UHF-Satcom.com - X-Band receive hardware                                                                                  updated: (30/10/2008 11:23:40 -0000)

Introduction

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 on the left 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 8000.000MHz to 8500.000MHz 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 UCS7-101-2 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.

(below) 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.

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

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.

The business end of a SCOT system, this is what is inside the radome pictured to the left. On the side of the radome is an access door that you can open, there is probably just enough room to get inside. The dish is on an az/el/roll mount that is electrically controlled.