How Good is your VHF Radio/AIS Installation?
VHF radio has traditionally been thought of primarily as a means of communication for club racing due to its shorter range compared to HF radio. However, with the advent of repeater networks and significantly improved coastal networks, the role of VHF radio for coastal sailing has greatly increased. Likewise, AIS has become an indispensable tool for off-shore sailing. Proper installation and maintenance of both VHF radio and AIS systems is key to being able to fully realise the significant benefits of these technologies.
2. Underlying Technology
Marine VHF radio and AIS operate in the very high frequency radio band with VHF radio broadcasting at 156-157 MHz and AIS at 162 MHz. These frequencies correspond to wavelengths of about 2 meters, so the antennas are typically 0.5-2 metres in length. The propagation of these frequencies is essentially line of sight which means that the range of transmission and reception is critically dependant on the height of both the transmitting and receiving antenna above the ground, the higher the antennas the better. The important characteristic of both VHF radio and AIS is that, in addition to the basic RF frequency, the transmitter is able encode information onto the underlying RF frequency (voice information in the case of VHF radio and navigational information in the case of AIS) and then receiving stations are then able to decode, or interpret, this information.
Marine VHF radios have a maximum transmission power of 25 Watts which means at full transmission, they require something like 5 A of current, while more like 0.5A in standby mode.
AIS comes in two flavours; Class A and Class B. Class A, used by the larger ships, is able to transmit at higher power (12.5W) and has a higher priority than class B (2W power, average current of about 0.2A). They do this by communicating with all other nearby AIS systems the time when they will be transmitting and so “reserve” a time slot in which to transmit which other nearby AIS systems cannot override. Class B systems, typically used by racing yachts, have to “negotiate” to find a time slot to transmit that has not been “reserved” by the higher priority Class A systems in the area.
3. System Components
VHF radio and AIS systems can be broken down into three basic components:
- the electronics
- the antenna
- the interconnecting cables and connectors
Let’s look at each of these components in turn.
A. The electronics
The electronics of VHF radio and AIS are incredibly reliable and very rarely are the cause of poor system performance. They are usually housed in the navigation table area where hopefully the possibility of water ingress is very low. Power requirements are low, so battery power is usually not problematic but ensuring clean connections is still good housekeeping.
The key objective of any antenna is that it be capable of transmitting into the atmosphere, all of the power sent to it by the transmitter. To do this, the antenna needs to be tuned to the frequency of interest and electrically matched to the transmitter and cabling. If either of these conditions are not met, the transmitted power will be lower with consequent reduction in range. There are ample commercial antennas that work well for both VHF radio and AIS.
When both VHF radio and AIS are involved, it is possible to use the same antenna for both functions using a device called a splitter to share the antenna but there are some aspects to consider. Firstly, a splitter, being an active electronic device is a potential point of failure so thinking about how one might provide VHF/AIS capability in that event is important. Secondly, antennas tuned well for VHF radio may not work optimally for AIS as the higher AIS frequency may well be on the edge of their tuning range with a resulting loss in performance. It is simple task for a radio specialst to test the tuning range of a given antenna. One advantage of using a single, presumably masthead antenna, is the extra range due to the height of the antenna. Another point to keep in mind is that the Australian Sailing Special Regulations require that a boat must be able to maintain all communication modalities (HF radio if being used, VHF radio and AIS) in the event of dismasting. So, one needs to carefully think through the antenna configuration both with regard to overall performance and redundancy.
B. Cable and Connections
Cables for VHF radio and AIS come in various sizes and levels of performance. The most common, and cheapest, 5mm diameter cable used for VHF radios, over a run of say 15 meters, can lose up to half of the power available from the transmitter, even when perfectly installed. There are some higher performing, more expensive, 5mm cables and even better thicker cables but they are heavier and racing sailors are always concerned about weight aloft so the bottom line is, don’t skimp on the choice of cable because you get what you pay for!
Invariably, a VHF radio (same comments apply to AIS) involves a number of connections; one at the radio itself, often one at the base of the mast (to allow for easy mast removal) and one at the masthead to the antenna. The bane of VHF/AIS performance is water getting into the connectors and cable. The water ingress and subsequent corrosion acts as big sponge to the RF power, all of it going into just heating the cable and less of it radiating out into space where we actually want it! Make sure all of the connections (the one at the radio should not be a problem) are well protected by amalgamating tape (amalgamating tape is a rubbery tape that when stretched and wound tightly around a connector, bonds to itself and prevents the ingress of water into the connection) to prevent water getting into the connector/cable. For any connector at the mast base, not only tape it up well but support it in a way that it cannot fall into a wet bilge! These steps are very important!
A very good way to test an overall VHF radio is, on Port Phillip, to set the radio to VHF 82 (Arthurs Seat Repeater), low power (1W), international mode, set the squelch to the minimum to remove background noise, and then press the microphone transmit button to send a signal to the repeater and then release the button. If everything is working well, you should hear a splash of noise coming back confirming that the repeater has “heard” your transmission and is replying. One should be able to do the same sort of test using an AIS antenna though it may not work quite as well at the higher frequency. This test does not test everything in the VHF radio system but it does check for the most common faults.
Another problem that has come to light in recent times is the potential for LED lighting to interfere with VHF/AIS performance due to them radiating undesirable signals. The VHF radio can detect this interference and if strong enough, will reduce the sensitivity of the radio making it unable to detect the weaker signals from distant yachts and only the very strong signals from nearby yachts. The best way to see if this is a problem on your boat is, with all boat lights off, to set your VHF radio to a quiet channel, lower the squelch button until you just hear noise and then raise it until it just stops. Turn on your boat lights and if you hear the radio noise again, you have an LED interference problem which needs to be fixed!
5. Monitoring VHF Radio
The Australian Sailing Special Regulations and all ORCV Notices of Race require yachts to monitor VHF Channel 16 at all times. When sailing off-shore, your most likely source of assistance in an emergency will be from a nearby yacht and VHF radio will be the preferred means of communication. This can only happen if the radio is switched on and can be heard! VHF radios consume very low power and when set up correctly (squelch level set carefully etc.) they are quite unobtrusive. It is critical that the radio be audible under all sailing conditions. Most VHF radios allow for an external speaker that will typically be mounted in the cockpit area. The choice of speaker and its location are all key to ensuring that the radio can he be heard under all sailing conditions. Most speakers have magnets in them so be mindful of possible compass effects. It is the skipper’s responsibility to ensure the radio can always be heard and keep in mind that it might actually be your boat asking for help!