MF ANTENNAS AND A2 STATION RANGE

The simple A1/A2 station described previously uses 125w transmitters for A2 DSC and radiotelephone services.

 A2 station range is determined by the effective radiated power of the transmitter and the noise performance of the receiver.

The limiting factor for A2 system transmitting performance is invariably the antenna.  Large vertical antennas are used at 2 MHz to achieve proper ground wave coverage. 

 

 

 The effective radiated power of MF antenna systems is a function of: 

  • transmitter power;
  • antenna length (height);
  • antenna coupler efficiency; and
  • the ground radial system (basically - the more radials, the better).

These are all a trade off – i.e. a higher power transmitter may be used to overcome an inefficient (short) antenna system. 

The ideal antenna height/length is a quarter wavelength – at 2 MHz this equates to around 37m.  A lower power transmitter could be used with a 37m antenna (and a good ground system).

The other issue to consider is noise.  Noise is the mortal enemy of GMDSS Coast stations – particularly at 2 MHz.  Traditional Coast Radio Stations were sited in low noise areas – normally well away from suburbia.  This may not be possible with modern GMDSS stations. 

 It is strongly recommended that noise measurements be made before a new site is selected for a GMDSS Coast Station – this may be as simple as listening on the MF/HF marine channels with a temporary/portable radio/antenna system.

  

Separate transmit and receive sites?

As discussed above, traditional Coast Radio Stations were located in electrically quiet rural areas. 

Because the stations ran a number of transmitters on different frequencies, they also operated with separate receive and transmit sites – often separated by as much as 10 km.

Unfortunately, many of these traditional large stations have gone the way of the dodo.  Modern economic constraints mean that separate transmit and receive sites may no longer be available.

A single site may now be the only alternative for GMDSS Coast Stations. 

Modern MF/HF receiver front ends are designed to be immune to strong signals – as much as 50V RMS.  However, if the receiver is subjected to a strong signal the AGC will take time to recover, so we recommend muting receivers unless some equipment and antenna separation is possible.

 

NAVTEX systems - why are they so expensive?

DUNSTAN AND ASSOCIATES

The GMDSS specialists

 

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