FAQ: Antennas >>>
The combination antenna is easy to mount and works very well.The quality of the reception is very similar to a regular VHF antenna. The difference is the height of the antenna in which the combination antenna is mounted normally.
A typical mounting place of the combination antenna is the railing at the stern of the ship. This “mounting” height is much less than the height of the masttop. In our practical use we found out, that the transmission range and the reception range is more or less the half. With our easySPLIT OCB and the masttop antenna you have a typical range of app 8nm for the transmission of your AIS data and app 25nm for the reception of other AIS data (Depends of course of the antenna and cable quality, as well as of the weather!) This is simply due to the reason of the physical effect of the VHF (Distance is line of sight and you know, as higher, as better).
“There is not such a good amplifier existing than a high mounted antenna!”
easyTRX 1. Generation >>>
All class B transceiver have in common, that they need an own GPS.
This is necessary because the normally existing GPS antennas deliver NMEA GPS data. This NMEA datas are generally not exactly enough in reference to their time-base. A typical class B AIS telegram takes 25ms. Within 1 ms the class B transceiver has to deliver his „beam“. In order that the sent telegrams are not overlapping, every sender has to keep an exact for sending. The date for sending has to be more accurate as the telegram. The date of sending is determined at ca. 100µs by the class B sender. This is only possible, when the sender gets the raw data from the GPS antenna and not NMEA data, which are only accurate to 1/10 s. This is the reason. For a reliable operation GPS datas on a µs base are needed. With the explanation above you need 2 GPS antennas. One with NMEA data for your chartplotter and one for the easyTRX.
Please note that you need a separate GPS Antenna for the Class B transceiver.The antenna which is on board, does not work probably. The normal GPS antenna are „active“ antenna, which are already sending a coded telegram (NMEA 0183). These telegrams are perfect adapted for PC programs and cardplotter. Most telegrams are only closely for 1 second or 1/10 second. That is to incorrect for a class B AIS transmitter. The timebasis for AIS radio communication must be closely 1ms also a/1000 second. This accuracy will be reached when the GPS antenna is sending the raw data to the Class B AIS tranceiver also easyTRX. The raw data are transmitted from easyTRX into the correctly time base.
Thats why you need an separate GPS mushroom-type antenna, which send the raw data to the easyTRX.
easyTRX 1. Generation >>>
Following is the reason: Class B transmitters, as ours, work within a process which is called SOTDMA. (self organized time division multiple access). This means the big vessels listen to the traffic and the information containing in all other telegrams and defines it’s own raster of transmission timing, which is then distributed to all other participants within the radio range. Our class B transmitter, as all the others too, is using a “listen-before-talk-method”. This means, that before transmitting, a ship has to observe the radio transmission timeslots, whether to be allowed to transmit, if the timeslot is not occupied, or whether to wait for a free time slot. A timeslot is very very short (app. 25ms). Therefore the starting time of a class B telegram must be very exact on time. This is only possible by a microsecond exact timebase. Normal GPS antennas do have an accuracy of 1 sec., sometimes a little bit better, but not as exact as necessary. With the GPS antenna which can be purchased from us the microcontroller inside the easyTRX is calculating the timebase in microseconds and therefore it is proofed that the telegram is transmitted exactly when it is allowed. With a normal GPS antenna this would not be possible.
