...Trouble Shooting
Marine Installations

Obviously fitting quality antennas, insulators, connectors and cables is essential. The antenna system may be the least expensive part of your communications set-up but unless it is working efficiently, you will get poor results, no matter how sophisticated your transceiver and tuner are - sometimes no results at all! Poor antenna systems, insulation (and maintenance in the case of fibreglass antennas), have been known to result in the antenna catching fire with serious damage to the transceiver and tuner.

Power can be lost due to resistance in the antenna conductor, earth and antenna connecting leads, loading coils, and the like. This is often due to thin conductors, bad joints, and poorly designed loading coils. The power lost in this way is wasted on heating up conductors. You may be getting good SWR readings but is the power being radiated?

Deck feedthrough insulators, antenna support insulators, poorly insulated leads running close to metal decks, bulkheads or other wiring can cause arc-over and/or heat losses in the insulating material. Some insulating materials degenerate in sunlight causing high leakage paths through the insulation. Salt build-up on insulators can also lead to arcing if not washed clean periodically.

Silicone alone is sometimes used to insulate cable passing through bulkheads and the like rather than using quality feedthrough insulators. However, because the cable is insulated, the silicone isn’t in contact with the copper. This practice leads to hardening and cracking of the silicone and the cable itself, and results in arcing and severe loss of power through heat. It can also be the cause of elusive receiving problems.

Power can also be lost after the signal is radiated as the radiated power causes currents to flow in nearby objects such as metal masts, halyards, stays and electrical wiring. Such losses are wasted in resistance (heating), and some relate to re-radiation of the signal by objects causing the antenna to become directional in an unpredictable way, especially at higher frequencies. Power induced into electrical wiring frequently causes navigation lights to glow and instruments to read wrongly during transmission.

In the case of HF whips which normally work in what is known as the 1/4 wave grounded mode with the sea or earth system providing the extra 1/4 wavelength, making a total 1/2 wavelength, much also depends on the efficiency of the earth system. Poor earths can contribute to many complaint s of poor performance and interference.

In the case of metal hulls, the earth point can be made to the hull immediately adjacent to the ATU. With wooden or fibreglass vessels radio earth plates are required. These should be in direct contact with the sea under all sea conditions and have an area as large as possible with no high impedance joints and obvious homes for marine creatures.

Radio earths should be kept separate from any other equipment earths. The lead needs to be run direct from the ATU by the shortest possible path to the earth point using thin copper strip at least 50mm wide. Aluminium strip may also be used. It is surface area that counts due to skin effect. Woven braid is not recommended as this tends to have a high HF resistance and corrodes easily. Where it is necessary to run long leads to earth points, earth lead sizes should be increased.

The mass of wiring and machinery, all connected in one way or another to the sea, usually via sea cocks, propellers and/or rudders, create complications. For this reason, it is essential to run earth leads well clear of other wiring, otherwise serious interference problems to and from the vessel’s electrics and machinery can result, and over time electrolysis may also raise its head.

It is wise to keep antenna leads as short and direct as possible. It is also advisable to run leads clear of other ship’s wiring and other metal objects, and avoid running parallel to metal decks, etc., with less than 2cm (3/4in) clearance. Standoff and cable run insulators can be useful in achieving this. In the special case of GPS equipment, running the cable too close to other cables and mounting the GPS and other antennas less than 0.6m (2ft) apart can create noise interference in the form of a pulsing through the receiver.

That said, sometimes when you think you have covered all the bases, it still won’t work like it should. Or to quote Unlucky Pierre “My antenna is perfect, but I cannot talk to anyone.”! Many problems are quite easy to check the incident current or voltage flowing along the line to the antenna and any reflected current flowing backwards to disrupt the flow with the aid of a SWR Meter. Of course, these days many transceivers have an inbuilt SWR/power meter.

When powering up the transmitter, the VSWR should still read close to 1:1. If you find different VSWR readings depending on where you insert the meter in the line, this normally is indicative of standing waves on the line and a mismatched antenna. However, sometimes VSWR readings are not all they seem. Where there are devices such as antenna switches, etc., in the line, it is important to look at the antenna on either side of the device. Different readings from transmitter and antenna sides of the device could indicate a mismatch in the device.

Antenna currents can flow on the outside of the coaxial cable due to poor antenna design or poor installation technique (see Feeder Radiation - Striking a Balance ), so that the line constitutes an extra load on the line and becomes part of the antenna system. These parallel currents will often cause indicated VSWR to change. A simple way to detect this is by physically moving the transmission line around which will result in changes in VSWR readings if this is the case.

Moreover, with maladjusted or faulty transmitters, it is possible for harmonics and low frequency sub-harmonics to be fed through the final stages of the transmitter driving the VSWR meter, introducing considerable error due to mismatch. This is because, although the antenna may be matched at operating frequencies, it is unlikely to be matched at the frequencies of these spurious emissions. The resultant error can be considerable, depending on the amplitude of the emissions, and can lead to other EMC problems!

With HF whips it is important to remember that the total antenna length includes all wiring from the ATU to the far end of the antenna, and where the earth lead is long this most be considered as part of the total length. Excessive lengths will alter the effective antenna length, which means that it will not performed as it was designed to.

While the ATU may be able to compensate, it will decrease the power available for radiation. It is important to keep as much length as possible in the antenna where it contributes to the radiation. This is why Moonraker uses centre loading where it can do the most good.

Where only the lower part of the band is going to be used, the antenna can be loaded to highest frequency to be used 10MHz or below. This increases the electrical length to give better performance at this frequency and the frequencies below. However, physical length in the antenna itself achieves the best results, so when fitting an HF antenna, it is always recommended that you fit the longest possible antenna for the length and type of the vessel.

Some ATUs state in the operating handbook that they require a 7m (23ft) antenna to operate correctly. Obviously it is not possible to install an antenna of this height on all vessels. With this type of ATU, fitting a shorter antenna gives poor results on low frequencies (eg 2MHz). This is where a loaded top can be useful. Because the lead is part of the antenna, this can be accommodated to some degree by using a longer lead to create a longer antenna so to speak. If you do this, you will most likely lose some of the top end frequencies. And…. when all else fails, read the instructions!!