VHF Antenna Information

AIS Antenna Information

marine electronics navigation F&C Marine

Author: George R Groves

VHF Antenna

VHF…… What does it stand for, how does it work, why does the AIS use it? These may all be questions you are asking yourself. Well you have come to the right place.  In this blog I will explain everything about a VHF you need to know. 

Well, VHF stands for, Very High Frequency (VHF).  It is the frequency band that the antenna transmits its data on.  A VHF is one of the most reliable and cost effective ways to send information on-board a vessel.  Parred with a decent range of transmission making it the choice of antenna to transmit data for the AIS.

Marine Very High Frequency Information

VHF Frequency Band at first glance online will seem like it ranges from 30Mhz to 300Mhz;  Which is true but misleading for the marine industry as the marine industry uses a very small section of this bandwidth.  Marine VHF radio refers to the radio frequency range between 156 and 174Mhz, inclusive. The “VHF” signifies the very high frequency of the range. In the official language of the International Telecommunication Union the band is called the VHF maritime mobile band. This basically means the VHF radios work only between 156 and 174Mhz.  This is because the rest of the frequency range is reserved for other facilitys such as aviation and milatry use. 

VHF Antenna (Image 1)

Transmission Length

There is no one answer that would be correct in all circumstances.  There are so many variable factors that effect the transmission length of a VHF.  In this section I will talk about the main factors    which are:

  • The height of the antenna.
  • Curvature of the earth (Sorry flat earthers)
  • Power of the antenna

The curvature of the earth effects the transmission range because VHF works of line of sight. If something blocks the path of the wave then it can not transmit further so if the transmission hit the earth then it is blocked. This then leads onto why the height of the antenna is a factor as; the higher the antenna is mounted the greater the distance will be untill it is blocked by the curvature of the earth.  Please see Image 2, to the left for a visual representation of these factors.

VHF Range (Image 2)

If the ship had a higher antenna then the line of sight would reach the vessel.

Eventually you will reach a point where line of sight is not a problem and that is where power comes in.  The greater the power of the antenna  the further the signal can travel and clearer it will be at the receiving end. If you have a weak antenna it does not matter how high you mount it as the signal just does not have enough umpth to reach its destination.

GPS Antenna

Global Position service (GPS) is the leading Global Navigational Satellite System (GNSS) in the world.  This is an American Funded system available to the entire population.  The rival to the American GPS system is the Russian GNSS system called Globalnaya Navigazionnaya Sputnikovaya Sistema (GLONASS). GPS has the best coverage across the entire world for your AIS, except for in the very north and south poles of theworld.  This is due to the GLONASS satellite positioning in space.

GPS Antenna (Image 3)

 

Why is this important you may be asking for your AIS.  Well for an AIS to operate, it must know its current position.  This is obtained via a GPS antenna.  Your class of AIS installed will depend on the functionality and quality of your GPS antenna.  Below is a comparison between a Class A AIS and a Class B AIS requirements for a GPS Antenna

Independent GPS –

Internal GPS –

External GPS –

Backup GPS –

Required

Not Applicable

Required

Required

Independent GPS –

Internal GPS –

External GPS –

Backup GPS –

Not Required

Required

Not Required

Not Applicable

Independent GPS –               Required

Internal GPS –                              Not Applicable

External GPS –                         Required

Back Up GPS –                         Required

Independent –                           Not Required

Internal GPS –                           Required

External GPS –                          Not Required

Back Up GPS –                           Not Applicable

A Class A AIS is required to have an independent, external GPS solely for the AIS.  The AIS must then have another source of GPS being supplied from somewhere else on board the vessel. Higher end Class A AIS can operate off GPS and/or GLONASS satellites simultaneously.  This ensures that you will have the most precise location anywhere on the planet that your vessel may sail.

A Class B AIS must have a source of GPS.  Most come with an inbuilt, internal GPS.  However for some vessels this inbuilt GPS will not work very well for various reasons such as installation location.  Due to this Some Class B AIS come with an external GPS instead, allowing for a more accurate position.  A Class B AIS is not capable of taking a back up source of GPS either.