At X-band, you will need a reasonable size dish (1.2m +), a circular polarisation feed, a reasonably good LNA (< 2dB NF) and a down converter to a suitable IF (250MHz-750MHz). It's worth checking out eBay as a lot of surplus mil-satcom gear is appearing, from entire radome units to down converter units and LNA's etc. For the down converter, a couple of choices exist, either find a second hand professional converter, or roll your own. Details are included below for both types.
The picture above shows a typical first generation home-brew down converter. The local oscillator runs at 7GHz, and is phase locked to a 100MHz source. This phase locking arrangement ensures that the 7GHz LO is pretty stable. The 7GHz LO is mixed with the incoming 7.x GHz signal to produce an IF in the DC-1GHz range, which is amplified by an Avantek gain block 7.25GHz = 250MHz IF, and 7.75GHz = 750MHz on the IF. Two Ferranti 7.x GHz LNA's are used (from eBay), one at the dish feed, and another immediately before the mixer. The LNA at the dish feed has had a new gaasfet fitted, lowering the noise figure somewhat. It is estimated that the original LNA's were manufactured in the mid-80's and probably have a noise figure in excess of 5 dB. The entire down converter runs from 24v, and using linear regulators and dc-dc converters the +20v, +12v, -12v and +5v supplies are generated.
The photos below shows a commercial pair of Comsat / Vertex RSI DCS7-101-2 (UCS8-101-1) converters originally costing many thousands of $. These take an X-band RF input and convert it down to a 70MHz IF with a bandwidth of approximately 42MHz. The tuning can be controlled in 1KHz steps, and each unit has a 10MHz ovened reference oscillator built in. Remote operation is also possible as the down converters use RS485 serial control - work is under way to make automated near real-time spectrum analysis of X-band satellites which will generate an online plot. Units of this type are often seen on eBay selling for $50 to $150. These converters are designed to be used with a variety of PSK and FHSS modems such as the Racal 2636 FHSS, VSC330 FHSS and the SLM6650.
The IF levels nominally operate at 1mW / 0dBm so need some attenuation before connecting to a receiver, as the quiescent noise level is high. The down converter has a fixed tuning range of 7250.000MHz to 7750.000MHz but it may be possible to modify this by changing the firmware (on going work). The converter at the top of the stack is a 7900.000MHz to 8400.000MHz version. Further information on these converters, including documentation, is available via the restricted access pages. Several members of #hearsat IRC are using these devices for X-band receive.
Vertex / Comsat RSI DCS7-101-2 Up-converter / Vertex / Comsat RSI DCS7-101-2 Down-converter
Pictured below is a typical commercial X-Band LNA. This type has particularly high gain, and in fact can deliver over 100mW at the output connector, should it see a strong signal at the input waveguide. The waveguide input feeds a 2-stage LNA, which is then filtered. The output of the filter feeds a 4-stage buffer amplifier. The buffer amplifier is protected each side with circulators, ensuring a 50 ohm match is always presented. The model shown is an 'EF-Data A46277' LNA, specified as 65dB gain between 7.25GHz and 7.75GHz. Its noise figure is around 1dB. Click on the pictures of the LNA's below to get high resolution versions. The LNA needs 12v to 15v to run and provides an alarm output in case of a failure.
RAF Oakhanger - Ground station segment for the UK Skynet series of satellites. Dishes probably pointing at Skynet 4E at 53 East and Skynet 4C/4F around due south. There is also another ground station at RAF Colerne. Another RAF satcom site used to exist at Defford - this has now been decommissioned. The local.live.com images have been found using image recognition software programmed to search the UK for satellite antennas. Work is on going to review the findings and where appropriate, links will be included on this page. RAF Colerne will be the prime support site for Skynet 5A at 53E, Skynet 5B at 1W and Skynet5C at 17.8W, whilst RAF Oakhanger will become a remote site. Further details of uplink / downlink sites, including photos, spectrum analyser plots of uplinks and other technical information is available on the restricted access pages.
(above) RAF Oakhanger satellite ground station. The two large dishes are for operational use, and the smaller dish is for quicklook operations.
(above) VSC504 man-pack portable satcom terminal being tested.
(above) RAF Oakhanger satellite ground station TCS site. Far right steerable dish in dome, and steerable UHF-Satcom array of helicals.
(above) Talon tactical 1.85M X-band terminal
(above) Reacher 2.5M antenna on show at Brize Norton open day
NAVY SHF Terminals - some photographs of the well known Astrium SCOT (Satellite Communications Onboard Terminal) system.
(above) This is the SCOT below deck equipment. In this rack is the motor control system and the RF amplifiers and up / down converters. This rack takes a feed from the accelerometers and synchro resolvers mounted on the dish, and feeds corrections to keep the dish on satellite
(above) This is what a typical SCOT radome looks like. Its fed with waveguide from the below deck equipment, along with DC feeds and 110V for internal heaters etc. The radome is fibreglass and is sealed to protect the electronics from salt water corrosion.
(above) The business end of a SCOT system, this is what is inside the radome pictured above. On the side of the radome is an access door that you can open, there is just enough room to get inside. The dish is on an az/el/roll mount that is electrically controlled.
LXA7550 X-Band LNA
Some very nice milsatcom equipment has been appearing on eBay recently, including almost state of the art LNA's and down converters. Both units are made by Vertex RSI, the LNA is model LXA7550.0001 and LXA7550.0008, and the down converter is model TR0700-02. This particular LNA is centred on 7.5GHz with a bandwidth of +- 250MHz, a noise figure of 50K (0.7dB NF) and a gain of around 50dB. Both the LNA and down converter run from a 12v to 24v DC supply and are really super additions to your X-Band monitoring system. These units were originally used with the TSC-503 transportable satellite communication terminal.
Low Noise Amplifier
For the power supply input, the LNA uses a 6 pin connector as can be seen above. The pin outs are shown on the left. 12V DC is really the minimum acceptable voltage, any lower and the LNA shuts down. The LNA can also be powered up the coax with the same voltage.
The LNA draws 300mA at 12V. The LNA's LXA7550.0008 have the +20dBm output option installed, and are therefore suitable for use with very long cable runs between the LNA and the block down converter. A data sheet for the LNA is available here.
Block Down Converter
The down converter made by Vertex RSI, model TR0700-02, uses a 4 pin connector for power input. The pin assignments have been determined as;
- A - 12V to 24V DC input
- B - NC or Fault output
- C - ground
- D - ground
The down converter has an IF output of 950MHz to 1450MHz (L-Band). It is suitable for direct connection to satellite modems operating in this band. A reference signal of 50MHz needs to be fed into the L-Band output connector to lock the internal 6.3GHz oscillator. The beacon receiver output level is approximately 20dB below that of the main L-Band output. The converter draws 380mA in an unlocked state, and 520mA when phase locked.
So far, a data sheet has not been found for the block down converter - if you have one, please send uhf-satcom.com a copy! If you have additional info on these parts, please let us know!
Block Up Converter
X-Band LNA Mods
Surplus X-Band low noise amplifiers have appeared on the market in the UK, having been removed from Racal VSC501 type down converters. The converters have a waveguide input, a tuneable oscillator inside, and an IF output, all running from 24V DC. The amplifiers in an unmodified state cover 7250MHz to 7750MHz. Modified, they have a useful gain from 5.8GHz to 9GHz, and so may be ideal for DSN operations at 8.4GHz.
The X-Band LNA's are made by Ferranti, part number 5820-99-744-0560. The amplifiers are solidly built and housed in a machined aluminium casing which is of sturdy construction. The amplifier needs +15V and -15V DC supply, but with a simple modification, it can be made to run of +12V (see mod info below).
The X-Band RF output is an SMA socket adjacent to the waveguide input, and the output is driven with a pair of NEC9001 MESFET's. Power and control is via a 7 pin 'mil' connector.
A = +15V input B = Ground C = -15V input D = +5V output E = Possibly a 'fault' output F = No connection G = No connection
In the above picture, a DC-DC converter has been added, along with a 7805 regulator. Measuring the FET supply rails, it was found that +12V provided sufficient voltage to correctly bias the positive rails, and -9V from the DC-DC was sufficient for the negative voltage rails. The picture below shows the modified LNA. The 1st band pass filter has been removed and a SMA socket installed, so as to take advantage of the broad band front end. Since we are not interested in using these LNA's in conjunction with a transmitter, the out of band filtering is less important. The other modification performed was to remove the FET drain power supply lines for those device in the output chain, this dropped the LNA's power consumption by 100mA. Note that the 'plugs' in the metal casing need to be removed to allow access to the hex socket cap screws that hold the band pass filter in place - this is done by chipping away the paint, then digging out the filler from the plugs and unscrewing them.
A short test was conducted to check the useful gain over the 6GHz to 9GHz band, with the following results;
- Freq GHz 6 6.5 7 7.5 8 8.5 9
- Gain 42 50 50 45 43 37 20
It should be noted that the above gain figures are for the modified LNA with the RF output taken after the initial 3 gain stages, as per the above picture.
The band pass filter that was removed from the LNA has excellent pass band characteristics, allowing 7250MHz to 7750MHz to pass. These filters are therefore perfect for integration into your X-Band down converter unit in order to cover the milsatcom downlink band. These satellites which are mostly geostationary, make perfect dish alignment / calibration points. The following picture shows the BPF during the modification process;
The holes previously used to mate the filter unit to the LNA are ideally sized to be tapped with an M2.5 tap. This is what has been done to the unit in the above picture, in order to screw the SMA socket securely in place. As can be seen, the other SMA socket has yet to be attached.
A simple mod can be performed to make the LNA run off a single +12V supply, this involves fitting a DC-DC converter into the PSU side of the LNA; -12V is fed to the end of C5 closest to the PCB edge, whilst +12V is present at C1, on the pin also closest to the edge of the PCB.
The modified LNA was tested with 'off air' signals from the Stereo-A spacecraft, which at the time was around 130 million miles away from Earth. Reasonable quality signals were received, meaning that this LNA is an ideal first step for those wanting to start experimenting with 8.4GHz reception. The LNA was modified as described above. The next picture below shows the LNA mounted on the dish. A simple transition was made to convert the waveguide LNA input to a circular 28mm waveguide (copper pipe) as used for the X-Band receive system.
FFT Screenshot showing Stereo-A carrier as received (Note: Frequency step is a retune, not spacecraft)
If you are interested in obtaining one of these LNA's, please contact UHF-Satcom.com who will gladly put you in touch with the seller. (The LNA's do not come from uhf-satcom.com - we are simply a very happy customer!)