10 GHz Dish Antenna Feed from StarChoice LNB by VE2JWH (Jim)

edited by VE3CVG (Rick)

This describes how to construct a 10GHz dish feed from a StarChoice satellite TVRO LNB commonly found in Canada. The LNB that we start with is a model #150284, manufactured by California Amplifier. At VE2JWH the feed is used with the 24" offset feed dish that it came attached to, but it could be used with other dishes.

Take your time and work carefully and you will make it through this project.

First cut off the round wave guide at the point that it forms into the rectangular box. Use a grinder or file to dress the wave guide to a nice round surface.

Take the round wave guide to a machine shop and get the wave guide bored to .762" inside diameter and have a shallow, tight-fitting aluminum plug manufactured. This plug should have a slight shoulder as shown in the diagram. It has to be a snug fit.

Drill a 1/4" hole into the wave guide at precisely .575" from the aluminum plug, to accept a press-fit 1/4" brass rod, 3/8" long. The brass rod is flush with the inner surface of the wave guide. Drill this brass rod to accept UT141 or RG142 coax.

In the round wave guide, opposite the hole for the brass rod, drill a hole and thread it for a 4-40 brass screw. The brass screw should be 1/2" long, and be threaded with a lock washer and a brass jam nut.

Prepare the probe by removing the outer jacket of the RG142 for about 1". Tin the outer braid. Score and remove the tinned braid, so that 1/2" braid remains on the probe. Carefully remove the dielectric off the 1/2" of protruding centre conductor. Trim this centre conductor (probe) to 5/16". Insert the RG142 probe into the drilled brass rod and solder so just the centre conductor of the probe projects into the wave guide. If you use UT141 - you should end up with a similar configuration.

The newly modified feed.

picture of new wave guide

Close-up of the Probe

picture of tinned RG142

10 GHz Rx Pre-amplifier from StarChoice LNB by VE2JWH (Jim)

edited by VE3CVG (Rick)

This describes how to construct a 10GHz pre-amp from a StarChoice satellite TVRO LNB commonly found in Canada. The LNB that we start with is a model #150284, manufactured by California Amplifier.

The first step is to measure voltages at points A and B (see drawing) so that later, resistors may be added to the DC-DC convertor to produce the same voltages at the test points. To measure these voltages, remove the board from the housing and apply +18vdc to the "F" connector. This will energize the RIGHT side pre-amp. Lowering the voltage feed to +14vdc will energize the LEFT side preamp.

The very next important step, before starting modifications, is to short the GaAS FET gate to ground with a jumper, to protect it during modifications (see drawing).

Carefully mark the preamp section (see drawing) with a marker and cut out with a Dremel tool (cutter disk). Avoid inhalation of the fiber dust.

Close-up of the Pre-amp area

picture of the preamp

Clean this up as necessary. Then solder in a .015 thick brass wall, 1/4" high, all the way around the GaAS FET area as shown in the drawing. Do not place a cover over this as it is not necessary and it could start to oscillate. Solder the brass to ground wherever possible.

Use UT141 through the brass for the link from the preamp to the relay (RF IN) from the antenna. Install a 3.5pf SM coupling cp between the GaAsFET and the centre of the UT141. At the connection point of the UT141 to the cap, the centre conductor should be as short as possible and the shield should continue as close as possible to the connection point of the centre pin. Think 10GHz RF.

For the output side of the pre-amp, use UT085 bent 90 degrees and soldered on both sides of the PC board. Be sure to cut the pc board so that the UT085 can be soldered for good grounding and mechanical strength.

Mechanical Considerations

picture of the preamp

Biasing the FET

The negative supply can be mounted as desired, but in this version, it was soldered to the back of the pre-amp. This way the preamp only needs to be supplied with 12 volts during receive.

Use the negative supply (inverter) from the original feed.

From the 12v input run a 680 ohm 1/8w resistor to the drain +ve on the pre-amp. When in receive mode, the voltage should be about +2.89v.

Use a simple 2 resistor voltage divider from the -4v to produce the -.65v to the gate -ve of the pre-amp.

With this voltage setup, the current drain is 14 ma.

picture of the preamp bias supply

REMOVE the protection jumper on the gate of the GaAsFET before applying power to the preamp.

This simple pre-amp brought the receiver noise figure from 12dB (original) on a Macom WhiteBox transverter, down to a very acceptable 4dB.

Good luck.

picture of the preamp rear picture of the preamp front


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This page was updated on February 5th, 2003