Saturday, 15 October 2011

Very High Frequency (VHF)


Communication between boats and harbors or between boats and diving centers, coast guards or marine police is done through VHF Radio, which is obligatory in every boat. The VHF radio may be fixed r mobile and use radio waves with frequencies between 156 and 162 MHz ( Hz express the frequency F of the electromagnetic waves also called Hertz) or in other words the number of cycle represents the distance between two peaks, the prefix Mega (M) stands for one million Hertz ( 1MHz = 1.000.000 Hz).
The electromagnetic waves are emitted in direct lines and thus the VHF range is limited apart from the radio power (max.25 watt) as well as the height of the unit’s antennas emitting and receiving. In most cases, the range does not go beyond 50 nautical miles but is usually around 30 nautical miles.
Most of the marine VHF radios have 55 pre-programmed channels which can be either simplex or duplex. The simplex channels the emission frequency is the same as the receiving frequency, thus conversation takes place in only one sense. The one calling and the one receiving cannot talk at the same time. The frequency in duplex channels is different and conversations is similar to that of normal mobile phone as both receiving and calling can talk at the same time.
Channel 16 corresponds to 156.8 MHz which by international agreement is both the open communication and emergency calling channel. This is different in the area of southern red sea where open communication channel is Channel 14 (156.7 MHz). and the emergency channel remain Channel 16.

The Search and Rescue or The Coast Guard, prefers that you invest in a VHF-FM marine radio and transmit emergency calls over Channel 16. They reason that when you call on a cell phone, other boaters who may be near enough to provide assistance cannot hear your distress call, as they would if they were monitoring VHF Channel 16. Also, it is difficult to determine a caller’s location, not to mention it wastes precious time. Possibly the most frustrating reason not to use a cell phone is the “can-you-hear-me-now?” syndrome of poor reception and low cell phone batteries, also the fact that many areas aren’t covered with cell connection.

Procedure for Transmission and Receiving
After having switched the VHF to the channel 14 when calling the diving center or the marine office. Or channel 16 in case of calling the coast guard or search and rescue or the hyperbaric recompression chamber, and tuned the switch for background noise reduction (squelch) you proceed the follows:
1-Press the button that allows transmission and pronounce the name not more than 3 times of the boat or diving center you wish to contact.
2-You pronounce the name of your radio station ( diving center or boat) not more than 3 times.
3-Release the call button to allow reception.
4-Once communication is established, the channel for further communication is agreed. Both switch to this one thus keeping the calling channels 16 and 14 free.

Hertz Waves
Radio waves are part of the electromagnetic spectrum with wavelengths longer than infrared light. Naturally-occurring radio waves are produced by lightning, or by astronomical objects. Artificially-generated radio waves are used for broadcasting, mobile and fixed communications, navigation, computer networks and innumerable other applications. Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere; long waves may cover a part of the Earth very consistently, shorter waves can reflect off the ionosphere and travel around the world, and much shorter wavelengths bend or reflect very little and travel on a line of sight.
Radio waves (also called Hertz waves according to Heinrich Rudolf Hertz (February 22, 1857 – January 1, 1894) who was a German physicist who clarified and expanded the electromagnetic theory of light that had been put forth by Maxwell. He was the first to satisfactorily demonstrate the existence of electromagnetic waves by building an apparatus to produce and detect VHF or UHF radio waves ) expand at the speed of light (300.000 km per second) in linear direction, their length unit is measured in meters. In order to calculate the length L of a wave knowing the frequency F (in KHZ or kilohertz) you will need to apply formula L=300.000/F, and vice versa you will receive the frequency F (how many cycle per second) knowing the length L when calculating F=300.000/L.
Knowing the VHF waves have a frequency between a range of 156 and 162 MHz (Mega Hertz). 1MHz =1.000KHz and applying the above formula, we calculate that their length is 2 meters, therefore they are also known as metric waves.
If you want to identify the theoretical range R of a unit on board of a boat and of a coastal unit, knowing that the VHF wave expand in linear direction, you only will have to know the height of  the respective antenna (h and H) and apply the formula R=2.1 x(/H+/H).

Emergency Use of VHF 
If there any  situation maybe of life threatening and you can’t reach any of the medical service or search and rescue by Mobil phone then start radio call on Chanel 16 as follows:

1-    MADAY, MADAY. MADAY
2-    Pronounce your radio station (Daily boat, live board, Rib) name no more then 3 times
3-    Indicate your Position, Nature of emergency and kind of help needed……….

4-    Report the nature of the emergency and request the appropriate help and indicate whether diving emergency is involved.

If you are moored in a dive site and there is GPS available onboard then report your position as example,” One mile East of Ras Banas” Privide . Latitude …. , …. , …. N or S, Longitude …. , ….. , ….. E or W
If you are not moored to the site then report your position using GPS coordinates and visible landmarks.


References:
•    Jenkins, John D. "The Discovery of Radio Waves - 1888; Heinrich Rudolf Hertz (1847-1894)
•    UK Maritime and Coastguard Agency Marine Guidance
•    US Coast Guard, Department of Homeland Security, Navigation Center Official Web Site www.navcen.uscg.gov
•    Michael Russell Rip, James M. Hasik 2002. The Precision Revolution: GPS and the Future of Aerial Warfare. Naval Institute Press. ISBN 1557509735.
•    United States Naval Observatory. GPS Constellation Status.
•    "Publications and Standards from the National Marine Electronics Association (NMEA)". National Marine Electronics Association. http://www.nmea.org/pub/index.html. visited 3 feb 2010
•    Susskind, Charles. (1995).Heinrich Hertz :a Short Life. San Francisco: San Francisco Press. 10-ISBN 0-911-30274-3; 13-ISBN 978-0-911-30274-5
•    Hertz, H.R. "Ueber die Ausbreitungsgeschwindigkeit der electrodynamischen Wirkungen", Annalen der Physik, vol. 270, May, 1888. (WILEY InterScience)

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