Working the AO-40 Satellite

Scott F. Migaldi K9PO@amsat.org

One of the most exciting satellites to be launched in the past few years is about to becoming even more usable. AO-40, which had some difficulties since it's launch, is entering a new phase that will allow for much easier access and reliable communications. Passes for AO-40 will typically last hours instead of the minutes that we are use to on other satellites. The coverage is phenomenal and it is possible to work Japan, South America, the whole of North America and part of the Atlantic all at once. It is not difficult to get started on AO-40 whether you are already using satellites or just thinking about it.

Uplink/Downlink

To operate AO-40 there are two paths for uplink and one for downlink. Uplink can take place either with 435 MHz or 1270 MHz. The modes that can be used are SSB, CW, and PSK-31. The 435 MHz uplink is the easiest for most amateurs to establish as most already are using that band as a link on other satellites and it is included as part of most VHF/UHF equipment today. 435 MHz uplink requires an output of about 200-400 watts ERP whereas the 1270 MHz uplink requires ERP higher than 1kW. It is very easy to obtain the 200-400 Watts of ERP with the 435 MHz uplink using most UHF transceivers and a modest gain antenna.

The downlink for AO-40 is 2.4 GHz. To receive one needs to acquire on of the many surplus down converters and mount it at the antenna to mix 2401 MHz down to a lower frequency, normally 2m. The antenna gain for these systems is typically around 24 dbi (linear polarization) or 21 dbic (circular polarization). The best results are with a preamp that has less than 1 dB NF, mounted at the antenna feed.

The dish feed should be optimized for minimum noise pickup, NOT maximum gain. This means that it is important to keep this away for 2.4GHz noise sources such as cordless phones and Wi-Fi wireless ether -net devices. There are many MMDS bbq dishes available that are adequate for use with the AO-40 downlink. These can be used as is with linear polarization or modified for circular polarization.

The use of a power inserter to power the preamp/downconverter via the coax cable is required. By down converting at the antenna to 2m the use of less exotic transmission cable can be utilized. RG-6 TV coax is fine between the downconverter and the radio as the output of the downconverter and LNA is quiet high and losses due to mismatch are negligible. It is important during the cool weather of northern Illinois that one warm up the downconverter, then establish converter frequency by finding the 2401.323 MHz beacon. After you have done the warm up then compute your uplink/downlink pair only after the downconverter frequency is known. For Mode U/S2: (TX frequency) + (RX Frequency) = 2836.987 MHz - (downconverter frequency). Stay at least 50 kHz away from the beacon when doing your initial testing.

S-Band receiving equipment

Hams normally do not have 2.4 GHz receiving equipment laying around the shack so some investment is required. With the demise of MMDS there is a plethora of surplus equipment to be had and some of it only requires small modifications such as a simple Xtal and filter change. It is possible to spend around $150 to have a respectable receiving system. If you have a VHF/UHF radio that can be used for SSB or CW this will be the only investment you need. The use of complex azimuth and elevation rotors are not needed because the satellite travels very slowly and the dishes are small and may be mounted on photo tripods for a temporary set up in the back yard or patio. A 435 MHz antenna can also be tripod mounted. The Arrow antenna for FM voice repeater satellites will work for AO-40. Some hams have even modified this antenna for circular polarization to increase the reliability for AO-40 operation. A listing of some of the 2.4 GHz receiving equipment is:

Radios

To Operate AO-40 one must have full-duplex capability to hear/tune in your downlink signal.

Some typical radio that are used are:

Suppliers of preamps and downconverters:

Satellite Schedule and Parameters:

Get information from http://www.amsat-dl.org/journal/adlj-p3d.htm

Hints for a successful transponder operation in mode U/S2 and L/S2 (from AMSAT-DL)

S2-Transponder passbands

U-uplink frequency (MHz) = 2836.990 - S2 downlink frequency

L1-uplink frequency (MHz) = 3670.706 - S2 downlink frequency

You must correct for Doppler shift and downconverter offset frequency.