Antenna Update

Tuesday night is RSGB UKAC contest night, this week on 432MHz. Following the frustrations of the weekend I spent a therapeutic Monday evening cutting various lengths of RG213 and fitting N-Type connectors.

I have now got a coax run through the wall into the shack, with another to follow soon. To facilitate ‘switching’ between multiple aerials I have fitted each aerial with a length of coax running down the pole, terminating in an in-line N-type socket near the base. It is a simple case of connecting the appropriate shack coax, fitted with a n-type plug, to the appropriate socket.

To keep everything water and weatherproof I have opted for a DRi-BOX. These are inexpensive plastic boxes sold as waterproof housings for outdoor electrical installations such as garden or Christmas lights.

The lid has a silicon seal and there are a number of cable entry points with a flexible seal. When the lid is firmly clamped securely in place the box is effectivly watertight.

It is a bit of a fiddle with the thick RG213 but it seems to work well. There was a vicious thunderstorm and downpour yesterday afternoon and the Dribox lived up to its claims after sitting in a few inches of water.

Still awaiting the X-50 collinear on the top!

The pole now has the 2M Yagi and the 70cm 7-element ZL-Special fitted. The 70cm aerial is far from optimal but I was looking forward to giving a go with some decent coax.

Tuning around prior to the start of the contest and the band seemed quiet, hearing just a strong local operator. The contest start time passed and I was met with a wall of static only hearing the occasional very weak signal. I tried unplugging and reconnecting plugs, new patch lead, took the VSWR/Power meter out with no effect after 20 minutes I gave up. I decided something was obviously wrong with my new installation at the top of the pole.

I went back into the house where the wife was watching some dreadful house/diy/makeover program on TV which I could only manage for about 15 minutes. Grumpily I went upstairs and fired up the FUNCube Dongle and twitter and realised I’d forgotten and completely missed the first pass of the newly launched UKube-1, unlike some lucky ones. Idly I tuned to 70cm using the discone in the loft and could see a waterfall of signals! Including that local operator with a lot of splatter considering he was running just 10W

Going back into the shack and things had improved, so perhaps it wasn’t my setup! After missing nearly a hour I searched and pounced again, just making 14 contacts but achieved my highest score so far for a 432MHz UKAC contest, still operating as M6GTG in the low power section.

Various operators have commented on the weird/poor/flat conditions last night, so perhaps I shouldn’t have been so dismissive of my ability to put up a decent antenna!

My seemingly magic HAB antenna

Since visiting the UKHAS Conference and getting my Foundation licence I have come of out of my lull with a new found buzz and have been busy with my radio gear.

I have helped track a few more High Altitude Balloon (HAB) flights, uploading the received telemetry data to the Habhub server. Visit the UKHAS website for more information on how to become involved.

I am still surprised by the performance of the loft antenna I am using to receive the HAB telemetry. It was constructed as an experimental wide band antenna solution to use with my scanners back in 2007. I discovered some basic plans on the internet for this simple home brew antenna (originally hosted on a Geocities website, but is thankfully archived on Reocities here)

It is a modified bicone design and is constructed from nothing more than a couple of metal coat hangers fixed to a piece of pvc water pipe, with a 10m length of RG58 coax as the feeder. I used it successfully as a portable solution, mainly for airband listening and stuck it on top of a fibreglass fishing/flag pole when camping away at dog agility shows as these photos show.

Eventually it ended up being mounted up in the loft, suspended on a piece of string from the rafters and had largely been unused since I acquired a discone a few years ago.

When I first started tracking the HAB payloads I naturally used the discone but soon became frustrated by its variable performance and so tried this antenna instead and was amazed. Something appears to be just right with this antenna on 434MHz when used together with my FUNCube Dongle PRO+

I haven’t analysed why is seems to have such a sweet-spot on these frequencies and I am not touching it, moving it or even taking another picture of it in case it loses it’s magic properties! Remember these HAB flights are only 10mW and this antenna is under a slate roof!

On Friday (20-Sept-2013) Adam Cudworth (@adamcudworth) launched HABE-10 which involved a normal tracker on the balloon due to burst at around 35km along with a secondary tracker payload of a 3D-printed man that was separated at approx 27km, the two payloads being tracked separately as they fell to Earth. During the accent the main tracker also transmitted SSDV images from an on-board camera. Unfortunately this failed during the mission but some pictures were received before it did, as you can see I managed to successfully receive this 300 baud RTTY signal from home and uploaded packets to help reconstruct the images. The original images are at http://ssdv.habhub.org/HABE/2013-09-20

 


 

Friday afternoon also saw the launch of Leo Bodnar’s B-13 pico floater. Leo has become something of a HAB superstar following his amazing B-11 and B-12 balloons which broke the duration record for an amateur balloons (as reported on the Southgate ARC website) flying over many countries before contact was lost.

Sunday saw a launch of MOD-1 by Ugi, which again I received rather well as can be seen on this pie chart.

Today saw Steve Randall (G8KHW) launch two of his XABEN flights, XABEN-56 and XABEN-57. The main transmitters were on 434.250MHz and 434.300MHz and I was able to use the multiple VFO option of SDR-Radio.com V2 to track both launches, and as you can see from these pie-charts I made number three in the tracking charts! I achieved this while using a VNC connection to control the receiving station from work, just checking occasionally to correct for any drift in the frequency of the signal caused by the temperature variation of the transmitters.


Away from the noise

High up and a hillside in Cumbria at the moment, nine metre fiber glass fishing pole and 10 meters of wire, a ground spike and my FUNCube Dongle PRO+ and the laptop and what a revelation!
The entire HF spectrum spread out before me with little or no noise anywhere, signals booming in! and this was the view last night
As they say Location, Location, Location!
 

The joys of RTL-SDR and Taxi MDT Decoding

I have owned and used radio scanners for many years, and loved them as my posts before December 2011 will testify.

In that month I became the proud possessor of a FUNCube Dongle Plus and discovered the joys of software defined radio, since then I purchased a FUNCube Dongle Pro+ and extended my SDR adventures in to the realms of HF and I have several of the insanely cheap RTL2832 based dongles.

As much as loved my scanners there was a major flaw with them, which has been brought into sharp focus now that I have used SDR.

No matter how fast or as sensitive as the scanner is are you are still playing a game of chance. You are limited by the frequency steps, demodulation modes and scanning rate of the receiver and you could zip through the band all day and still miss those elusive signals.

SDR and the waterfall display is a revolution, you can view a portion of the spectrum in real time and actually see the signals, they may be short lived bursts of data and voice, or continuous data transmissions.

The RTL-SDR dongles excel in this respect with their wide sampling rate you can view up to 2MHz of the spectrum at once, the following images show typical waterfalls captured this morning using one of my RTL-SDR dongles.

The first one, shows the cluster of data channels (was the old Vodaphone Paknet system) around 164.2 – 164.4 MHz, a trunking control channel and various data bursts, which are mostly Taxi Mobile Data Terminal (MDT) transmissions.

A little lower down the spectrum and another trunking control channel, a speech conversation, more data bursts and a faint digital channel.

Further up the spectrum into the UHF, a cluster of data transmissions.

Simply moving the cursor on the display and you can hear the transmission, if necessary change the demodulation type, widen or narrow the filter bandwidth, save the frequency. If you capture the IQ file you can then replay it endlessly tweaking and refining until you extract the information you want.

During the weekend I was experimenting and noticed there was lots of data bursts in the 163-168MHz range, I confess that I already knew what most of them were as I have experimented before with a scanner (with a discriminator tap) and Ian Wraith’s Java based Taxi MDT decoder. I decided to reinvestigate them using the RTL-SDR as the receiver.

While many taxi companies still use voice transmissions, many have adopted automated data terminal systems, where the dispatcher sends information about jobs to terminals in the cars, the drivers then can accept jobs, get information and send information back to the dispatcher.

Ian’s decoder which requires the Java runtime environment decodes systems that use the same physical layer as MPT1327 i.e 1200 Hz and 1800 Hz tones transmitted at 1200 bps. The two main systems used in the UK, are the Autocab and Auriga. The Taxi MDT Decoder currenly decodes the Autocab, but the coding for the Auriga system is still an unknown, so just outputs the raw data.

More information about Taxi MDT Decoder can be found here I confess to having one slight niggle with it, often I couldn’t get it to accept sound from the selected input. A work around I found was to first open the Audacity sound editor which I had installed and select the input and start a recording, then opening the decoder seems to make it work!

Ian has also written the excellent DMRDecoder which allows analysis of the DMR digital mode which is becoming more widespread. I intend to post some details soon about decoding digital modes, keep watching.

I created a video showing the Taxi MDT Decoder in action, the quality is pretty dire but you can get the idea, I identify the Auriga as being encrypted, it might be but as nobody on the team knows the protocol yet!

Dongles, Dongles Everywhere!

It is dongle overload at the moment at Châteaux Nerdsville,

Firstly I purchased a new (and improved) RTL-based USB DAB/DVB stick.

Like the FUNCube Dongle Pro it seems the manufacturers of these cheap USB receivers have been hit by the shortage of the Elonics E4000 tuner chips. Up to now the E4000 has been the most desirable tuner to have in the devices as it’s the most capable, giving usable coverage from around 60MHz to 1.7GHz. However Elonics has been liquidated and the intellectual property is up for sale and supply and/or stocks of the E4000 have dwindled. Manufacturers have therefore resorted to different tuners.

One of the alternative devices is the Rafael Micro R820T which has support in Linux drivers so the source code was ported to the rtl-sdr project. Several postings I saw hinted that the device could preform down to around 24MHz (which I have yet to confirm) and was more sensitive, so when I saw a dongle for the grand total of £11 including postage I brought one. It is a Newsky dongle and looks exactly like my previous one.

In the meantime the newly designed FUNCube Dongle Pro PLUS has been in production and lucky customers have been slowly receiving theirs. With over a thousand people in front of me on the waiting list I wasn’t expecting one before the new year, so imagine my surprise when I received an email on Thursday inviting me to buy one, which of course I did without hesitation, and it arrived on Friday! So over the weekend I had chance to toy with it and the RTL-USB device.

The first issue I had was my favoured SDR program SDR-Radio doesn’t yet support the new FCDP+, but support is being worked on for the V2 release. There is support using a new EXT-IO dll for HDSDR but I was keen to reacquaint myself with SDR#.

The SDR# (SDR Sharp) project which has become a popular application and I have used it occasionally with the original FCD. The latest download comes complete with all the necessary drivers and libraries to use with the FCD/FCDP+ and RTL-USB devices, indeed it was quite painless and everything seemed to work straight out the box.

The most obvious new feature of the FCDP+ and one of the main reasons I got one is the extended frequency range. It works down in the LW/MW and HF bands and that is what I have spent most of the weekend doing, picking up a lot of SSB/CW Amateur contest traffic as well as other CB operators. The VHF/UHF preformance seems much better and less prone to noise that the original FCD, but have yet to use it in anger.

Here is a recordings made using the FCDP+ down in 20 meter band

and one made showing reception in the 10 meter band of what appears to be SSB transmissions from America.

The RTL-USB device unfortunately had less use over the weekend playing second fiddle to the much more expensive FCDP+, however I did have some attempts at using it and first impressions are that it is more sensitive than the older version, but that really isn’t much of recommendation as the older device was quite deaf, however it does look quite promising.

This is a recording made of some SSB TX by the RTL-USB in the 70cm band. (The recording that was posted earlier was recorded using the FCD)