic base antennas are probably the easiest way to get QRV quickly. The coaxial cable can be led into the vehicle either provisionally through a gap in the not quite closed side window, or in a special thin version past the sealing rubber of the closed door. A simple quarter-wave antenna already stands securely on the roof or the boot lid with a small magnetic base. Longer antennas with integrated extension coils have a higher wind load and require a stronger magnetic base for a secure hold. If the vehicle roof is already made of plastic, the magnetic base solution is of course out of the question.

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Coaxial cable to the antenna

An RG-58 coaxial cable is often used for the few metres between the antenna base and the mobile phone. However, it is advisable, especially in the interest of good shielding, to use a higher-quality cable with a high degree of shielding. On the one hand, to minimise EMC problems such as interference with the car's electronics, and on the other hand, to minimise interference with reception on the amateur radio frequencies.

The route of the coaxial cable to the antenna should be carefully selected and not necessarily longer than necessary. In most cases, the length is between about 2 and 3 m. For trouble-free matching to the antenna base, a cable length of multiples of half the wavelength multiplied by the shortening factor of 0.66 is advantageous. The SWV should not exceed a maximum value of 2.0.

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KW mobile antennas

A quarter-wave antenna for the 10 m band can be realised with a length of about 2.50 m without shortening. All other amateur radio bands below 28 MHz always require an extension coil, which is inserted either at the base or further up to the centre of the radiator. From a mechanical point of view, the footpoint coil is usually preferred; from an electrical point of view, a central coil with its lower losses would be "ahead of the game". While the mechanical requirements have to be met for the relatively short lengths of VHF mobile antennas, air resistance, wind load, elasticity and the lowest possible tendency to oscillate play a decisive role. The electrically better solution of a roof capacity is thus excluded from the outset. This already shows that it can only ever be a compromise between the mechanical and electrical properties. The efficiency of KW mobile antennas is consequently low, but can be partly compensated for by the fact that mobile antennas can be used in locations that are particularly exposed from a propagation point of view. In order to achieve maximum radiator length without exceeding the maximum permissible overall height of the vehicle plus antenna, it is exceptionally advantageous to have the lowest possible mounting point on the rear bumper or trailer coupling. Even more than with VHF mobile antennas, in the short-wave range, care must be taken to ensure an electrically flawless earth connection to the vehicle body! Although there are numerous variations of shortwave mobile antennas offered by different manufacturers, the range of products is limited with regard to their basic functional principle.

Source: Excerpts from Rothammel Antennenbuch, 13th edition, ch. 28, Amateurfunkantennen für den beweglichen Einsatz, pp. 909-924. To the products

Where is there room for the mobile antenna?

The quarter-wave antenna requires little space. Since it needs the car body as a counterpoise, only direct mounting in the centre of the roof is possible. From an HF point of view, this is the optimal mounting location, but apart from an old car, hardly anyone will want to use a drill with a circle cutter or a hole punch. Fortunately, there are numerous reversible mounting options for VHF mobile antennas, such as the magnetic base, the holder on the roof rack or on the roof rails (see several different versions of a roof rail clamp on the Diamond antenna product page), the boot or tailgate antenna, the window clamp antenna, and, as an inconspicuous solution, an adhesive glass antenna to stick on the inside of the windscreen or rear window.

Older vehicles had a car antenna for the car radio installed on the mudguard. The existing opening could be used for mounting a VHF mobile antenna. The car radio could be connected to the same antenna via a crossover.

Power supply from the vehicle's electrical system

Special attention must be paid to the wiring in the vehicle from the radio to the battery. Mobile operation of a radio station in a vehicle places special demands on the power supply. Especially if it is not only a VHF radio, but also an HF transceiver with 100 W transmitting power. One is dependent on an on-board battery (accumulator) with sufficient capacity, the continuous recharging of which must be ensured. An older battery quickly reaches its limits with the additional load. After a longer period of radio operation while stationary and with the engine switched off, you are often no longer ready to start with a deeply discharged battery.

The cigarette lighter is absolutely not suitable for connection to the 12 V on-board network. Its supply cable has too small a cross-section for currents of around 20 A. The supply cable must come from the transceiver. The supply line must be connected directly from the transceiver to the positive pole of the battery and a central earth point by the shortest route.


In German usage, we distinguish between a single-use, non-rechargeable battery and a rechargeable battery. Typical capacity values of lead batteries in closed, maintenance-free design are between 30 and 50 Ah for passenger cars and up to 180 Ah for truck batteries.


A fuse holder with a fuse in the positive wire is inserted as standard in the power supply line of current mobile phones. In the meantime, it has become common practice to insert a fuse in the negative wire as well. It is advisable to retrofit a so-called "voltage monitor", an electronic overvoltage protector that switches off the supply voltage when 13.8 V is exceeded and protects the equipment from "overvoltage death". Voltage and current monitors switch off at a defined undervoltage of, for example, 9 V, as well as when the nominal operating voltage of 13.8 V is exceeded, and protect the station in addition to the fuse, which reacts more slowly anyway. Even if transceivers and mobile radios have a reverse-polarity protection diode in the reverse direction to the operating voltage, an additional external reverse-polarity protection cannot do any harm.

2-pole fuse protection

A 2-pole fuse protection of DC connection cables in positive and negative line is required for DC low-voltage systems in motor vehicles and on watercraft without a fixed earth or ground reference for safety reasons. For example, there are still some older, English (vintage) car models which, in contrast to the worldwide standard, have the positive pole of the battery connected to earth. In these cases, a short-circuit current could flow completely unprotected, mainly via the negative cable. In this case, positive and negative cables are always fused. The additional fuse in the negative cable prevents the radio from being destroyed, the cable from burning and the battery from blowing up.

Voltage drop

The results of a cable calculator using three common cable cross-sections as examples give an idea of the voltage drop on the connecting cable of a shortwave transceiver. 300 W DC input power at 13.8 V, corresponding to a current consumption of 21.7 A, was taken as a basis.

Conductor cross-section Output voltage Voltage drop

6.0 mm² 13.67 V 0.13 V = 0.94 %

4.0 mm² 13.61 V 0.19 V = 1.41 %

1,5 mm² 13,28 V 0,52 V = 3,75 %

A cross-section of at least 6 mm² is recommended in this case. Below that, the voltage drop increases. The jump from 4.0 mm² to 1.5 mm² is already much larger and shows: 1.5 mm² is clearly too little. In the range of a few amperes, it should be 1.5 mm². To supply a transceiver with 20 to 30 A, at least 6 mm² are required.

If the voltage drops significantly under load and the LED displays and instrument lighting become dimmer, the power supply has a serious problem. Either the battery is not sufficiently charged, or it is already stricken. Voltage drops at contact resistors are also a possibility. Therefore, the low-voltage line to the mobile phone should be kept as short as possible. Furthermore, a cable cross-section of 4² or 6² is already quite appropriate for the standard output power of 100 W.

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Safety instructions and accessories

Numerous distraction possibilities while driving are a current topic. It is therefore important that the mobile phone or its control unit is placed in an ergonomically suitable position and also in such a way that you can "keep an eye on the traffic" when looking at the display.

The swivelling mounting brackets and holders for the control unit allow the radio to be optimally positioned on or on the dashboard. Multi-function microphones, which also allow the operation of the most important device functions, possibly even wirelessly, as well as headsets and, even better, a hands-free system, are an essential contribution to road safety. A mobile loudspeaker improves intelligibility and thus safety when driving. It can usually be placed in an acoustically optimal position, whereas the possible position of the mobile phone via the built-in loudspeaker often does not allow good audio reproduction. If the mobile phone is in a slot, a separate loudspeaker is required anyway. Mobile loudspeakers have a mounting bracket that allows them to be mounted in an adjustable position.

Electromagnetic compatibility

When installing and mounting the mobile antenna and radio, the EMC Directive 2014/30/EU on electromagnetic compatibility.

must be observed with regard to possible interference with and radiation into the on-board electronics. It may also be the case that the vehicle's on-board electronics are the source of reception interference.

StVZO Road Traffic Licensing Regulations

For understandable reasons, the total height of the vehicle and mobile antenna is limited. According to StVZO § 32, Dimensions of Vehicles and Vehicle Combinations, Para. 2, the maximum permissible height overall may not exceed 4 m. This applies in our case to vehicle antennas. In our case of vehicle antennas, this applies to the height of the mounting location plus the length of the antenna! Or put another way: the maximum permissible radiator length is 4 m minus the height of the antenna base above ground.

Source: Federal Ministry and Federal Office of Justice
Manufacturer info

Each vehicle manufacturer explicitly specifies the mounting locations for antennas, the routing of RF cables and power supplies, frequency ranges and permissible transmitting powers for its models. Many factory customer services have these documents available as PDF files. A look at the vehicle manual may also be helpful. DARC members have access to these documents in the member area.

Source, Legal situation for mounting antennas on vehicles
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