Monday, 24 March 2014

Mobile antenna mount repaired

Since I had the car apart to replace a headlight bulb, I decided to find out just what was wrong with the HF antenna mount. I managed to remove the mount only having to unfasten the very ends of the side trims and the last two fixings of the head lining, which gave just enough room to squeeze my hand through to retrieve the SO239 fixing.

As expected, the center pin connection had separated. Luckily not the pin sheared, but the soldered connection to the coax. It seems under enough torque the pin can turn, and over time the wire has split.

After recutting the coax and inserting it back into the connector, I managed, somehow, to resolder the center. Then after a fair bit f swearing I finally managed to get the mount back through the hole in the roof and have it stay there without falling back out for long enough for me to get hold of it from the top of the car and get the nut on! A few minutes of the good old antenna fitters rooftop pump pliers dance, and its secure again.

Tested with the 40m antenna, found enough activity to verify it was operational again.

Mobile Intermittency

Shortly after obtaining a 40m mobile antenna, it became apparent that there was a problem. Often I would be able to hear nothing on the mobile HF.

It turns out the roof SO239 connector has gone intermittent. I suspect wind loading stresses have fractured the center connection. As I have other vehicle repairs to do tomorrow (yet another blown headlight!) I intend also taking a look at the antenna connection, and seeing if its repairable.

Because of this problem, ive gone much of the winter (what we've had of one!) without a reliable HF mobile, and so made little use of the 40m antenna.

I suspect, that by the time ive fixed the problem, the higher bands will be back in play. One of my tasks will be to build an efficient antenna for the one favorite band I dont have one for - 10m!

Sunday, 23 March 2014


Two things have occupied my technical time today. These were lighting up the shack, and making sure I can deal with it if the workshop catches light!

My workshop also doubles as much needed storage for the rest of the family, so one of my ongoing jobs is to improve this storage (or, as Julie puts it - 'tidy up'). Ive bought six plastic storage boxes today (stackable) to help with this. Part of the problem is I have lots of 'junk', otherwise known as salvaged equipment. These boxes will help me store such things as salvaged circuit boards, RF/Coax patch leads etc.

One of my obsessions is with fire safety. This stems from my earlier, childhood obsession - fire.

When I was younger, fire fascinated me. I would sit watching candles burning. I would also set fire to anything and everything, from dry grass, to cardboard boxes in the woods, to 'volcanos' made from weedkiller and sugar. But I soon came to realise that the real buzz from pyromania isnt destruction - its control. I learnt to respect, but also control fire. From this led to my unending interest in fire extinguishers. From a teenager I knew the classes, colour codes etc. I have already taught Sam how to control a fire, and soon will teach him how to extinguish one!

The workshop is a particular worry, due to the expensive electronic test gear, soldering irons, flammable solvents etc. The two choices are dry powder and CO2. Because of the risk of damage to the equipment, I opt for CO2. Today I finally put my extinguisher into its permanent  fixture on the wall, within very easy reach! (I have another CO2 for domestic use, plus a 1kg powder. I carry a 2kg powder extinguisher as part of my mobile set-up)

I need to go through a lot of the stuff I have and decide if I am ever really going to use it, and if not to dispose of it. Ive already stacked a load of casings/cabinets up to be recycled.

Onto the shack lighting. A strip of LED cool white self adhesive flexible lighting was cut and stuck tot he underside of the top shelf of the shack desk. I just need to solder on the power wires, add an on/off switch, and wire it up to either the shacks solar power system or the main PSU. This three foot strip of LEDs is bright enough to light up the whole desk and most of the rest of the shack.

As I share the shack with the rest of the family (Im in the corner!), and as I had bought the 5m long RGB LED strip just for a bit of a laugh, I also today ran this strip around the ceiling coving, attached the control box and held it in place with a couple of map pins, and ran it to a small 12v SMPSU. These LEDs are enough to light to whole room, and when all the LEDs are on it does in fact result in a general white illumination. But the real fun is with the control box and its little remote! The LEDs can be made to flash in RGB sequence, or to fade in and out all colours in all combinations, etc etc, and the speed or brightness can be adjusted. Ideal for a room the boys play in. I just need to lengthen the power cable and hide it in a bit of trunking.

I still have a fair bit of the cool white LED strip left, I think I will put this in the under stairs cupboard, along with a 7Ahr 12v SLAB and a microswitch on the door! It will make finding my next beer much easier!

Monday, 17 March 2014

USB power monitor

I bought this as my 'frivolous' purchase on ebay. It just sort of took my fancy, and I thought it could be reasonably useful.

Its nothing complex, looking through the translucent blue plastic case, it looks to be a simple VOM/Display Driver chip and a 4x 7-seg LED display block. Im guessing that the current monitoring is done after the supply for the LEDs is taken, as with no USB load it does read 0A.

I think its name of 'Charger Doctor' is supposed to indicate its intended use of checking when a USB port is used for charging a device. I have it now connected charging my Kindle

80-90mA seems to be what the Kindle is taking, and with this load the USB port is running at 5.07v.  I did try earlier with one of the RTL SDR dongles, which took about 110mA, and the voltage dropped to about 4.9v

Yes, fairly frivolous. Yes, reasonably useful occasionally. Worth the £1.99 from the far east, free shipping? Absolutely.

Thursday, 13 March 2014

DAB RFI Issue Investigations

Attempt to stop the RFI at DAB frequencies by decoupling and filtering the driver PSU have been ineffective because the RFI is being generated by the LED lamps themselves.

This photo shows one of the MR16 LED lamps opened up. The boost regulator boards can clearly be seen siliconed into the connector pins housing.  This is the source of the problem. A direct DC test was set up as below

A very simple test, nothing more than one of the lamps croc clipped up to a 12v 7Ahr SLAB, with a DAB receiver a few inches away. What cant be seen here, other than the telescopic antenna positioned above the lamp, is the spectrum analyser.

With the lamp powered, there was a definite set of spurii between 250 - 300MHz indicated on the analyser. DAB transmissions are just a little lower than this, around 220MHz (this will have significance for US radio hams wishing to use the 1.25m band!) There was no obvious peaks anywhere else from 0 to 1000MHz! That makes it a particularly awkward problem, as its affecting just this spot on the dial!

Although the peaks were not measured, only observed, the addition across the lamps terminals of a 100nF ceramic capacitor dramatically reduced their amplitude. The DAB receiver, which was almost entirely wiped out six inches from the lamp, was now usable somewhat closer. Only by bringing the antenna within a few inches of the lamp was the RFI apparent.

This lamp has now been refitted into the holder in the bathroom, with its 100nF capacitor wire-wrapped to its pins with minimum lead length. With this lamp in place, and the DAB radio back in its usual location on the windowsill, powering the lamp circuit results in badly mushed DAB reception, but it doesnt now drop out entirely.

The position of the receiver is almost equidistant between the modified lamp and the next unmodified one of the three in this circuit. I anticipate that adding the 100nF decoupling capacitors to the other two lamps, plus the addition of a few snap on ferrite beads to the positive supply wire close to the lamps, will resolve the RFI issue.

Really though, the 100nF decoupling capacitor and the ferrite chokes should have been included by the manufacturer within each lamp. Another case of saving a few pennies in production, at the expense of increasing the RF noise floor and causing unacceptable RFI.

DAB interference from MR16 12v LED lamps

Since installing the 12v LED lighting in both the bathroom and the kitchen, i've been aware that the circuits cause interference at around 220MHz, sufficient to render DAB reception impractical. As VHF FM is not affected by this, I havent been particularly quick in getting around to solving it, but Radio DSO is approaching, and I prefer several DAB stations for my normal listening anyway, I thought it time to have a go at it.

My circuits use 12v MR16 lamps fed with one switch mode LED driver per circuit. It was my conviction that the drivers were the most likely source of the interference. Today, I spent some time adding a 10nF bypass capacitor and several ferrite snap-on beads to the wiring close to the driver transformer of the bathroom circuit.

This has made no difference to the level of interference at all.

Further online research leads me to suspect that, contrary to engineering reason that LEDs are passive and cannot cause interference if DC fed, the lamps themselves may be the problem.

It seems that some of these, and quite likely the ones I use, contain a boost regulator to step the 12v supply up a bit, usually to around 18v. These being a class of switch mode supply, will produce pulses in the area of a few tens of kHz, but these being square waves will produce all manner of interfering harmonics.

What I need to do now, is take one of the lamps and feed it with pure DC (easily done from one of my large SLAB backup batteries), to see what the level of interference is from a single lamp. It should then be possible for me to develop a filtering regime per lamp that should cut the level of radiated and conducted interference.

I will also report the problem to the supplier.

The ADS-B collinear is now in its tube, just awaiting some end caps and sealant. Performance at ground level inside seems poor, but of course that isnt its intended operating position! Once sealed up, that will mark the endpoint of my experiments into ADS-B antennas.

Sunday, 9 March 2014

Note to self...

A bit of a design change to the CoCo ADS-B antenna

Change the whip section at the top to a 1/4 wave section.

Use thin (2mm) slices of spare dielectric to from spacers between element section, to prevent the braid of each section from touching.

Cut the end of the feedline sheath back by 1/4 wavelength, remove the braid and fold back the foil shield. This coax (CTF100) uses a copper foil rather than a metalised film. Form the folded back foil into a
1/4 wave sleeve. Remove the excess dielectric and inner.

I should COCO!

After a bit of playing, I found that none of the matching techniques tried on the ADS-B ground plane antenna gave a usable device, and with the DVB-T stock mini mag-mount antenna being actually usable at 1GHz, as evidenced by having the system running on my laptop autonomously over yesterdays night shift, and it being perfectly capable of tracking at least those aircraft within a close enough range (about 10NM) despite the very much indoors location, the very high RF environment, the huge amount of local RFI, AND the antenna being stuck on a bit of a bolt head beside me on the desk, I have decided to abandon this design direction in favour of gain antennas.

The wire collinear proved to be a usable design, but is remarkably flimsy in its open state. Putting it in a PVC or fibreglass tube would solve this, but the ground radials would need replacing with thicker rods to survive any knocks.

So the idea is to make up a Coaxial Collinear (CoCo) antenna. I have investigated these before, but only at VHF, where unfortunately the dimensions become a bit too long when aiming for decent gain. At 27cm though, the dimensions for even a high gain version become quite manageable.

A CoCo antenna uses the collinear principle of feeding halfwave sections out of phase by 180 degrees, but whereas in a normal collinear this is achieved by quarterwave stubs, here it is done by reversing the connections between the half wave sections, which are formed from coaxial cable.

Designs online suggest a CoCo formed from eight 1/2 wavelength coaxial sections will have a gain of about 3.5dBd. But how long physically would that be?

Well, our design frequency is 1090MHz -

300/1090 = 0.275m

so a halfwave is 0.138m

Now, we cant calculate the length from this, as theres a couple of other things to take into account. One is the addition of a whip section on the top, which adds another half wavelength. The other is the cables velocity factor.

The cable im using is CTF100, which has a VF of 0.82 (Rf travels through it at 82% of free space velocity), so we must multiply our halfwave by this, except for the whip, which is 'free space'

0.138 x 0.82 = 0.113m = 113mm

we have 8 half wave sections, plus the whip,

(113 x 8) + 138 = 1041mm = 1.04m

Thats a pretty manageable length for a decent bit of gain. If we improve the match by including a quarterwave tube at the feedpoint, we'll only increase this by about another 7cm. But for receive only, this isnt vital.

I have the various coaxial sections cut ready, including an extra 20mm on one end and 15mm on the other. The reason for this is to allow enough of the inner conductor to connect between the sections (the difference in end lengths makes the next job easier!). To join these, the bared inner conductor is simply inserted between the outer sheath and the braid. This creates the alternating connection which provides the phase change.

I would normally prefer soldered connections, especially at high UHF! But the outer sheath will be tight against the connections and it seems provides a decent usable joint. I will cover each joint with heat shrink tubing anyway which will provide even more of a squeeze onto the connections.

I have a few feet of extra CTF100 to act as the feedline. This has an F connector on it at present, but I might replace that with a BNC to make interconnection easier. Once built and tested, the whole antenna will be slipped in and sealed into a length of PVC conduit, it will then be mountable somewhere up clear.

A worry with these DVB-T dongles is that the receive front end is absolutely wide open. To help prevent desense of the receiver from high powered broadcast stations, I will be adding to the antenna one or more quarterwave stubs. A quarterwave coaxial stub, open circuit at the end, will provide a notch filter at the design frequency. Not what I need here. But if it is short circuit at the end, it forms a band pass filter at the design frequency. Taking velocity factor into account, a coaxial stub filter for this frequency is a piddling 5.6cm long!

Apart from building one of them into a shielded box, this will be my last build as goes the ADS-B system. I need to get on with other projects, especially the 10m WSPR transceiver,

I have spent too long trying to get the Wispy design working for me, so have decided to abandon that also. Not in its entirety though. I will replace the mixer with an SBL-1, so I have then confidence in that part of the circuit. I am happy with the performance of the oscillator/doubler, so will keep this part of the design, except for adapting it to feed the SBL-1, which I expect will require a 5dB pad. Knowing that the parameters of the modulator are fixed and out of my control, means I can concentrate of setting the drive levels such that I have a working DSB modulator. Once this is done, I can build on it with the amplifier chain to get the desired output power. Whether I modify the Wispy board, or start a fresh layout I havent decided yet, but a fresh start will allow me to better shield the oscillator, and control the harmonic output.

Friday, 7 March 2014

Plane Weirdness

The original, suspected faulty dongle is currently plugged into my main PC, like so

The cheapo stock mini mag-mount that came with the new one is plugged into it (the modified antenna connector is open) and the antenna is set up two feet from my feet, and at least 12ft inside with the tiniest view of clear sky, atop a cable drum (empty!)

Yet, ADSBscope and ADSB# between them are tracking a good few aircraft!

So, im really none to sure why it wouldnt work before! RF overload? possibly, when I tried it at work. But maybe it doesnt like the grounded antenna? If so, why did it not work on the colinear yesterday? Maybe, the modified antenna socket is naff? Ok, i'll accept that possibility!

A few people have expressed an interest in what im doing here, so I will give a quick condensed overview, enough at least to get someone started -

ADS-B stands for Automatic Dependent Surveilance - Broadcast. It is a form of Secondary Surveilance Radar. Primary radar works by detecting the reflected pulses of a high power transmitter, and by measuring the propagation delay, working out the range to the aircraft. By using rotating antennas with known take off angles, the height and bearing of the aircraft can be determined. This requires a lot of power, and some accurate and hence expensive mechanics, so primary radar is a very expensive bit of kit. It also has very limited range as the detectable range is about half the transmitted range.

Way back in the Battle Of Britain, it was realised that our RDF (Radio Direction Finding) operators could not easily tell an RAF Hurricane from a Luftwaffe BF109 on a radar screen. This led to the invention of IFF (identification, Friend or Foe). This was the very first Secondary Surveilance Radar system. The IFF unit on the aircraft was a transponder. When 'interrogated' by a ground station, it transmitted back the aircraft callsign.

Through various incarnations of SSR to the present, and with the advent of small, cheap and amazingly accurate aeronautical GPS systems, we have ADS-B. This form of SSR requires no interrogating ground station, only receiving stations. Each equiped aircraft regularly broadcasts its callsign, flight designation, height, bearing and position, in a brief data packet, at 1.090GHz. This is known as 'squittering'. Ground receiving stations detect and decode this data burst, and via the magic of the internet, not only can plot the aircraft received on screen, but can look up all the flight details, airframe details etc. And by uploading the decodes to a server, from many different locations, a radar image of the whole airspace can be created.

The professionals use very expensive, ultra high accuracy and reliability equipment for this, but amateur plane spotters can do just as well with less money than a couple of pints! Skip a canteen breakfast two days on the trot and buy a DVB-T USB dongle!

Everything you could possibly want to know is available on but I will briefly describe my set-up

I have two dongles, both were bought off ebay from a seller in the Far East by the name easabonn. They take about a month to arrive, but cost me £5.37 each, including shipping. They come with a remote control (no use), a miniCD (no use) and a mini mag-mount antenna (reasonable for ADS-B but no use for anything else!)

OK, so you have your dongle. Now, plug it in and let windows install its driver. As soon as its done that, go online and download a program called Zadig. Run this, and follow the instructions you can find on the above website to change the driver!

With the driver changed, the dongle is now no longer a DVB-T telly stick, its a 24 - 1700MHz Software Defined Radio!

I use three programs now -

SDR# is an SDR control program. This allows you to not only receive and listen all over the dongles coverage, but to see the spectrum up to 2.4MHz wide as a waterfall. Tune to 1.090GHz and you should see a trail of brief bursts of signal go down your screen (depending where the antenna is, get it high and clear if you can), these are the ADS-B squitters.

Close SDR# and run ADSB#. This is a dedicated ADS-B receiver decoder. You wont see much happening, other than the frame rate counter hopefully go up. If you telnet to it ( you should see a stream of hex values, these are the decoded data. I have my ADSB# controls set to 'Tuner AGC'

With that running, now you need to visualise the data. Run ADSBscope. Go to Other > Network > Network setup. Click the ADSB# button, and the 48706 port number should appear. Set the URL to local. Then click on the RAW data CLIENT button. All being well, planes will start to appear on your screen! Its really as simple as that.

I have found that minimizing ADSB# stops it working, so keep that on screen.

Ideally, you want a better antenna. Get the soldering iron out and have a look at the various ideas online.

And thats all there is to it to get your very own ADS-B SSR ATC console!

Thursday, 6 March 2014

ADS-B colinear success

With the new, working dongle, and a new MCX to TNC adaptor, I am now connected up to the colinear. This is hanging from the telly, well inside the house, and is still giving 80% data quality, and tracking a good half dozen aircraft

Thoughts turn now to why the other dongle seems to have failed at 1GHz. I wonder if it doesnt like DC grounded antennas? Before using the ground plane antenna on this one, i'll look at a DC blocking capacitor!

Thats it for radio today, I slept in until midday anyway, and other than this the day has been taken up with giving my fire extinguishers their annual service.

Receiver Woes! Dongle not Antenna!

Oh flippin' heck!

As well as the ground plane antenna, I decided to knock up a colinear, to try and increase receive aperture and get better ADS-B signals. This being nothing more than some strategically bent wire, took little time to create. The most effort was drilling a small bit of PCB material to mount the socket and radials on. A bit of nylon cord was attached to hang it up for testing

I tried this antenna with the modified RTL dongle - nothing! No matter what I did, neither the GP or the colinear would give me any usable decodes.

I started thinking then it must be a software config issue. But try as I might, nothing could be found amiss with the software. I even tried different software, I tried the firewall settings, I tried Telnet (on the main PC, the Vista laptop was told to enable the Telnet client about 2h ago and is still trying!). I could not get any received data. Looking at the spectrum on SDR#, I was only seeing occasional pulsed of ADS-B at 1.09GHz, surely theres more planes within range than that!?

Suspicion began to fall on the receiver...

By a lucky chance, the postie came with a couple of parcels, one of which was - my second RTL dongle! I set the software back to SDR#, put the cheap and tacky stock mag-mount on the new dongle, plugged it in and started the software...

Blimey! A MASSIVE column of ADS-B bursts at incredible strength filled the screen! Switching to ADSB#, and starting ADSBscope, and a few seconds later this was my laptop screen

Dozens of aircraft! So the problem lies in the RTL receiver stick, or the antenna modifications. I have one adapter for the MCX connection, but I also have the MCX plug and a bit of coax cut from the first dongles stock antenna, so I will find make up some sort of adaptation for the TNC connectors on the antennas.

I wonder if the problem is down to the way I just patched my 50 ohm coax across the 75 ohm MCX antenna?

Wednesday, 5 March 2014

Yet Another ADS-B Antenna

Even as I play with the groundplane design, it is fairly clear to me that I will probably need a greater aperture for receiving at 1GHz. This means, simply, just as it does at HF - more wire in the air!

Theres lots of designs on the 'net, and there doesnt seem to be any obvious comparisons between them. I fancy a 12 section COCO (COaxial COlinear) but the necessary cutting of bits of coax isnt really a quick evening job. So, I thought i'd try this

My version doesnt follow exactly, but im hoping it will be 'in the ballpark'. For a start, im using 1.5mm wire, not 3mm (because I have some), and im also not worrying too much about weather protecting this one. So the specified dimensions will probably not be quite right for my version, but should at least give something on receive.

Ive cut, measured and bashed the wire into the required shape. Tomorrow i'll knock up the groundplane section as a few bits of wire soldered to a bit of PCB with a hole in it, and solder the lot to yet another TNC socket nicked from an old RC-690!

Partial Success with ADS-B Antenna

At lunchtime test today, in the meagre time I have before the laptop's battery dies, and with the ADS-B antenna wedged on the car roof on top of a pair of binoculars and a sponge, I gave reception on the newly built antenna a go.

SDR# clearly showed bursts of signal at 1.09GHz. They seemed a bit weak, whether this is true or whether the proximity of the tower caused desense of the receiver im not sure yet. A bit of fiddling to get ADSBscope and ADSB# playing together and a few aircraft were identified and their altitudes listed. Unfortunately, this test didnt go as well as hoped, and no position data was recovered, meaning none of the aircraft could be plotted.

But, its a start. It shows the antenna does work, as does the software. Whether the antenna works well enough, will take a few more tests to decide.

Tuesday, 4 March 2014

An ADS-B receive antenna from scrap - 2nd Attempt

Having eventually got sorted out in my head what this hairpin was and how to implement it in a bit of wire, I have rebuilt the ADS-B antenna

The separation of the hairpin wires is not perfect, but it gave me the ability to squeeze the loop with pliers to gently extend the hairpin length until it is about as close to 10.8mm as its likely to get.

In fact, remeasuring it just now, from side of vertical element to tip of hairpin is actually about 10.0mm. This is mostly due to the position of the vertical element. As the wire of the vertical element is 1.5mm diameter, just moving this about 45 degrees around the center pin will adjust the hairpin length to 10.8mm. Thats a job for tomorrow.

A quick knock up test, with the antenna clamped to a bracket on the side of the workshop, with a clear view of maybe 30 degrees of south western sky, and well into the evening when most aircraft are not flying, did show some ADS-B signals! They were not strong, and I lost them again after fiddling with the software, but they were there!

Antenna Cock-Up

Well, what a bloody muppet!

AC6LA has informed me that, as impressively built as the ADS-B antenna is, its wrong. It seems that what ive built isnt a beta match (hairpin), but a gamma match! This might go some way to explaining why im still not picking up any signals!

After a fair bit of head scratching and looking at lots of drawings online, admittedly mostly of VHF hairpins on beam antennas, I think I know how its done!

Time will tell. I think though, that I will also build a simple wire colinear, just in case!

Monday, 3 March 2014

An ADS-B receive antenna from scrap

Playing with this little SDR dongle, ive discovered that it should be possible to use it to monitor aircraft ADS-B 'virtual radar' signals.

Ive tried this with the dongle working from the loft mounted discone, and received nothing. Now, the discone if I remember rightly has an upper frequency limit of about 800MHz, but the ADS-B signal is at 1090MHz! So an antenna for 1.09GHz was needed.

I didnt fancy a dipole, or a little groundplane antenna, even though these are very quick and easy to make. I wanted something a bit more challenging, and sturdy. My thoughts went to the groundplane antennas I used to see on fire stations, with a folded driven element.

After some questions on, and a bit of research online, I found these are called 'folded monopoles', and have a feed impedance of 100-150 ohm. I needed 50 to 75 ohm. It was suggested to me that I shunt feed the antenna with a hairpin. AC6LA kindly modeled the antenna on EZnec and made the dimension adjustments for 75 ohm. Whilst I began building the groundplane

Using a spare sheet of 1.5mm anodized aluminium, I marked a 13cm diameter circle (1/2 wavelength) using a pair of compasses, and cut this out using my nibbling tool. This tool is invaluable for this sort of work. You could use a hacksaw, but the nibbler will follow the curve. If your sheet metal is thinner, of course, tin shears would work. Aviation types will follow the curve as well, so long as you use the correct 'handed' ones!

Also, If you do make one of these, take special care. Sheet metal is not forgiving of skin contact! You dont want your new antenna stained with your claret!

Once cut out,  the edges were filed down to exactly on the line of the circle, the disc having been cut out a little wide to allow for my cack-handed cutting. The edges were then smoothed with a sanding block to remove burrs, and the central hole drilled for the antenna connector.

The connector in this case is a panel mount TNC type taken from an old Marconi RC-690 (lots of good bits in a 690!) This matches well to the TNC used on the dongle. A flat was filed on the side of the connector to solder the element to.

I spent a very long time trying to solder the main element, getting nowhere, when it occurred to me that the socket was attached to a bloody great radiator! So I removed it from the disc and soldered it on the bench! The main element was then cut to length and the hairpin soldered to the pin of the socket. A bit of measuring, cutting, bending and remeasuring later, and it was time to solder the stud tap point.

The resulting antenna looks like this

It being dark now, theres not many planes about that will be transmitting an ADS-B signal, and its too late to go outside and test properly line-of-site, so proper testing will have to await daylight.

I also have a 3m USB extension cable for the dongle now, but havent tested it with this yet.

Saturday, 1 March 2014

Dangly Dongles

Because of work, ive not had much time for radio for a few days. But I have been playing with a bit of SDR.

For a bit of fun, I ordered a few of these DVB-T USB dongles from China

These are intended for use in receiving off air digital TV, but the antenna that comes with them, a piddly little magmount, is frankly dire. Of far greater interest is that these have very wide band software defined radio receivers. The R820T tuner chip in these can cover from around the mid 20 MHz, up to about a gig and a half, if used with suitable SDR software.

An inside view shows how these are made up. The little biddy chip to the left of the 28.8MHz clock crystal is the Rafael R820T tuner. The other square IC to the right is the Realtek RTL2832U DVB-T demodulator and ADC. These sticks are popular because of the wide tuning range of the R820T and the very low cost of the device. However, as with most things bought from the far east, quality control is rather variable. This particular model seems to be a good one, but if nothing else, always check that the little SMT dual schottky diode that can be seen just above the MCX connector is present. This is ESD protection and is crucial, as the chip is rather sensitive!

As I said, the stock antenna is abysmal. But I dont have an adaptor from MCX to anything useful, and I certainly wasnt going to buy one, as an adaptor is almost as expensive as the dongle! So I had a hunt about for something to use. As it happens, I intend building this one into a die cast box, with a USB-B socket and proper antenna connector, so it can be mounted as far as possible from the PC. In its present state, and plugged direct into a computer, it is very susceptible to RF noise from the PC and power supplies. On the casing of the old Marconi RC-690 ive been dismantling, there are three TNC connectors. OK, so not as easily used as BNC or N types, but far better than the MCX. As it happens, one of these is a bulkhead socket with a tail of RG-178, perfect for this task. So, a little cutout into the plastic case (which incidentally, is a silk matt black and feels so smooooth!) and I modified the dongle for an external antenna

I like to use RG-178 as its silver plated and makes for a very good and neat job. Be aware these are RoHS compliant and use lead free solder, so are a bit tricky to get melted. Ive left the MCX connector on, as it might come in handy, and its too big a pain to take off.

Im using this with SDR#. This software is fairly easy to use, but there is a specific version to use for these dongles, otherwise it doesnt give you the USB option! Setting up the software is a bit of a bother until you find your way about. With the dongle on its supplied antenna, you can barely get even the strongest of FM broadcasts, but on my discone, things are much different. It is prone to easy overload though, which results in a lot of spurious indications on the waterfall.

But for under £6 shipped, its a bit of fun, and the things it can be used for are almost endless! I have a 3m USB extension cable coming. Keep it away from RFI sources, shielded, and with a proper antenna, and it promises to be an interesting bit of kit.