Room to move

When we first moved into this place about seven years ago, I built some basic desks into my study/workshop. Well I keep expanding into unwanted parts of the house so it's time for some renovations. 

First some "before" shots! This was the work area in the old study, way too small to be useful and a huge bookshelf took up a 3m long wall.

Well both of those are gone and being replaced by proper kitchen cabinets giving me a short 1.5m x 650mm wide bench and a long 2.9m x 850mm wide bench with upper and lower cabinets!

As you can see it will make a huge difference. More to come.....

A microPLC..

In a moment of recent madness I said I would design a very very small programmable-logic-controller (PLC) for someone, including writing the language interpreter / byte code generator for it....

When will I learn!!!!

Hopefully the final product will run on something like the ATTINY84 or similar.

Defining the syntax rules

Portions of dodgy C

The byte code decompiler listing what the compiler produced.

Mini eBay monitor

Last week I ordered a 7" HDMI/VGA/Composite monitor from an 'Australian' seller 'Globedealmark' for about $64 including postage. It comes as a bare PCB and LCD module with no documentation so they expect you to mount it in a case etc yourself.

Screen, controller and user controls

The board requires 12V DC at about 1amp. I first tried it on an old Windows 7 laptop using the VGA interface and a screen resolution of 1366x768 scaled automatically to the monitors native 800x480 pixels. It looks very good!

VGA input from laptop at 1366x768

Then I fired up the pcDuino Nano3 ARM board and tried that via HDMI. This time I set the resolution to match the display and the result is very nice.

Native 800x480 via HDMI

This is the combination I intend to use, mounted in a briefcase along with some SDR hardware. Altogether a good buy I think!

800x480

I forgot to mention that the display supports a reversed composite input for a car reversing camera etc too. Very hand all round.

Airprobe-rtlsdr DC spike

HackRF with DC spike

I've been experimenting with the Airprobe-rtlsdr utility which is designed to demodulate non frequency hopping gsm signals and stream the demodulated data to Wireshark. Then in Wireshark you can use the built-in GSM decoding functions to extract cell tower identification etc. You CAN NOT decode the SMS or voice content so stop getting all excited!! That requires a whole lot more effort which would take you from the slightly illegal to the totally busted zone quick smart. Anyway when using the HackRF or it's clone the HackRF-Blue you face the usual large DC spike issue at 0Hz. This stops the demodulator from working so what I have done is add the GNU Radio DC blocker block to Airprobe-RTLSDR and now it (theoretically of course) decodes just fine.

Modifications to airprobe-rtlsdr.py

The modifications to the airprobe-rtlsdr.py file are shown in the above screen shot. I commented out the old code (three lines starting with #) and added the four new lines which insert the dc blocker into the signal chain. You also need to instantiate the dc blocker which is what is happening at the top of the image. Another thing not shown is that you must include the filter code from GNU radio so you need to add an import statement at the start of the code which says:

from gnuradio import filter

HackRF with DC spike removed

Up market SDR dev kits - AD9361

AD9361 SDR on AD-FMCOMMS2-EBZ

I'm lucky enough to have access to a range of different toys (err tools) and the latest gadget that someone has kindly lent me is an Analog Devices AD-FMCOMMS2-EBZ and a matching Digilent ZedBoard. This evaluation board features the AD9361 two channel 70-6000MHz SDR transceiver chip. The AD9361 has dual 12 bit ADC and DAC giving 72dB dynamic range (compared to the the HackRF's 48dB). Maximum bandwidth is 56MHz. The Ettus E310 is also based in this chip and that's a $4200 AUD radio. (about $2600 more than this solution)

Once of the best things about this evaluation kit is that Analog Devices fully support it and a number of different motherboards with a prebuilt Ubuntu ARM image which contains example applications and a fully functional GNU-RADIO source and sink block. This means you hit the ground running and you are not restricted to vendor specific applications. What will I do with it? Good question and hopefully I will be in a better position to answer it intelligently after I complete a week of FPGA/VHDL training next week :-) (Stay tuned for updates to this post)

Digilent ZedBoard ARM/FPGA combo.

Happiness is cheap connectors

I bit the bullet and trolled eBay for a reasonable dealer who offered a range of SMA connectors at sensible prices. I came across a store called av-rf who had a very good selection of connectors, adapters and ready made semi-rigid patch leads.

It has taken about two weeks for the order to arrive and for what they are the quality seems pretty decent. The bits and pieces in the photo came to under $50 AUD. There are four each SMA male-male, female-female, N-male-SMA-female, two BNC male to SMA female, four 15cm and four 30cm semi rigid patch leads, two SMA male to N male patch leads and not in the picture an SO239 to SMA.....

Homebrew Discones

I need a broadband antenna for my SDR experiments, something that will cover 2M, 70cm and 23cm all in one would be nice. The discone seems like a reasonable general purpose omni-directional antenna with something resembling a 10:1 useable frequency range.

A bit of Googling and I came across an online calculator  so I used 140MHz as my lowest operating frequency (I had read somewhere that design figures can end up a bit short so start lower than you really want) and I came up with about 600mm for the cone sides and 428mm diameter for the top hat. I used copper coated steel TIG welding rods which are strong but also cheap and solderable. A tube of 28 2.4mm x 1m long rods was about AUD $19 from the local GasWeld store.

The eight radials forming the cone are soldered to a 15-20mm copper plumbing adapter, the inside of which is just right for sliding a bit of RG213 coax through. The top elements are soldered to a large washer. All a bit rough but this was only meant to be an experiment.

0-1400MHz sweep.

How did it sweep? Well, as you can see from the Rig Expert screen shot it's not as flat as I expected! The lowest usable frequency is about 152MHz where the VSWR is about 3:1 so really I should have designed it for about 130MHz and it may have come out usable on 2M (144-148MHz). 70cm and 23cm are fine. More experimenting required!! By the way I discovered these great heavy-duty microphone stands at Swamp in Fyshwick here in the ACT they are over 2m tall, very solid and well priced at $35 each. I also use them as speaker stands form a couple of studio monitors I have.

An update

I decided to build another Discone which would start a little lower so as to cover the airband. I also found the construction of the previous one a little fragile because I had soldered the ground radials to the outside of the copper fitting the coax passes through. So below is version two...

 

Discone version two - 120MHz up.

The top disc is again made up of copper coated steel welding rods, this time soldered to a brass disc. The overall diameter is 500mm. The ground radials are 694mm long and soldered into a gap formed by two copper plumbing fittings fitted together (see the pictures). This makes for a much more rigid assembly but the it could still do with a nice machined part to hold the disc and ground radials together. Now lots of photos which should be easy to follow.

Copper fittings. ID just right for RG213

Radials poked in gap

Soldered using 50/50 thick solder

Radials soldered into place.

RG213 fed through fitting ready to solder to top disc.

Top disc assembly

The resulting antenna has a frequency response very close to the design, the SWR drops sharply at 120MHz and they stays below 3:1 right up to about 1400MHz. I would have expected it to be a little flatter in places but perhaps their is some Discone matching black-magic I have not discovered yet. On air it seems to perform well.

This is the final frequency response of the new discone when swept from 1MHz to 3GHz. Quite usable on 2M (146MHz) up for transmit.