I get quite a few emails from readers of my blog asking how my QO-100 satellite station is put together and so, I thought perhaps now is a good time to put together an article detailing the complete build.

My QO-100 satellite ground station is built around my little Icom IC-705 QRP transceiver, it’s a great little rig and is ideal for the purpose of driving a 2.4Ghz transverter/up-converter.

Of course all the software used for the project is Opensource and freely available on the internet.

The station comprises of the following building blocks:

• Icom IC-705 Transceiver
• DXPatrol 28/144/433Mhz to 2.4Ghz Up-Converter
• DXPatrol GPSDO Reference Oscillator
• DXPatrol 2.4Ghz 5/12w Amplifier
• Nolle Engineering 2.2 turn 2.4Ghz IceCone Helix Antenna
• 1.1m (110cm) Off-set Dish
• Bullseye 10Ghz LNB
• Bias-T to feed 12v to LNB
• NooElec SmartSDR Receiver
• PC Running Kubuntu Linux Operating System
• GQRX SDR Opensource Software
• Griffin Powermate USB VFO Knob
• QO-100 Ground Station Dashboard developed using Node-RED
• LMR400-UF/RG58 Coax Cable

To get a good clear view of the QO-100 satellite I have the dish mount 3.2m above the ground. This keeps it well clear of anyone walking past in the garden and beams the signal up at an angle of 26.2 degrees keeping well clear of neighbouring gardens.

The waterproof enclosure below the dish houses all the 2.4Ghz equipment so that the distance between the feed point and the amplifier are kept to a minimum.

The DXPatrol amplifier is spec’d to run at 28v/12w or 12v/5w, I found that running it at 28v produced too much output for the satellite and would cause the LEILA alarm on the satellite to trip constantly. Running the amp at 12v with a maximum of 5w output (average 2.5-3.5w) is more than enough for me to have a 5/9+10 signal on the transponder.

The large 1.1m dish gives me quite an advantage on receive enabling me to hear the very weak stations with ease compared to other stations.

The photo above shows the 2.4Ghz equipment mounted in the waterproof enclosure below the dish. This photo was taken during the initial build phase before I rewired it so, the amplifier is shown connected to the 28v feed. To rewire the amp to 12v was just a matter of removing the 28v converter and connecting the amp directly to the 12v feed instead. This reduced the output from a maximum of 12w down to a maximum of 5w giving a much better (considerate) level on the satellite.

It’s important to keep all interconnects as short as possible as at 2.4Ghz it is very easy to build up a lot of loss between devices.

For the connection from the IC-705 to the 2.4Ghz Up-Converter I used a 7m run of
LMR-400 coax cable. The IC-705 is set to put out just 300mW on 144Mhz up to the 2.4Ghz converter and so it’s important to use a good quality coax cable.

Once again the output from the 2.4Ghz amplifier uses 1.5m of LMR-400-UF coax cable to feed up to the 2.2 turn Icecone Helix Antenna mounted on the dish. This keeps loss to a minimum and is well worth the investment.

The receive path starts with a Bullseye LNB, this is a high gain LNB that is probably one of the best you could use for QO-100 operations. It’s fairly stable frequency wise but, does drift a little in the summer months with the high temperature changes but, overall it really is a very good LNB.

The 12v feed to the LNB is via the coax and is injected by the Bias-T device that is in the radio shack. This 12v feed powers the LNA and associated electronics in the LNB to provide a gain of 50-60dB.

From the Bias-T the coax comes down to the NooElec SmartSDR receiver. This is a really cheap SDR device (<£35 on Amazon) based on the RTL-SDR device but, it works incredibly well. I originally used a Funcube Dongle Pro+ for the receive side however, it really didn’t handle large signals very well and there was a lot of signal ghosting so, I swapped it out for the NooElec SDR and haven’t looked back since.

The NooElec SmartSDR is controlled via the excellent Opensource software GQRX SDR. I’ve been using GQRX SDR for some years now and it’s proven itself to be extremely stable and reliable with support for a good number of SDR devices.

To enhance the operation of the SDR device I have added a Griffin Powermate VFO knob to the build. This is an old USB device that I originally purchased to control my Flex3000 transceiver but, since I sold that many moons ago I decided to use it as a VFO knob in my QO-100 ground station. Details on how I got it working with the station are detailed in this blog article.

Having the need for full duplex operation on the satellite this complicates things when it comes to VFO tracking and general control of the two radios involved in the solution and so I set about creating a QO-100 Dashboard using the great Node-RED graphical programming environment to create a web app that simplifies the management of the entire setup.

The QO-100 Dashboard synchronises the transmit and receive VFO’s, enables split operation so that you can transmit and receive on different frequencies at the same time and a whole host of other things using very little code. Most of the functionality is created using standard Node-RED nodes. More info on Node-RED can be found on the Opensource.radio Wiki or from the menu’s above.

I’ll be publishing an article all about the QO-100 Dashboard in the very near future along with a downloadable flow file.

I’m extremely pleased with how well the ground station works and have had well in excess of 500 QSO’s on the QO-100 satellite over the last last year.

More soon …

I’ve been wanting to tidy up the cabling to the 12v DC PSU for some time in the radio shack as like many HAMs I have a number of radios/devices that all need a 12v feed but, only two connectors on the front of the PSU. The net result was a birds nest of wires all connected to the PSU making it impossible to disconnect one device without others getting disconnected at the same time.

Looking online I found that many of the HAM outlets stores sell nice little 12v DC distribution boxes that would be ideal however, they’re all priced somewhat high for what they are so, I decided to purchase the parts and make one myself.

Searching on Amazon I found all the necessary parts for less than a quarter of the cost of commercially made units. A couple of days later the parts arrived and sat on my desk in the shack for a few weeks. Yesterday I finally found the time to make a start on the project.

After much drilling and filing I had the necessary holes/slots cut in the plastic box for the 4mm connectors and fuse holders and started wiring them up. Part way through my 30 year old soldering iron decided to die and so I had to stop and wait for a replacement to arrive.

With the new soldering iron in hand it only took 30mins or so to complete all the joints and I soon had the box together ready to test with my multimeter to ensure I didn’t have any shorts or crossed wires.

With testing complete and fuses in place I connected it up to the PSU and then connected all the devices one by one checking for voltage drops as I went.

I now have my CG3000 remote auto ATU, GPSDO, QO-100 ground station and IC-705 all nicely connected in a much tidier fashion than before, all for considerably less than the commercially available alternatives.

More soon …

Fig 1: The Hammond 1590 aluminum case housing the FE-5860A rubidium osc- oscillator and other circuitry - the markings faded by time and heat. Click on the image for a larger version.

The first of these - my Efratom LP-101 - fired up just fine, despite having seen several years of inactivity.  After letting it warm up for a few hours I dialed it in against my HP Z3801 GPSDO and was able to get it to hold to better than 5E-11 without difficulty.

My other rubidium frequency reference - the FEI FE-5680A - was another matter:  At first, it seemed to power up just fine:  I was using my dual-trace oscilloscope, feeding the 'Z3801 into channel 1 and the '5680A into channel 2 and watching the waveforms "slide" past each other - and when they stop moving (or move very, very slow) then you know things are working properly:  See Figure 2, below, for an example of this.

That did happen for the '5680A - but only for a moment:  After a few 10s of seconds of the two waveforms being stationary with respect to each other, the waveform of the '5680A suddenly took off and the frequency started "searching" back and forth, reaching only as high as a few Hz below exactly 10 MHz and swinging well over 100 Hz below that.

My first thought was something along the lines of "Drat, the oven oscillator has drifted off frequency..."

Fig 2: Oscillogram showing the GPS reference (red) and the FE-5680A (yellow) 10 MHz signals atop each other.  Timing how long it takes for the two waveforms "slide" past each other (e.g. drift one whole cycle) allows long-term frequency measurement and comparison. Click on the image for a larger version.

As it turns out, that was exactly what had happened.

Note: 

 I've written a bit more about the aforementioned rubidium frequency references, and you can read about them in the links below:

• A portable 10 MHz Rubidium Frequency Refrence using the Efratom LPRO-101  - LINK 

• A portable 10 MHz Rubidium Frequency Reference using the FE-5680A - LINK.
  

Oscillator out of range

While it is the "physics package" (the tube with the rubidium magic inside) that determines the ultimate frequency (6834683612 Hz, to be precise) it is not the physics package that generates this frequency, but rather another oscillator (or oscillators) that produce energy at that 6.834682612 GHz frequency, inject it into the cavity with the rubidium lamp and detect a slight change in intensity when it crosses the atomic resonance.

In this unit, there is a crystal oscillator that does this, using digital voodoo to produce that magic 6.834682612 GHz signal to divine the hyperfine transition.  This oscillator is "ovenized" - which is to say, the crystal and some of the critical components are under a piece of insulating foam, and attached to the crystal itself is a piece of ceramic semiconductor material - a PTC (positive temperature coefficient) thermistor - that acts as a heater:  When power is applied, it produces heat - but when it gets to a certain temperature the resistance increases, reducing the current consumption and the thermal input and the temperature eventually stabilizes.

Because we have the rubidium cell itself to determine our "exact" frequency, this oven and crystal oscillator need only be "somewhat" stable intrinsically:  It's enough simply to have it "not drift very much" with temperature as small amounts of frequency change can be compensated, so neither the crystal oven - or the crystal contained within - need to be "exact".

Fig 3: The FE-5680A itself, in the lid of the case of the 1590 box to provide heat- sinking.  As you can see, I've had this unit open before! Click on the image for a larger version.

What is required is that this oscillator - which is "pullable" (that is, its precise frequency is tuned electronically) must be capable of covering the exact frequency required in its tuning range:  If this can't happen, it cannot be "locked" to the comparison circuitry of the rubidium cell.

The give-away was that as the unit warmed up, it did lock, but only briefly:  After a brief moment, it suddenly unlocked as the crystal warmed up and drifted low in frequency, beyond the range of the electronic tuning.

Taking the unit apart I quickly spotted the crystal oscillator under the foam and powering it up again, I kept the foam in place and watched it lock - and then unlock again:  Lifting the foam, I touched the hot crystal with my finger to draw heat away and the unit briefly re-locked.  Monitoring with a test set, I adjusted the variable capacitor next to the crystal and quickly found the point of minimum capacitance (highest frequency) and after replacing the foam, the unit re-locked - and stayed in lock.

Bringing it up to frequency

This particular '5680A is probably about 25 years old - having been a pull from service (likely at a cell phone site) and eventually finding its way onto EvilBay as surplus electronics.  Since I've owned it, it's also seen other service - having been used twice in in ground stations used for geostationary satellite service as a stable frequency reference, adding another 3-4 years to its "on" time.

As quartz crystals age, they inevitably change frequency:  In general, they tend to drift upwards if they are overdriven and slowly shed material - but this practice is pretty rare these days, so they seem to tend to drift downwards in frequency with normal aging of the crystal and nano-scale changes in the lattice that continue after the quartz is grown and cut:  Operating at elevated temperature - as in an oven - tends to accelerate this effect.

By adjusting the trimmer capacitor and noting the instantaneous frequency (e.g. adjusting it mechanically before the slower electronic tuning could take effect) I could see that I was right at the ragged edge of being able to net the crystal oscillator's tuning range with the variable capacitor at its extreme low end, so I needed to raise the natural frequency a bit more.

If you need to lower a crystal's frequency, you have several options:

• Place an inductor in series with the crystal.  This will lower the crystal's in-circuit frequency of operation, but since doing so generally involves physically breaking an electrical connection to insert a component, this is can be rather awkward to do.

Fig 4: The tip of the screwdriver pointing at the added 2.2uH surface-mount inductor:  It's the black-ish component at sort of a diagonal angle, wired across the two crystal leads. Click on the image for a larger version.

• Place a capacitor across the crystal.  Adding a few 10s of pF of extra capacitance can lower a crystal's frequency by several 10s or hundreds of ppm (parts-per million), depending on the nature of the crystal and the circuit.

Since the electrical "opposite" of a capacitor is an inductor, the above can be reversed if you need to raise the frequency of a crystal:

• Insert a capacitor in series with the crystal.  This is a very common way to adjust a crystal's frequency - and it may be how this oscillator was constructed.  As with the inductor, adding this component - where none existed - would involve breaking a connection to insert the device - not particularly convenient to do.
  

• Place an inductor across the crystal.  Typically the inductance required to have an effect will have an impedance of hundreds of ohms at the operating frequency, but this - like the addition of a capacitor across a crystal to lower the frequency - is easier to do since we don't have to cut any circuit board traces.
  

With either method of tweaking the resonance of the oscillator circuit, you can only go so far:  Adding reactance in series or parallel will eventually swamp the crystal itself, potentially making it unreliable in its oscillation - and if that doesn't happen, the "Q" is diminished, potentially reducing the quality of the signal produce and furthermore, taking this to an extreme can reduce the stability overall as it starts to become more temperature sensitive with the added capacitor/inductor than just the crystal, alone.

In theory, I could have placed a smaller fixed capacitor in series with the trimmer capacitor  - or used a lower-value capacitor - but I chose, instead, to install a fixed-value surface-mount inductor in parallel with the crystal as it would not require cutting any traces.  Prior to doing this I checked to see if there was any circuit voltage across the crystal, but there was none:  Had I seen voltage, adding an inductor would have shorted it out and likely caused the oscillator to stop working and I would have either reconsidered adding a series capacitor somewhere or, more likely I would have placed a large-value (1000pF or larger) capacitor in series with the inductor to block the DC.

"Swagging" it, I put a 2.2uH 0805 surface-mount inductor across the crystal and powered up the '5680A and after a 2-3 minute warm-up time, it locked.   After it had warmed up for about 8 minutes I briefly interrupted the power and while it worked to re-establish lock I saw the frequency swing nearly 100 Hz below and above the target indicating that it was now more less in the center if its electronic tuning range indicating success!  As can be seen from Figure 4, there is likely enough room to have used a small, molded through-hole inductor instead of a surface-mount device.

Fig 5: The crystal is under the round disk (the PTC heater) near the top of the picture and the adjustment capacitor is to the right of the crystal. Click on the image for a larger version.

With a bit of power-cycling and observing the frequency swing while the oscillator was hot, I was able to observing the electronic tuning range and in so-doing, increase the capacitance of the trimmer capacitor very slightly from minimum indicating that I now had at least a little bit of extra adjustment room - but not a lot.  Since this worked the first time I didn't try a lower value of inductance (say, 1uH) to further-raise the oscillator frequency, leaving well-enough alone.

Buttoning everything back up and putting it back in its case, everything still worked (always gratifying!) and I let the unit "burn in" for a few hours.

Comparing it to my HP Z8530 GPS Disciplined oscillator via the oscilloscope (see Figure 2) it took about 20 minutes for the phase to "slide" one entire cycle (360 degrees) indicating that the two 10 MHz signal sources are within better than 10E-10 of each other - not too bad for a device that was last adjusted over a decade ago and as seen about 15000 operational hours since!

 

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Follow-up:  A few weeks after this was originally posted I had this rubidium reference with me at the Eclipse event as a "hot standby", its frequency being compared to the LPRO-101 - which was the active, on-the-air unit - using an oscilloscope.  This (repaired) unit fired up and locked within 5 minutes at the cool (45F/7C) ambient temperature and remained stable for the several hours that it was powered up.

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This page stolen from ka7oei.blogspot.com

 

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