- HF on the move...


Once upon a time the only way to communicate over long distances while on the move was HF and despite the advent of VHF then UHF networks, HF remains a very useful tool for long distance operators, especially when compared with satellite systems and their high running costs. With HF, once you have installed the equipment, then your communications are free to air.

Cellular phone networks suffer from propagational issues like multi-band fading and areas of poor or no communications. Moreover they do not provide blanket coverage over long distances, even along some major highways. And then there are operating costs to contend with. VHF/UHF radio networks cannot provide the range and also face the same propagational problems.

Before the invention of the Moonraker Auto Tune system, which provides continuous frequency coverage throughout the HF band (2 to 30 MHz), changing frequencies necessitated getting out of the vehicle and physically changing taps on the antenna itself – a process that was time consuming and, in military situations, could be life threatening. With the advent of Moonraker Auto Tune, it became possible to leave this behind.

Moonraker Auto Tune technology provides continuous frequency coverage on all channels throughout the HF band with a possible communications range of over 3,000 kilometres for vehicles, including 4-wheel drives, buses, truck and vans. Communications for distances up to around 100km are via the ground wave. For much longer distances (thousands of kilometres) it provides low angle radiation utilising reflections from the ionosphere.

Communications at intermediate distances, which are subject to signal loss due to the skip zone (say around 100-400 km) can be enhanced by pulling the top whip section of the antenna down to a near horizontal position and securing with a non-conductive material. This produces a more vertically directed radiation pattern and helps to fill-in the skip zone by reflecting from the ionosphere at steeper angles (NVIS).

Technically the AT230 antenna is a centre-loaded whip which operates in the "¼-wave grounded" mode. Tuning is accomplished by a special movable element, within the antenna, driven by a stepper motor under the control of the antenna interface, which is provided by the AT104 control unit, or by software incorporated in the Icom IC-F7000 transceiver. The control system optimises tuning by looking for the best SWR. Changing to a new channel is typically achieved within 1 or 2 seconds depending on the frequency separation of the two channels. The control unit can be installed anywhere in the vehicle other than the engine bay.

Most transceivers, including crystal channelised sets, can use the Moonraker Auto Tune system so long as they are able to provide continuous RF for tuning purposes by arranging the control unit to control the transceiver by way of a remote PTT line, an external initiate switch and an audible tuning indicator. Other sets allow operation from the set itself.

To improve the reception mode a broad band amplifier housed in the base of the antenna is automatically switched on after a call is completed (or approximately 20 seconds after transmission has finished). This amplifier is automatically switched off when the radio transmits.

The system is designed to use an earth system, normally provided by the vehicle’s metal chassis. For satisfactory operation it is important that the actual earth system is connected directly to the base of the antenna. The antenna may also be connected to the metal roof of a building or similar ground plane of wire radials if it is a minimum of 4m x 4m. It is also be possible to achieve satisfactory operation on a steel or aluminium vessel/boat where the base of the antenna is bolted directly to the steel or aluminium.

The use of centre loading is a unique feature of Moonraker Auto Tune and means that, unlike other systems, the antenna, when tuned, is always at a nominal 50 ohms at the feed point and therefore is low impedance. And because of the low impedance, the antenna is not “hot” or over sensitive to objects close to the base of the whip. As a result, the auto-tune system can automatically compensate for extra capacitance that might be introduced by mounting the antenna close to a vehicle's bodywork.

This makes it possible to mount the antenna close to and low down on the vehicle, provided that the top whip section is in free space.

This means that for front mounting, the antenna can be mounted on the bumper bar between the bull or crash bar and the bodywork, rather than, for instance, on the top of the bull bar. In the case of buses and other forward control vehicles the antenna may be mounted directly in front of the central windscreen divider or at one edge of the windscreen with the removable whip section above the roofline.

Thus the antenna has a lower profile and only the slim portion is in the field of view of the driver. This is an important safety advantage for the driver and one that regulatory bodies are turning their attention to more and more. Moreover, the antenna itself is subject to considerably reduced vibration from the vehicle compared with bull bar mounted units and less likely to hit overhead obstructions such as power lines.

The antenna is also well suited to mounting directly onto a tow bar (or onto a bracket attached to the tow bar). When mounted in this position on 4-wheel drive vehicles and vans, normally only the removable whip top extends beyond the roofline.

The AT230 antenna is of very robust construction and has been designed to withstand the harshest of conditions. It is sealed against the ingress of dust and water and requires no routine maintenance. Each antenna is pressure tested during final inspection. Its reliability has been proven by numerous installations particularly in off-road remote and tough environments.

Construction is of anodised aluminium alloy tubing with a sealed lower section housing the electronics and the control devices. The mounting base includes a solid block of special shock absorbing synthetic rubber which provides a rugged yet flexible and effective anti-vibration mount. The sealed midsection houses the main radiating and tuning elements whilst the top section is a removable stainless steel whip, 1.3 metres long, supported by a special stainless steel barrel spring.

Some transceivers, such as those designed for use with automatic tuning antennas and others designed to interface with automatic antenna tuning units (pulse tune) allow full front panel transceiver control of the AT 104 Control Unit and hence the auto tuning antenna. With these transceivers tuning indication is also via the transceiver, with either transceiver controlled, or automatic operation of the scan receive amplifier, which is housed in the base of the antenna.

Other types of transceivers may also take advantage of the automatic tuning antennas by arranging the AT104 Antenna Control Unit to control the transceiver through the use of an external tune initiate switch and audible tuning indicator.