Originally published in Electron April 2000
Bluecap lnb as 10 GHz tx PA3GCO

Introduction
... If you're a regular visitor of radio fleemarkets, you've surely noticed the growing number of second hand lnb's being offered. One of them is called Bluecap. Seeing the picture below, its name is no surprise. Ten years ago they were sold with nearly every satellite dish, especially in the UK. Nowadays you can buy them for the price of a glass of beer.

In the area where I live, they're used as transmitters for atv, 76K8 packet radio en wideband hi-fi audio.

I realize it's not a new technique: Several times it has been described how to convert this lnb ino a transmitter. Yet it appears to be difficult to complete this conversion successfully. Many people with two left hands, like myself, need additional hints from those who've done it before.

 

... ...
...
... ... A friend of mine, Marco Merks PE1PUW, gave useful hints which resulted in several new 10 GHz stations in my area.

Marco doesn't like writing stories. That's why I'm doing it, to increase activities on 10 GHz.
 

...
.
....
.
Background information
...
The Bluecap lnb was designed to receive satellite televisions, downconverting signals between 11 and 12 GHz. The lnb has a local oscillator, a dro on 10 GHz. Input signals are converted to the range 1 to 2 GHz where a standard satellite tuner can handle them. The lnb has two separate input stages, one for horizontal and one for vertical polarization, T1H and T1V in the diagram. By selecting the right input stage you can choose the desired polarization. T2 and T3 further amplify the signal. After a 11-12 GHz bandpass there's a notch to prevent local oscillator radiation leaving the lnb. Then there's the mixer, after which the resulting signal is amplified in the if-amplifier, needed to compensate the coaxial loss between lnb and satellite tuner.

Click here for the functional diagrams in PDF.

It's not difficult to convert this lnb into a 10 GHz atv transmitter. Usually amateurs use horizontal polarization in the 10 GHz band. Therefore the vertical part is not used. Transistor T1V is spare. The amplifier stages are rotated 180 degrees. T3 is swapped with T1H. Bandpass and notch are shortened, the mixer is replaced by a condensator. Where the if-amplifier used to be, a new simple modulator is built. Via this new modulator both supply voltage and baseband signal are applied. So only one cable is needed.

The modifications are described step by step. After each step, there's a check. If the result isn't satisfying, it's clear which step is causing the problem.

 

...
...
...
.
Necessary tools
.
Small soldering-iron, 15 W
Large soldering-iron, 80 W
Drill
Scalpel
Pair of tweezers, made of synthetic material
Voltmeter
10 GHz diode-detector
10 GHz rx station (around 10.45 GHz)
...
.
...
...
.
Preparations
...
There are several different types of Bluecaps. The modification in this document is for one particular type only. This doesn't mean that other Bluecap types can't be converted. It only means more effort is needed for those other types. I don't discuss other types, I can't do everything! After opening the lnb, you can see which type you've got.
...
...
...
...
...
...
It's important the lnb is still functioning. Check it by receiving one of the Astra satellites. If your Bluecap is receiving well, it means all vital parts, like oscillator and amplifier stages, are still ok.
...
...
...
...
.
Modification itself
...
Verify the lnb receives correctly.

Remove the four blind rivets with a 3 mm drill.

Remove the outside cover, it can be luted as well.
 

... ...
...
... ...
Now the inside cover comes in reach.
Remove the eleven screws in the inside cover.
Check if you have the "right" type, like mentioned above.
Remove the srew through the voltage stabilizer 7805.
Desolder the connection between F connector and pcb.
Now the pcb lies loose. Although there may be some glue under the pcb near the oscillator, it must be possible to take the pcb out now.
...
...
...
...
...
Write down what the four transistors T1H, T1V, T2 en T3 look like. Often they've got a dot or a line in a particular color so you can tell which is which.

Mark the gate on every transistor housing, using a pencil.

Now take the large soldering-iron. Coat the bottom side of the pcb with tin-solder, exactly UNDER the four transistors. Place the soldering-iron back in its holder so you have both hands free. Now heat the pcb from below by pressing it against the soldering-iron, exactly on those spots you've coated with tin-solder before.
 

... ...
...
...
The tin-solder on the topside of the pcb will melt and the transistor you've chosen will float in its own tin-solder. Pick the transistor up with a pair of tweezers and place it on a piece of aluminum foil.
This way all four transistors can be removed without damaging them.

The corner of the pcb with the if amplifier stages must be made empty. This is done the same way: Heat the pcb from below and just sweep the parts of.

The empty space is used for some new parts. They make it possible to apply voltage as well as modulating signal via the same F connector, so you only need one cable between shack and transmitter. Schematics available in PDF.

Remove the three-pin mixer. Place a 4.7 pF smd condensator at the empty spot.

Remove the 10 GHz notch with a scalpel.

Remove the bandpass filter with a scalpel and replace it with a brass foil strip, 1.2 mm width and 0.05 mm thick. Use some glue to position it correctly and coat it with solder-tin afterwards.

Solder 4.7 pF smd condensators at the positions where T3, T2 and T1H where placed before. Now the output of the oscillator can reach the horizontal antenna in the feedhorn. The signal travels via the brass foil and the 4.7 pF smd condensators.

Place the pcb back in its housing. Connect the F connector and place back the inside cover.

Apply voltage to the F connector: Between 12 and 15 V. Use your 10 GHz rx equipment to see where the oscillator is transmitting. Adjust the screw above the puck to change the frequency. Turning it clockwise results in the frequency going up. It appears you have maximum output power between 10.4 and 10.5 GHz, so make sure it's in this frequency range.

Connect your 10 GHz diode detector to a voltmeter. Place it against the blue cap of the lnb in such a way that the polarization matches. Although the amplifier stages aren't placed yet, you already measure a little output power. Not much, but you see the needle moving. Find the position where the meter shows the highest voltage.
Write down what the meter shows. You need this information later!

Now take the pcb out again and concentrate on the position where T3 used to be, so the first stage after the oscillator in the 10 GHz transmitter. Remove the 4.7 pF smd condensator and place T1H or T1V at this position. Notice that the gate of the transistor points towards the oscillator, so exactly the opposite of the original situation: Gate and drain are switched. Via a track the drain is connected to an smd condensator and an smd resistor. The same goes for the gate. Swap both resistors and swap both capacitors. Cut the track that goes to the drain, cut the track to the gate. Now make a cross connection. Study the following pictures.
 

...
...
Original
... Cross connection
...
...
Near the drain is a stub, a small track on the pcb. Make this stub 2 mm longer by soldering brass foil on it.

Place the pcb back in its housing and check the output power. It must be more this time. Write down what you measure this time.

Remove the pcb and concentrate on the next amplifier stage. T2 used to be there, now replaced by an smd condensator. Remove the smd condensator you've placed there before and place back T2. Notice that the gate points towards the oscillator this time, so exactly opposite to the original situation. Swap the smd resistors on gate and drain, the same for the smd condensators on gate and drain. Make the drain stub 2 mm longer using brass foil. Make a cross connection like you've done before.

Place the pcb back in its housing and check the output power. Again it must be more than before. Write down what you measure.

I'm sure you've read this before: Remove the pcb and concentrate on the position where T1H used to be. Now there's a 4.7 pF smd condensator, placed by you. Remove this capacitor and place back T3, with its gate towards the oscillator. Attention: Don't swap the smd condensators of gate and drain this time. Remove both smd resistors. The one that used to be at the drain is placed back at the gate. The drain gets a new 10 Ohm smd resistor. Make the cross connection for this stage.

To make sure the final stage always has a negative voltage on its gate, two smd resistors have to be removed: Rh in the photo. Both resistors are 160 kOhm. There's "164" printed on top of them.

 

...
...
...
...
Place the pcb back and verify that the output power has increased again. Get maximum power by adjusting the three smd variable resistors. After closing the lnb with four M3 bolts and nuts your transmitter is ready!
...
...
...
...
Parts list


Number Part
4 condensator 4.7 pF smd
1 elco 10uF/16V
2 elco 100uF/16V
1 resistor 10 Ohm smd
2 resistor 47 Ohm
1 resistor 180 Ohm smd
1 resistor 470 Ohm smd
1 transistor BC547B
1 rf choke 3 mm with wire
1 brass foil 0.05 mm
1 Bluecap lnb
.
Closing remarks and hints
...
Your Bluecap transmitter uses horizontal polarization. If you want vertical polarization, there are two possibilities: Use the vertical antenna of the Bluecap instead of the horizontal one. In that case the last transistor T3 is placed at the position where T1V used to be. A more easy way is to turn the whole transmitter over 90 degrees.

It's possible you can't make enough deviation, in spite of the fact you're applying a lot of baseband signal. In that case you can experiment with the position of the oscillator puck.

If you have a simple spectrum analyzer up to 500 MHz, use an original Bluecap as an extra converter. This way you get the range 10.0-10.5 GHz and you can see your own transmitter.

Several Bluecaps have been converted yet. Output power is about 35 mW. More output power is possible: See PE1MMI's high power Bluecaps.

In my area Bluecaps are used to transmit 76K8 G3RUH compatible packet radio on a subcarrier that can be modulated from a few Hz up. It's also possible to apply the packet signal directly by connecting the lf output of the modem to the transmitter. Problems may occur because modulating signal and voltage travel over the same cable. In that case you should use separate cables for lf and voltage.

The pictures show that the track to the not used horizontal antenna is cut. This is not necessary. It doesn't result in extra output power via the horizontal antenna. You can only find this out by trying, that's why the track is cut in the picture!

The transmitter can be positioned in every commercial 10 GHz dish. Without dish is also possible, which can be seen in the first picture of this document. Those transmitters carry a 76K8 packet radio link over 20 km distance. The receiving side uses a 30 cm dish. The transmitters are 35 m above ground level on top of a 9 story building. The receiving site, 20 km away, has its 30 cm dish 24 m above the ground.

With a Bluecap transmitter also in a 30 cm dish we make qso's over 40 km distance, from home to home. Both tx and rx dish 15 m above the ground. Nice to live in a country that's so flat!

Many thanks to Twan van der Meer, PE1NHL, for making several of the pictures. Also thanks to Elmar Leinen, DL5OBW, for sending improved pictures.
 

...
Back
www.pa3gco.com