VHF: performance and procedures
This post is also available in: Italiano (Italian)
VHF is the device ‘par excellence’ for safe navigation, effective when used in full knowledge of its performance and procedures.
The range of VHF radio communications doesn’t come first and foremost from the device power but above all from the position height of the transmitter’s and receiver’s antennas. Of course it is compromised by the terrestrial curvature and interposed obstacles.
The calculation is easy: for each antenna, the communication range (to the horizon) is 2.5 times the square root (SQR) of the antenna height. The communication range between two antennas is the sum of the two results.
a) Sailing boat whose antenna is on the mast at a 16-m height: SQR of 16 = 4. 4 x 2.5 = 10 nautical miles (to the horizon).
b) Vessel whose antenna is located at a 49-m height: SQR of 49 = 7. 7 x 2.5 = 17.5 nautical miles (to the horizon).
The two vessels will communicate at a distance of 17.5 + 10 = 27.5 nautical miles.
In particular weather conditions this distance can increase considerably.
Portable VHF used at 1 metre above sea level, Monte Cavo Coast Radio VHF antenna being mounted at a height of 900 metres above sea level.
a) Range of the portable VHF antenna: SQR 1 = 1
b) Range of Monte Cavo antenna: SQR 900 = 30
The two antennas will communicate at up to a distance of (30 + 1) x 2.5 = 77 nautical miles (in radio noiseless conditions).
In short a portable VHF able to communicate at a distance of as little as 12 miles with a vessel whose antenna is installed at a 16-m height will also easily communicate up to a distance of 77 miles with Monte Cavo Coast Radio.
These examples lead to major observations:
a portable VHF usually has a 5W emission power while for a fixed VHF it is 25W. As stated above, the communication range of VHF emissions does not depend on its power: a low power emission will reach another low power station unless full power transmissions or other nearby stations cover the communication.
|This is the reason why radio silence is compulsorily applied on VHF channel 16 for the first three minutes of each hour, as indicated by a green wedge on radio room clocks. Radio silence is used to allow faint distress calls to be heard.
Therefore maritime radio stations are required to always use the minimum power needed to ensure communication with the desired station in order not to disturb communication between other vessels.
These brief comments should make us understand that the effectiveness of radio communications for safe navigation relies on the strict respect of precise procedures, which the average pleasure boater often disregards.
Each member of the ITU (International Telecommunication Union) has issued a Master Plan with which it engages to ensure over the years the radio coverage of its VHF radio stations whose transmission distance depends on the installation height of its antennas.
Link to IMO/Masterplan: http://www.imo.org/blast/blastDataHelper.asp?data_id=30623&filename=13.pdf
Italy has several high mountains overlooking the sea on top of which VHF Radio Stations monitor antennas have been installed.
The picture here below shows that the average coverage all along the Italian coastline is of approx. 60-70 miles (called Sea Area A1), to which the result of the calculation of the vessel’s station antenna height range has to be added.
As opposed to what many think, maritime radio communications aren’t only useful for our safety but most of all for those who need help (we might be the ones to have asked for rescue).
Listening to the VHF channel 16 is mandatory yet most of the pleasure boaters keep the radio switched off at all times!
In short: widespread information has to be made available.
In past times distress calls caused confusion and radio traffic on the distress frequency, generating congestion of the communication and interfering with the Search And Rescue (SAR) activities. Since 1995 the GMDSS (Global Maritime Distress Safety System) – an internationally agreed-upon set of communication protocols – must be complied with: first of all whatever vessel at sea must make sure that any distress call is received by a Coastal Radio Station relating to the Maritime Rescue Coordination Centres and managing the combined intervention of SAR crafts.
Suggestions and notes:
1) As indicated above, the range of a portable VHF compares to that of a fixed installation but is limited by its emission power and the radio traffic. Unfortunately the rules on radio silence are often disregarded, which makes communication via a portable device little effective. It is highly recommended to equip your vessel with a fixed device with a masthead antenna.
2) Most vessels have a fixed VHF station which can only be used from the inside of the boat. It is highly recommended to choose a fixed radio that can be connected to an external station using the same masthead antenna and the same functions. If the device features a DSC function, the external microtelephone will always display the ship coordinates. In the unfortunate event of a fire onboard, the underdeck station will not be easy to reach; the same happens in case of adverse weather conditions not allowing to leave the steering wheel.
An external station prevents dangerous words of mouth.
3) The same is true in case of motorboats with flybridge: a station underdeck and a repeater on the fly.
4) Often check the device correct operation and its connection to the antenna. Replace the cable of the antenna every 10 years.
5) For different reasons all people onboard, and not only the Captain, should be able to manage maritime radio communications: in fact in case of distress the Captain will be most probably at wheel or trying to fix the problem and will delegate radio communications to others.
6) VHF radio communications reach any listening and coverage stations, that is all those nearby and able to help. Sea traffic is nowadays very thick and even when sailing in the ocean it is practically certain to find another vessel in a 30/40-mile range and listening.
Communications via cell phone (i.e. italian coast guaild 1530) are only available for the receiver without involving vessels that might be at few hundreds of metres afar.
The international IMO, ITU and CEPT provisions that have long been adopted by most European countries (Belgium, the Netherlands, the UK, France, etc.) require the maritime radio operators on board pleasure boats to be adequately qualified, i.e. having passed at least the GMDSS (SRC) examination for General Operator. This ensures that the operator knows the correct procedures and practices, and can offer effective support to distressed vessels without interfering but rather assisting rescue activities.
In Italy – which nonetheless complies with the GMDSS International Convention – the GMDSS (SRC) certificate is compulsory for radio operators that use a VHF-DSC radio installation; yet the GMDSS (SRC) certificate will soon be compulsory under international agreements also for the use of simple VHF installations also in Italy.
The examination syllabus guidelines are defined by the CEPT (Conference of European Post and Telecommunications Administrations) therefore a GMDSS (SRC) certificate is valid internationally no matter which country issues it.
The examination requirements for the certificate of GMDSS (SRC) maritime radio operator – for pleasure boating – can be downloaded from the website http://www.erodocdb.dk/Docs/doc98/official/pdf/REC3104E.PDF
The main examination topics are:
1) General knowledge of the correct voice procedure for distress, urgency and safety calls.
2) Distress call and message, procedures to follow when acknowledging the receipt of a distress call, and transmission of a distress message by a station not itself in distress.
3) Dedicated VHF channels for intership and ship-to-coast station voice communications as well a port operations.
4) Usage and destination of VHF channels (ITU Appendix 18).
5) Procedures to follow when a false or inadvertent Distress Alert is transmitted.
6) Propagation and range of communications; The concept of radio channel: simplex and duplex.
7) System overview of the GMDSS structure.
8) Avoidance of transmissions in guard bands.
9) Procedures and operation of DSC equipment.
10) System and functions of NAVTEX system.
11) System and functions of Emergency Position Indicating Radio Beacons (EPIRBS).
12) System and functions of Search and Rescue Radar Transponder and Transmitter (SART).
The examination consists of theoretical supplemented by practical tests on the knowledge of radio equipment.
As the examination syllabus guidelines indicate, the aspects relating to phone communications prevail over the DSC functions, to which mere practical tests are dedicated, and account for the largest share in the evaluation.
The GMDSS (SRC) examination in Italy can only be sat at the Ministry for Economic Development in Rome. Waiting time is quite long, one to two months, due to the elevated number of applications.
In Italy many British RYA Centres recognized by the MSI (Marine Coastguard Agency) offer one-day courses at the end of which attendants can sit the GMDSS examination.
English is the official language of maritime radio communications.
Just have a look at the http://www.marinetraffic.com website and you’ll realize that vessels flying the flag of foreign countries account for most of the marine traffic even in our territorial waters.
The GMDSS system highly recommends the use of the DSC technology since it effectively reduces traffic congestion on channel 16 and makes out for difficult communications in different languages and dialects in case of distress calls. The transmission of a DSC Distress Alert shall include the vessel’s MMSI (Maritime Mobile Service Identity), last known position, time the position was valid and, of course, the nature of distress. All coastal and ship stations (within the coverage area) receive the distress alert and the distressed vessel’s data even if the installation is tuned on any other VHF channel, the squelch is badly adjusted or the volume is low.
Each DSC Distress Alert must be followed by a voice message in compliance with the official protocol.
To better understand how useful DSC technology is, try to record your voice simulating a voice distress call: imagine you’re sailing in adverse weather conditions, you’re cold and tired and strained. Ask someone to take notes of your transmission and evaluate its result. Of course, all in the English language. Your message will most probably be unintelligible or wrong.
Inizia a navigare dal 1967 sulla barca del padre, l’8 msI Bamba del 1927 Cant. Baglietto..
Nel 2004 fonda l’associazione U.N.I.C.A. Unione Nazionale Imprenditori Charter Nautico Associati e nel 2012 l’associazione Universo Mare per lo sviluppo della Nautica da diporto.
È autore di diversi libri editi da “Il Frangente” su normativa nautica, sicurezza e comunicazioni radio marittime.