10Ghz transmitter from standard satellite LNB.

[dr pepper]

I have been reading an article on how to convert an old marconi 'bluecap' lnb into a 10Ghz transmitter, only these are now long obsolete.
I have aquired a cheap LNB and pulled it apart, the technology unsurprisingly is now much smaller.
The regulator, tone LO decoder and fet biasing is now done inside one IC.
I was wondering whether I could do the same mod to this later LNB and convert it into a transmitter, so long as I can actually see to do the work.
The output of the local osc goes to the mixer, if I was to reroute this so that it goes through the amplifier mosfets back to the antenna after removing the bandpass and notch filters from the pcb I'd have a 10.7 Ghz transmitter (the freq of the LO), modulation comes from injecting audio or video onto the drain of the LO to create FM.
What I was thinking was whether this woudl work with the control IC generating bias for the fets, and whether I could replace the bias current programming resistor to up the current a little.
Anyone any ideas, and anyone who's messed with these your input welcome.
Heres a naff photo of a new LNB guts:
**broken link removed**

And heres a schematic, not my actual LNB but mines very similar, the device is a znbg4000, the reg is built in, the rest is pretty much similar:
**broken link removed**

 

[jiripolivka]

I know a number of people converted satellite LNBs into transmitters. I am only not sure how successful the modification was over time.
Modulating the oscillator to obtain FM is possible directly for NBFM. If you need a wideband FM like in satellite TV, I would rather advise to use a varicap. There are some small SMD varicaps than can be used at 10 GHz.
"Reversing" the LNAs to make them power amplifiers may be possible to get ~0 dBm; I am not sure if you can replace the low-noise transistors for some capable of a higher output.
If your receiver to which the "reworked" transmitter will send a FM signal, will be left unmodified, please remember that most LNBs are designed to keep their local oscillators tuned e.g. to 10.5 or 10.9 GHz. Their ceramic resonators are temperature-stabilized for these frequencies. You can easily tune them far away, but then this thermostabilization will not hold and the oscillator frequency would drift with temperature while receiver LO will not.

 

[dr pepper]

Interesting.
Yes I read somewhere that the supply voltage will only push the oscillator by a few hundred kc's, and that would mean that the receiving end would need afc to keep track as the tx osc could drift so much to lose reception (depending on the bandwidth of the rx).
I didnt know that thermal stability would be lost by adjusting the resonant freq of the oscillator puck, I did read somewhere that the frequency can be adjusted as well as increasing modualtion deviation by shifting the position of the puck on the board instead of 'bashing' it, my microwave experience doesnt go far enough to design an oscillator using a varactor, vhf maybe, microwaves are fairly new to me.
My intention was to use a 'universal' lnb one side at 10.7 Ghz LO tx and a 9.75Ghz lo rx at the other side of a test link, with a receiver connected tuned to 10.7 - 9.75Ghz therefore 850 megs, then once this works adjust the transmitter a bit at a time untill I get it to tx on the ham 10 Ghz band.

 

[jiripolivka]

I think your idea can work. I saw several similar attempts; certain worked and still do, judging by that such devices generate interference to satellite TV receivers and some of my friends failed to operate solar radio telescopes at ~11 GHz due to this interference )

The dielectric resonators in the oscillators are optimized to selected frequency at which the frequency holds when temperature varies from -30 to +70 deg.C. If tuned away by the mechanical piston tuner, the frequency stability worsens.
If you can install an AFC in receiver IF, this may still work.

At 10 GHz band, radio amateurs use similar front-end receivers like satellite LNBs, but the LO signal is rather generated by multiplication from a quartz oscillator somewhere ~ 30-80 MHz for stability. Also, radio amateurs prefer SSB or CW operation , when narrowing the bandwidth allows for DX operation with only several mW of RF power.
NBFM can work over a short range. Good luck!

 

[dr pepper]

Excellent I'll continue then.
Yes I dont want to generate interference for anyone, that is a concern, the reason for the device is to gain some experience in microwaves and if successfull as a security device for a building a few miles away, apparently a few mW can go this distance easily line of sight even without a dish, I dont know but aim to find out.
I originally intended on trying to cheat and leave all the devices in place just reroute the output of the lna string to the feedhorn and the input to the local osc, but I've decided against that now, I suspect if I do that I'll get feedback oscillation, so I'll unsolder all the fets and reverse them, I'll also need to reverse the bias supply connections.
For a RF detector probe I was thinking of a typical diode probe with a bas70 microwave diode and a scrap LNB with the guts removed and the probe cicruit connected to one of the antenna's.

 

[jiripolivka]

I hope you are successful with the modification. Satellite receivers are pretty sensitive (I used LNBs in radio telescopes), so it may be possible to use small horns as antennas, or, a small dish on one end only.
For RF detector I can recommend to install a diode in a half-wave dipole; instead of two chokes to a mAmeter, I use a quarter-wave section ending in a 1 nF ceramic capacitor.
I used the same structure as a noise "radiator", with an e-b diode of a RF SMD transistor fed with ~ 5 mA from a 9V battery (a series resistor ~ 1..2 kOhm).
You could start with the simplest experiment; add an IF amplifier with a detector after a LNB (I can recommend a Sat-Finder or an in-line amplifier, both have ~20 dB gain), then an audio amplifier and a speaker. Then you can modulate the local oscillator in an open LNB with ~1 kHz, and see how far such transmitter can be received by your receiver. Adding small horns can add 16-20 dB each, a small dish up to 35 dB. A selective amplifier can help, too. I am not sure how easy it is for you to down-convert the high IF down to FM radio range, and use FM to transmit. \Narrowing the bandwidth may help.

 

[dr pepper]

Yes now theres a thing, good bit of lateral thinking, a sat finder and standard LNB would give me a simple cheap ready built RF meter within the freq range of the LNB (there would be a null at the lo freq I presume), why didnt I think of that.
I'm not sure if its what you meant in your message but also AM modulation simplifies the receiving end at least for testing purposes, like you say a rf filter supplying an audio amp is a wide band am receiver, modulating the supply volts to the local osc is going to produce both am and fm so the transmitting end wouldnt need modifying for am or fm.
And yes I was thinking of using a dish at the receiving end for various reasons, aiming one of them, the tx would be on a 20 meter high silo which would be a help.
I just got a set of surgical style craft knifes so operation on the lnb will begin shortly.

 

[betwixt]

I did this but instead of reversing the FETs and bias connections, I cut the front end stages out of the PCB and physically reversed the whole section so the input went to the mixer filter and the output went to the dipole in the horn. With the filter adjusted to allow higher frequencies through and the IF stages bypassed, I sent a modulated 1.2GHz up the cable and had it radiate at 11.2GHz from the horn. I would not say it is easy and some PCB tracks still have to be rewired but it certainly worked.

Brian.

 

[dr pepper]

Yes I see what you mean, I think on my new LNB with a bit of looking into it, its going to be too tricky, the smd's are ridiculously small, the board is only 14mm by 35mm, so I'll get something a little older and larger to work on methinks.
I'm hoping on being able to apply data direct to a simple transistor modulator, similar to a couple of articles on the web have done.
The fact others have done it is inspiring.

 

[vfone]

Another cheap option with better performances is to use a frequency multiplier as HMC444 from Hittite (about $10).

https://www.hittite.com/content/documents/data_sheet/hmc444lp4.pdf

Providing a clean 1.296GHz at the input you get a good phase noise, +5dBm, and good stability signal at 10.368GHz, using just one IC.

 

[dr pepper]

An interesting circuit, looks all self contained, I could rob the bits from a satcan for a 1.2 Ghz source, or this Ic looks as though it'd so the job:
**broken link removed**
All looks traight forward 2 ic's and a few passives however my microstrip and microwave construction techniques arent good enough.
Neither is my soldering skill, I broke one of the legs of one the hemt's trying to get it off the board, scrap LNB.

 

[betwixt]

I found the easiest way to salvage HEMTs is to cut the PCB into a small square using scissors so it just surrounds the tips of the transistors legs. Then turn it upside down and hold the square in tweezers. As the underside of the board is usually copper ground plane, it will spread heat quite efficiently so place the tip of a soldering iron on it for a few seconds then gently tap the board on safe surface. The transistor will drop off undamaged.

Brian.

 

[dr pepper]

I'll keep the LNB I just wrecked then, as from what you said I could get another 4quid LNB the same type, chop it up and remove the fets and fit them to this one I busted the transistor on.
It'll get around worrying about esd on the one I have now too.
Swapping the bias voltage to the fets on the board looks fun, with the fets pointing the other way on the board the bias voltage will obviously be reversed, do you think I'd be able just to bend up the dc voltage pins on the smd's and swap them around back to the board with a bit of tinned copper wire?

 

[betwixt]

It was some time ago that I last did a conversion but if I remember correctly, I reversed the whole section of PCB then linked the original gate and drain supplies back to the uncut section where they originally came from. I used thin enameled wire, probably salvaged from a transformer winding to make the links. It worked afterward, in fact so well that I could 'feel' the radiation from the horn and it completely wiped out all reception on other 10GHz receivers in the room. Unfortunately, my spectrum analyzer only goes up to 2GHz and I do not have a microwave power meter so I couldn't tell you anything definitive about power output or purity. I can tell you that mounting it in a 35cm (ex Sky) dish sent the signals more than 50Km very easily.

Brian.

 

[dr pepper]

So theres a good chance it'll work then.
Better not look down the waveguide while its transmitting either.
Some geyser on the web, looks dutch used a marconi bluecap but replaced the last 2 fets before the aerial and theres a big fat stud diode where the IF used to be, and claimed 500mW, unfortunately theres only some photos, no description of what he did.
Linky:
Oude foto's PA3GCO

 

[betwixt]

That web page almost exactly shows what I did 10 years ago!
Incidentally, there are several other people experimenting with the same ideas in Lancs. A good place to look for discussion is at: **broken link removed**

Brian.

 

[samanthajones]

I have been searching unsuccessfully for a DBS feedhorn to use on a prime focus dish.
This is to be used with a single polarity LNB covering only the DBS-11.7 to 12.2ghz band.
They do exist in offset versions but cannot be used because of the different f/d ratio.

 

[betwixt]

The feedhorn is integral to most DBS LNBs so you may not need one. If you are sure you need one, are you looking for a scalar ring type or a regular conical horn?

Brian.

 

[jiripolivka]

Most LNBs are integrated with a choked small horn designed as the primary feed of an offset or prime-focus dish. The typical radiation pattern is ~ 90-110 deg., so you can use it with any suitable reflector.
I used many LNBs of various types as receivers for small radio telescopes, with 1-ft symmetric parabolic dish, 2-ft dish as well as with a 60x80 cm offset. All could be focused well, and I did not see any side lobes over -13-15 dB down from the main lobe.
Mostly I used the LNBs with a choked circular horn but other ones had a rectangular waveguide input. No problem with any one.
If you use it with a "transmitting" LNB, focusing is easier with a horn-detector. You can take the radiation pattern while adjusting the focus and reducing the side lobes.