LNB tests – indoors

With the bias tee providing 12V down the LNB feed it was time to set up and test things. My plan is to see if I can receive signals from Es’hail 2 with a really simple system, consisting of the 60cm dish plus LNB, an RTL-SDR dongle, and a Raspberry Pi running Gqrx.

I have already used the RTL-SDR plus Raspberry Pi setup to receive Funcube in the 144MHz band, so had the software already set up (fortunately I always document how I do stuff, otherwise I would never have remembered how to set it up!). Here’s a screenshot of the system looking at a test signal on the LNB:

Screen grab with Gqrx running

The first test was with the original LNB that came with the 60cm dish when I bought it as a complete satellite reception system in a B and Q offer many years ago, thinking that the dish might be useful. It will have a DRO local oscillator, so frequency accuracy and stability are not expected to be that good.

I tested that (a) it was receiving, and (b) how stable it is, using a little signal generator I made for my microwave tests – it’s a PLL synthesiser generator that gives out about 7dBm at 1296MHz. A little diode multiplier on the end of a bit of semirigid coax gives harmonics through the higher microwave bands, and the one at 10368MHz is useful here, as it will give output from the LNB at a nominal 618MHz, assuming the 9750MHz LO is accurate. Here’s the test signal source:

Harmonic generator on 1296MHz source

The base signal level is at about -50dB, compared to about -80dB with nothing plugged in to the RTL-SDR. The old LNB works fine, though it does drift a bit and the frequency is not that accurate. Here’s the gqrx screenshot:

Test signal received on the old LNB

After Brian’s talk I had also ordered a modern Octagon LNB (model OSLSO) with the PLL local oscillator. This also works fine, though initial drift is even more than the DRO based LNB. It does stabilise quite well after half an hour though. Here’s the screenshot after just a few minutes from switch on:

Test signal received on the Octagon PLL LNB

The frequency accuracy is better than that with the old LNB too. I also established, without taking it to bits, that the internal crystal oscillator is a 25MHz one – the signal is easy to receive on my FT-817!

For these tests the LNB was parked about 1m from the signal source, roughly pointing towards it. Doppler effects from moving around the room are clearly apparent.

All looking good! Next test will be out of doors, trying to find geostationary satellites.