AirNav Systems Forum

AirNav ShipTrax => AirNav ShipTrax FAQ => Topic started by: AirNav Development on July 11, 2011, 09:01:58 am

Title: Where should I site my Antenna?
Post by: AirNav Development on July 11, 2011, 09:01:58 am
ShipTrax comes with a small magnetic base antenna and this antenna has been selected for its excellent performance when used with the hardware receiver and AirNav software. Correct positioning of the antenna is essential in order to gain maximum performance as AIS signals are transmitted in the Marine Very High Frequency band.  At this frequency, signals travel by "line of sight" and are affected by any obstructions such as buildings, trees and hills. This means that if anything comes between you and the Ship, then that Ship will probably not be displayed by ShipTrax. In order to achieve maximum performance and to ensure good range and coverage, the following points should be noted:


· The ShipTrax antenna is not an External antenna.

· The higher the antenna, the better the performance.

· Performance is enhanced by a "ground-plane" which is simply a metal disk the size of a CD or larger. Attach the magnetic antenna to the center of the disk

· The longer the feed cable, the poorer the performance.


Whilst ShipTrax can work perfectly acceptably with the antenna sitting on a window ledge, you should always try to find the highest and clearest position.  A skylight window is an ideal location for the antenna. If you have ShipTrax installed on a laptop computer, then you will be able to move the laptop, ShipTrax and antenna to find the position that gives the best performance. An external antenna connected by a long feeder cable can improve performance, however, the advantage of the high antenna can be completely overcome by losses in the cable. If you wish to use a long feeder cable, you must use high quality, low-loss cable. Note that such cable tends to be thick, inflexible and expensive.

A signal pre-amplifier may improve ShipTrax performance, however, the amplifier must be capable of working at 162 MHz. The performance of a poorly sited antenna will not be improved by a pre-amplifier.  It is recommended that you experiment with the standard ShipTrax antenna before considering an external antenna and/or a pre amplifier.  Moving the antenna only a small distance can make a significant difference in performance.  Remember that height is everything, but not at the expense of a long 'lossy' feeder cable.

Use the Maximum Range Polar Diagram to see how well you have positioned you antenna.  Clear the polar diagram before starting this test - File | Preferences | General | Clear/Reset Polar Diagram.

Leave ShipTrax running for at least 24 Hours to allow the polar diagram to build up.