Don't forget to vist our other pages before you leave !
Although the technology is advancing towards totally digital transmissions there are still a very large number of narrow band analogue FM signals using Inmarsat type 'A' terminals, which can be monitored relatively easily, providing you have access to the following equipment :-
A good quality receiver capable of tuning across the 1525-1545MHz frequency band. e.g. AOR 3000/3000A/5000 or ICOM IC-R7000 (with pre-amp)/7100/8500
A directional antenna with at least 20dB gain
A compass to point the antenna in the right direction
And possibly a Pre-amplifier if you have a deaf receiver or long cable runs
The easiest antenna to make withouth having to have access to expensive test gear is a parabolic dish. These are becoming very cheap to buy secondhand as people upgrade to digital services which use much smaller dish sizes. The minimum size you need is 0.6 metre, try and get hold of a 0.9 metre one if you can.
First you need to construct a circuarly polarized feed tuned to 1.5GHz. Such as the one shown below (suggested by Paul Marsh - Thanks Paul) It really is very easy to make and the results are excellent.
Mount the feed in place of the orignal LNB on the dish. You can leave the LNB in place if you wish. Point the feed assembly towards the middle of the dish and then position it more accurately once you start to hear some signals.
In the UK the easiest satellite to try for is the one positioned to give coverage of the Eastern Atlantic region.
The required direction can be determined from the charts shown below, but for the UK a bearing of 195 degrees relative to magnetic North and an elevation angle of 30 degrees is required.
As the beamwidth of the dish is not as narrow at 1.5GHz as it is in the frequency bands used for satellite TV it is only nessesary to get the direction and elevation roughly correct.
Once you start to hear signals you can experiment to find the best position and then bend the feed to optimise the signal strength.
The following charts allow you to work out the correct azimuth and elevation required for each of the satellites from any location.
Simply find your location on the map, see which lines cross and read off the required values.
Make sure you don't get confused with the overlapping coverage areas.
Once you have your antenna assembled you need to see if you can hear any signals.
Turn the squelch off and use narrow band fm or ssb and with tuning steps of 1KHz to initially find the signal, you may well find that your receiver is slightly off frequency and that you have to tune with a slight frequency offset from the nominal 25KHz channel centre frequencies. As an example my AOR 3000 tunes 8KHz low, and a friends IC-R7000 tunes 6KHz high of the actual frequency.
Once you have found a signal start tuning in 25KHz steps and optimise the antenna for the best signals.
The actual transmissions use companded fm to obtain the best signal to noise ratio and to minimise satellite power consumption. This means that the signal level tends to reduce when there is no speech present. Correctly setting your squelch control is critical to obtain good results.
Most voice traffic on AOR-E can be found between 1537-1539MHz and 1540-1544MHz. The rest of the channels tend to carry high speed data or digital modes used by later generation mobile terminals.
Unfortunately the American Forces Broadcasting Network which used to have a continuous programme running on 1537.00MHz broadcast from AOR-E have now ceased using Inmarsat and reverted back to HF radio
Dramatic improvements can be made by adding a pre-amp directly at the feed, especially if long coax cables are required to connect to you receiver.
The cheapest method I have so far found is to use the guts from a 'Lowe' (or Watson GPS-150) GPS antenna by removing the inbuilt bandpass filter and connecting the helical feed in place of the inbuilt patch antenna (which has the wrong polarisation in this application).
This provides as good a performance as a purpose built GasFet pre-amp (also OK for meteosat and 23cms) and has a lower noise figure than could be obtained by using the i.f. amplifier stages from a scrap satellite LNB.
The unit needs a 5 volt power feed via the coax and I use a satellite TV D.C. injector to supply it.
You can add an on board 5 volt regulator if you wish to supply it from 12 volts or use it with a satellite TV set top box for Amateur TV reception on 23cms.
Inmarsat themselves and some useful data !
Receivers from AOR
Receivers from ICOM
What other people have heard (V. Good)
Paul Marsh's Inmarsat Pages (Brilliant)
Another way of receiving Inmarsat from Timestep
Commercial preamps and antennas from Swagur
Some 1.4GHz components which may be of use
A commercial receive system and G3 Fax decoder
Some other pages
Inmarsat 15.5 West recent channel usage log