UHF-Satcom.com - Simple wideband LNA                                                                                                        (17/02/2012 22:30:30 -0000)

So, your interested in receiving some of the L-band satellite traffic, be it Inmarsat, the Meteosat weather satellites or World space, but you don't have a preamp to attach to your feed horn? Well if this is the case, get an old LNB and start modifying it! The LNB's in this article are Marconi types often found for next to nothing on eBay. An up-to-date LNB was modified along with one of the original single band Marconi's. Voltage switching LNB's are usually referred to as "Marconi" LNB's.

You will need to drill out the rivets that hold the case together. Do this with some care as LNB's are often odd shapes and hard to hold in the average home vice. Once you have the outer cover off, remove the inner cover so you can get at the PCB. The output from your L-Band antenna or feed horn needs to be fed into the LNB at the start of the IF amplifier chain. Generally, this is right after the 10Ghz DRO oscillator and mixer diode.

If you look at the pictures below, you can see a couple of exploded views of the tap points. It's important to remove the supply voltage to the DRO oscillator, and also to remove the mixer diodes. Doing this will prevent noise being down-converted and being added to your signal. Better still, remove the supply rails to all the 10Ghz side amplifiers, i.e. front end and buffer gasfets.

Usually, there are one or two surface mounted capacitors after the mixer, so in this case, remove the capacitor nearest the mixer, and feed the IF into the non mixer side of this cap. I.E. the side that goes to the rest of the IF amplifier chain. Try to use good quality coax for connection to the IF input. This means no RG58!. Instead, use UT141 semi rigid, and terminate it with a properly fitting type N socket. If no UT141 is available, then a short piece of PTFE coax will suffice. The amplified IF output will come down the coax cable normally connected to your satellite TV receiver. You will need to make a DC injector if you are going to use this preamp with your scanner. Using this preamp technique will result in a fairly low noise, but hi-gain solution. This has been used successfully to receive Meteosat and Inmarsat with small diameter dish antenna's. For the best results, you should ideally use a dedicated L-Band LNA, the output of which can be fed directly into this preamp.

Definitions:

LNA Is a Low Noise Amplifier - small signal amplification with minimum of noise.

LNB is a Low Noise Block-Down-Converter - This comprises of an LNA plus mixer and Local Oscillator. The most common form of LNB is that used in TVRO applications for converting a block from 11Ghz down to 1Ghz.

LNC is a Low Noise Converter - this is a mixer and local oscillator. Typically in the C-band days, an LNA was mounted at the dish focalpoint, and the 3Ghz signal was taken via coax to a LNC that was mounted behind the dish.