QO-100 Experiments – LNB Modifications

I have been using a Goobay 67269 LNB with a small Horn antenna on 10368MHz for a while to monitor beacons via rain-scatter. This was a fairly basic setup, the Chaparral horn had been cut-off and the LNB mated to a length of 22mm copper pipe that had a WG16 flange fitted at one end. The LNB had a cover made from an old plastic milk bottle that provided protection from the rain and some limited shielding from the effects of temperature change from the wind and the Sun. This was clamped to the cross-boom of my 144MHz EME array so I had control of where the horn pointed both in azimuth and elevation. A NooElec SDR Dongle and SDR-Console were used at the shack end. This simple setup worked surprisingly well, once the LNB frequency drift settled. This frequency drift was most apparent at initial power-on and when the ambient outside temperature changed.

As I was using round pipe and 3D-printed clamps it was also easy to rotate the entire horn and LNB assembly for either V or H polarisation. This enabled me to switch to V polarisation and listen to the QO-100 satellite. I found another “hidden” bonus from this setup, I could use the satellite to check and confirm the alignment of my EME array.

For serious monitoring of beacons etc. the LNB ideally needs to be modified to overcome the inherent drift associated with the standard onboard Xtal oscillator. Like most modern PLL LNB units the Goobay uses a 25MHz Xtal. This can either be removed and the 25MHz derived from an improved source (external or onboard TCXO) or it can be left in situ and injection-locked using an external 25MHz source. There are many blog posts that describe the various ways of doing this. Two that I followed were by Andy, G4JNT and John, G4BAO. The Goobay single port LNB has lots of space around the Xtal making it easy to work on.

Here’s my modified Goobay LNB:

I recently purchased an Octagon OTLG Green twin-port LNB to use with my QO-100 system. Like the Goobay LNB I chose to use injection-locking using an external 25MHz source derived from my shack G3RUH 10MHz GPSDO. This 25MHz source will be described later in another blog post. I did not want to use one of existing twin-ports to inject the 25MHz so I drilled a small hole in the back cover directly above the Xtal solder pad. I then soldered a short length of PTFE insulated wire to the Xtal pad and that passes through the hole in the cover. A second, hole was drilled and tapped in the cover and this is used to secure a small piece of copper clad board. This board provides a ground plane for the 1k + 10nF used for the injection-locking plus a 20dB attenuator. The external 25MHz source is fed to this LNB board using a separate coax lead. All of this fits nicely on the inside of the LNB cover.

 

One small tip should you modify the Octagon LNB, the metal cover has 5 screws holding it in place and 3 different length screws are used. Make a note of the screw / position when taking the cover off.

With the frequency stability issues resolved the LNB unit combined with an SDR dongle and software make a relatively low-cost receiver that’s excellent for monitoring the QO-100 satellite or 10GHz microwave band.

I recently read an excellent blog post by Bob, KA1GT where he describes a low cost 10GHz EME receive setup using an LNB. He describes a number of ways to optimise the LNB. In addition to addressing the usual frequency stability issues he wrote about tuning the LNB so it’s optimised for use at ~10.4GHz rather than something like the 11.7GHz or so that the typical LNB is designed for. This optimisation is done by tuning the LNB probe(s) using a screw through the wall of the LNB housing directly opposite the probe. I decided that I would try this on a second Goobay LNB I had available in the spares box.

Here are a few photos showing the tuning screws. I used M2.5 brass screws. Unfortunately I didn’t have any brass nuts available so it’s a bit of a mix of materials which is not ideal.

 

A quick test using a low power 10368MHz signal source in the shack and the LNA connected to my spectrum analyser (displaying the peak at 618MHz) resulted in a noticable improvement in peak signal level as the LNB tuning was optimised by adjusting how far the screw penetrated into the housing. The next test is to set the LNB up outside on a tripod and recheck the measurements, but this time peaking on Sun noise. Even with the very basic test the results indicate that this simple modification optimises the LNB for 10GHz operation / QO-100.

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