April, 2008 Getting in Tune V – functions of the ATU For correct operation, antennas at HF and below are often dependent on antenna tuning units or ATUs or other matching devices such as transformers and baluns. ATUs make it possible to tune over a wide frequency band and provide two necessary functions. Firstly, because a range of frequencies is normally required and in each case the antenna needs to look like a different length electrically, the antenna must be tuned or made resonant at the frequency required. Secondly, the feed line impedance needs to be matched to that of the transmitter, usually 50 ohms... Installation wise, the required antenna feed at HF is high voltage silicone cable. The actual antenna feed point is at the insulator on the ATU case. For this reason ATUs should be located as close to the antenna feed point as possible to keep losses to a minimum. Extended antenna feed lines form part of the antenna and will to radiate RF. This reduces the strength of the radiated signal, and alters the intended radiation characteristics. Therefore, it is generally unwise to exceed recommended lead lengths. Frequently manufacturers of ATUs will specify a minimum antenna length of 2.4m (8ft) for operation on 3-30 MHz and 7m (23ft) for operation on 1.8-30 MHz. Having a short antenna, therefore, will make it difficult to tune on the lower frequencies. This is more important in fibreglass whip antennas which only have thin wire radiators, compared with metal rods which have large radiating surfaces. Lack of antenna length can be compensated for by increasing the antenna feed line slightly, although this may affect antenna radiation characteristics. Best performance is achieved when an antenna is naturally resonant at the frequency you are using. An ATU requires power to tune and the greater the amount of adjustment it has to make, the more power is used up. An antenna naturally resonant at around 20 MHz has to be adjusted electrically from about 3.65m (12ft) to 26m (118ft) to tuned to 2 MHz. This results in power lost to radiation of the signal. You can use a tuned antenna but you will still need an ATU for impedance matching or there will be considerable losses on the feed line due to standing waves. In the case of antennas that are physically short for the frequencies to be used, it is possible to minimise losses in the ATU by incorporating centre loading coils and capacity top hats to make antennas look longer electrically than they actually are, thus maximising the power available for radiation. When an antenna is to be used over a range of frequencies, choosing the highest of regularly used frequencies for loading can make a significant difference to performance on the lower frequencies. This, of course, works in the receive direction as well. Earths must be sufficiently large and connections suitably bonded or problems will arise. When an earth system is too small, the earth mat may radiate instead of the antenna and you may get electric shocks from touching metal objects connected to the ground. Where the antenna and the earth system are made of different metals, you should be aware that they have different resistances. Aluminium, for example, has a significantly higher internal resistance than copper, so aluminium earth systems used with copper wire antennas should be larger than copper types. At frequencies where the earth system is small enough to be near resonance, it can be difficult for the ATU to decide whether the earth or the antenna should radiate, creating a loop. If you are connecting to an existing ground source, it is important to make sure that it is suitable and does not have poor connections. Lastly, ATUs can provide a certain amount of filtering to eliminate interference on frequencies higher and lower than the one you are using due to their inherent selectivity, and this can be very useful. So, if you are faced with interference within the band, you should consider using an ATU and whip rather than a broadband antenna. Lightning Protection
- for 12-24V DC and 110-240V AC external cables
 DC and AC external cables feeding navigation lights, wind speed and direction indictors, etc., can carry transient over voltages to other instruments and electronic devices such as chart plotters, computers and the like, to cause damage. The Moonraker DCL Protector, fitted at the cable entry point to the vessel, acts to prevent this happening by clamping any over voltage on 12 or 24 volt DC and 110 or 220/240 volt AC cables and conducting surges safely to ground. - type DCL or type ACL Versatile HF Reception
- for manpacks
 The HF MRA RXMP 2-30 MHz broadband tactical receiving system can be used either vertically or horizontally. High angle NVIS angles, short range ground wave or long distance skywave communications are all possible depending on the configuration and height above ground. The system is designed for fast deployment in the field. It uses Kevlar strengthened tinned copper braided wire and comes complete with camouflage satchel. and waterproof instructions. - type MRA RXMP Close in Performance
- for HF Base Stations
 The Moonraker HFB D/S semi delta antenna system is designed to give excellent broadband performance from 2-14 MHz for links up to around 1700km (1000 miles). High angle radiation eliminates the skip zone for local area (NVIS) communications. The system is supplied complete, including halyard pulley. Only one 15m (50ft) mast (or similar suitable support) and one earth stake required. Power capability is 400W PEP. VSWR is normally better than 1.8:1. An earth mat is recommended over dry arid ground. The antenna base line is 20m (66ft). The radiating element is multi-strand insulated hard drawn copper. - type HFB D/S
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