UHF-Satcom generally refers to frequencies between 240MHz and 270MHz. To receive it, you'll need a reasonable antenna, perhaps a LNA, and a receiver that covers the relevant frequency range. To get an idea of what sort of antenna you might like to build, check the uhf-satcom equipment gallery which has pictures submitted by readers of this site.
Generally you will not hear much clear audio from legitimate users, although there are many pirate users on the satellite, mostly from Brazil and Portugal. Occasionally you hear interesting transmissions, but the really interesting stuff is that which is unintentionally relayed. Some orbiting satellites use UHF for downlinks too, in particular the Russian Gonets / Strella satellites. The new British Skynet 5 satellites, in line with many other X-Band satellites, also have UHF transponders; this includes the MUOS constellation which supports both legacy UHF and new CDMA transponders.
Details on the experimental TacSat-4 LEO can be found here
The frequency list above shows all Atlantic and Indian Ocean transponders that have been discovered through off-air monitoring.
Thanks go to xBr, blh, Freq, pjm, Nils, Maik, Gonzo, muf, Herrmann_s and other hearsat'ers for input to this list.
Your antenna is the most important part of the reception system - you have to be able to efficiently collect the signals in order to feed them to your preamp or directly to your scanner. UHF-Satcom transmissions tend to be right hand circular in polarity, and as such the most efficient antenna is a helical. It is possible to use linearly polarised antennas and suffer the 3dB loss in signal. Typically, DAB antennas are freely available and operate in the 220MHz - 240MHz so just under the band we are interested in. They do however work reasonably well.
(above) UHF-Satcom helical antenna.
The picture above shows the second version of the satcom helical. The boom is made from 65mm square plastic drainpipe, the helix supports are made from 10mm diameter plastic rod, all easily obtainable from your local DIY store. The plastic rod is drilled in one end at the centre, and at the other end drilled across the diameter. This rod is used to support the helical element. The helix element is held in place with a cable tie.
(below) Close up of reflector mounting scheme.
As depicted in the picture, the plastic boom goes through a square hole cut in a 300mm diameter aluminium disc which has 8 sets of holes drilled to support the radials.
The disc is mounted on the boom with four right-angle brackets. The wire mesh is attached to the back of the radials with cable ties. The matching section is the usual 1/4 wave transformer, which in this case has been made adjustable in order to get the lowest SWR. The diameter of the back plane should be in the order of 1 wavelength.
The next pictures are close-ups of the backplane, but concentrating on the matching section. The end of the helical is soldered to the centre pin of an N or other suitable connector. The outer / ground of the connector should be connected directly to the backplane.
(above) Helix matching section.
The matching section is made from tin-plate and is cut to be a quarter of a turn, about 60mm wide. It's soldered or bolted to the ground plane at the connector end, and supported by an adjustment screw at the other end. Other matching methods are possible, including bending the first 1/4 turn of the helical such that it runs parallel to the backplane, or using an external coaxial matching transformer. Note: When looking at the pictures, please take note that the matching section is extremely reflective, and as such may deceive you!
Using an antenna analyser, it's possible to get the match very close to 50ohms, thus presenting a good match to your preamplifier.
(above) UHF Helical Dimensions.
Yagi antenna design for 260MHz.
This antenna was designed for 260MHz by scaling a common NBS yagi. The design is based on the NBS standard 6 element yagi, and if built correctly will provide good results. The diagram below shows the dimensions for the antenna. The boom is made from 20mm UPVC plastic tubing. The elements are made from 8mm micro bore copper tubing and the cutting lengths are indicated below. When the elements have been cut to length, mark them at the centre and also 10mm either side of the centre - This will help when fixing the elements to the boom and aligning them to the centre. Drill holes in the boom for the elements, and insert the elements. Once in position, use 2 pairs of pliers either side of the boom to crimp the copper tube into an oval shape, thus keeping it tightly fixed in the boom hole.
(above) UHF Yagi Dimensions.
All element spacing are taken from the reflector.
- Reflector Datum.
- Driven element 23.5cm from reflector
- Director 4 52.9cm from reflector
- Director 3 82.3cm from reflector
- Director 2 111.7cm from reflector
- Director 1 141.1cm from reflector
Yagi Design for 278MHz Meridian downlinks
On the 2nd of December 2010, 'skyking' reported to the Hearsat-L mailing list that around 278MHz, US air traffic control was being heard whilst beaming North from the UK. After some coordinated monitoring of the downlinks, members of the #Hearsat IRC channel determined that the most likely source of the signal was the Meridian Russian military communication satellites that were built as a replacement series for the Molniya satellites. A suitable antenna for 278MHz reception is described below, it is a scaled version of the WA5VJB 'cheap' Yagi originally designed for 222MHz ham radio operation.
(above) 278MHz Yagi.
|Element||Reflector||Driven||Director 1||Director 2||Director 3||Director 4|
From regular observations of the Meridian-2 UHF downlink, it has been noted that during the satellites ascent out to apogee, the transponder is activated when the satellite reaches about 61 degrees North, it then continues to transmit out to apogee, and on its descending leg, until around 36 degrees North when the transponder turns off. A good variety of traffic has been heard relayed through the satellite, including radio station 'studio to transmitter' links, Air Traffic Control centres, and various FM communications links.
(above) Meridian UHF transponders.
GONETS / STRELLA Store & Forward
Gonets is a Russian low earth orbit, store and forward data communications system. It uses satellites which downlink in the 259.5-265.2 MHz, 387-390 MHz and 1541.0-1541.9 MHz. Uplinks are (from sources on the Internet) 312-315 MHz and 1624.5-1643.4 MHz. The satellite orbits are 1,400 km high inclined at 82.5 degrees, with only some of the satellites carry digital store and forward payloads. Data rates are 2.4 Kbps for the uplink and 9.6 Kbps for the downlink, with GMSK modulation. Strella satellites are basically the military equivalent of Gonets.
Gonets and Strella transmissions are relatively short and finding them is difficult. #hearsat members use SDR-14 receivers to watch 1MHz of spectrum in order to locate the transmission bursts from the satellites, these bursts when out of range of a ground station are 300-500mS long. The pictures below show two waterfall plots of the frequencies in use by Gonets, as can be seen, the bursts are short, but the GMSK modulation sidebands can clearly be seen. There is no doubt that without the SDR-14 these signals would take a lot longer to locate. Even with a commercial spectrum analyser, because the lack of waterfall display functions, the signals would only appear for less that one second, and would be impossible to catch.
(above & below) Typical UHF bursts from Gonets / Strella satellites.
(below) User terminals for Gonets. On the left is a typical antenna, on the right is the modem interface unit.
Rasad-1 Iranian Satellite
RASAD-1 (Catalogue # 37675) is the latest satellite launched by Iran on the 15th of June 2011 at about 0914z. The satellite is in a low earth orbit, approximately 236Km x 299Km. The satellite has 2 UHF downlinks, 401MHz and 465MHz. To date, only the 465MHz downlink has been heard at good signal strength, as per the FFT below;
(above) RASAD-1 UHF Telemetry.
The 465MHz telemetry downlink is not a continuous signal, but has been observed at various ground stations around the world. Media reports on this Iranian satellite speculate that the satellite has image acquisition capabilities on board, if this really is the case, there may be another unknown downlink for high rate image data.
Here is an example of the UHF telemetry downlink:
(below) Two pictures from the FARS news agency showing the Rasad-1 satellite.
Further info on the telemetry format can be found at http://www.r00t.cz/Sats/Rasad
Old Molniya 3-x 1GHz downlinks
The picture below shows the basic design for a 1GHz LHCP helical that is suitable to feed a dish antenna for RHCP Molniya reception. You'll need to make a supporting arm to hold the actual helix, as having it dangling in free space isn't pretty. (see pictures below). The ground plane for the helical can be a tin lid of about 30cm diameter.
(above) 1GHz helical for Molniya 1- and 3-.
Since many of the Molniya 1- and 3- series have been monitored, the table below shows the operational status of each of the recent satellites:
Satellite Name Status Frequency band Molniya 3-53 OK C-band Molniya 3-51 No signals - Molniya 3-50 TBA - Molniya 1-89 No signals - Molniya 1-90 OK UHF (possible local scatter from 1-92) Molniya 1-91 OK UHF (Strong DSSS 990.6MHz Molniya 1-92 TBA - Molniya 1-93 OK UHF (1000.45 and 992.45)
The last list of active frequencies found is as follows:
Molniya 1-91, 1-90, 1-89
Freq MHz Modulation Comment 991.083 DSSS 490KHz wide 992.443 Mux / Multi carrier tt&c? 992.454 Mux / Multi carrier tt&c? Molniya 1-92 994.306 PSK (New 19/09/2006 Molniya 1-91 possibly) 996.013 framed PSK 996.037 2PSK 996.133 PSK 996.193 PSK 996.313 PSK 996.373 2PSK 996.413 PSK 996.533 PSK 996.593 PSK 996.653 2PSK 996.785 PSK 997.033 2PSK 999.775 Framed PSK (New 19/09/2006 Molniya 1-91 possibly) 1000.444 Mux / Multi carrier tt&c? 1000.450 Mux / Multi carrier tt&c? Molniya 1-92
(above) Molniya PSK downlinks.
The FFT's below show plots of the strong 'telemetry' that was observed from each satellite. Left FFT is 1000.450MHz and right FFT is 992.454MHz.
(above) Molniya analogue telemetry.
This is an audio recording of the above analogue telemetry: