Stackmatch Binocular Matching Transformer

Many stackmatch designs use a toroid core, but I’ve decided to  instead investigate using a multi-aperture “binocular” core.

There are not many manufacturers of large-ish binocular cores that can take the full VK HF limit of 400W PEP, let alone a full kilowasp amplifier !

So to start with I’ve chosen two well known binocular cores that you can obtain from various online suppliers;

  • Amidon BN-61-002
  • Fair Rite 2861010002

Both are the equivalent of each other in terms of price, size and material.  Both are made from Type 61 NiZn Ferrite material with a ui of approximately 125.   This material has good low loss properties and is essentially what others have used for their toroid designs.  It seems like a good place to start.

The transformer is wound the same way as if on a toroid, so take three wires, twist them together (battery drill helps) and wind the desired number of turns through the holes.   It seems sensible to start with a full core and take turns off if I achieved too much inductance.  Both of these cores hold ~4 turns of trifiliar wound 20AWG silver plated PTFE wire, it might add the last turn is hard to do.  All that is left is to series up the windings and tap at the appropriate positions, the schematic is to the right.

To make my measurements I decided to use a variable resistor (R1) to check the effect of load change on the transformer.  I had also seen some designs using a small amount of shunt C across the output to neutralise the output leakage inductance as well, so I placed a variable cap across the variable resistor when needed.

To make these measurements I’m fortunate to have a Spectrum Analyser with tracking generator and a suitable Return Loss Bridge.  The return loss bridge has a directivity of > 45dB at HF.  This means I can see how much energy is being transferred to the load resistor (R1), any RF energy that is being reflected back to the spectrum analyser is therefore wasted, so in all of the screen captures below the lower the Return Loss the better !

You can see the experimental setup in the image below and my initial measurement on the kitchen table…

I’d start these measurements by adjusting R1 until I achieved the lowest Return Loss and then remove the resistor temporarily and measure it separately with a 4-digit ohmmeter.  I wanted to check that I was close to 22.2 ohms in all cases.   With such a low resistance don’t forget to deduct the resistance of the multimeter leads.

Once I had confirmed the impedance transformation was correct I would then tack the trimmer cap across R1 and start with it completely un-meshed (min C).   The cap on the secondary side of our transformer will assist in “tuning” out the output reactance (Xs) of the transformer a little and improving the return loss at the upper end of our plot.

However there is a null that will occur at the low end of VHF that you need to be wary of as it can be unstable, you can if you have enough capacitance bring it into HF spectrum which is not advisable.   You will easily see the point where I’ve added too much in the sequence below.   What you’re looking for is just enough turns to give you the broadest match possible with the smallest value of C across it’s output.

Here’s a sequence of plots showning what happens as you increase C from min (5p) to too much (120p).  This was with 3 turns wound through the core, R1 approx 22.3 ohms;

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So in the above sequence you can see what the effect that the capacitance has across the output.  As we keep adding capacitance the return loss kept getting better and better before the null appears.  The ideal compromise was somewhere between 65p-80p.

What you can also do is widen out the frequency from 1-200MHz and watch what effect any nulls will have on the response.  It also pays to vary the load (R1) and see what happens as the load decreases below and above the desired match.   I found that as the output impedance goes up the null has a much more prominent effect as the load is varied.

So starting at 4 turns I checked the impedance ratio and then wen’t looking for the best compromise in terms of response, stability and lowest capacitance.   Below are the three best candidates that I found;

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So as we reduce the turns (inductance!) we see the return loss decrease at the low end, but we also see the required capacitance to flatten out the upper end also reduce.   Based on these measurements the best compromise I found for this transformer is 2T C=30p.   Ideally a return loss less than -20dB is a good target to aim for, I’ll let the reader work out what the equivalent VSWR is for this RL value (hint: it’s low, for additional points also take a look at a RL of -10dB for reference !).

Now to check that this is stable I’ve included a plot that is much wider in terms of checking the VHF region for signs of that null we wish to avoid.  Hmm, this combination is potentially usable up to 6m, now there’s an idea for another day.

Sweet no signs of the null being anywhere close to the MF/HF bands I want to operate over.

Now the ultimate test is to make two of these transformers, place them back to back into a 50 ohm load then measure the insertion loss. That will give me some idea on what sort of loss this transformer will have an ultimate how hot it will get when I attempt to pass 120W CW/FM or 400W pep of SSB.

More to come.

Yet Another Stackmatch Design

In October each year I join my local radio club AREG and participate in the Oceania DX (OCDX) contest.   For the past few years I’ve been the band captain for the 15m/10m station.  We started in 2014 with just an Icom IC-706mk2 and 2 element broadband Hexbeam, but this quickly morphed into a complete Elecraft K-line, Amplifier, SDR’s, filters and Spiderbeam on a 8m pump up mast; as a portable station.

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So to take our station yet another step forward we need to add additional antennas like mono-band verticals to take advantage of lower propagation angles. Here in VK we are along way from anywhere, so the majority of our propagation on most bands is below 20 degrees.   However before you can take advantage of these additional antennas you need to switch between them.

Trolling the internet for inspiration we discovered the 3 to 1 Broadband Stackmatch.  This is a simple 3 port antenna switch that can individually switch each antenna to the radio, but it can also be used to parallel up any two or all three antennas ports together.

It exploits the fact that two 50+j0 ohm antennas in parallel is 25+j0 ohms and three in parallel just shy of 16.7+j0 ohms.  So by inserting a transformer with a fixed impedance matching ratio we can bring the parallel antennas effective impedance back up close enough for many amplifiers/transceivers not to care about the minor mismatch.   So that is deviously clever and simple.

The equivalent circuit of a stackmatch transformer is shown to the right, you’ll see it drawn many different ways but I find it easier to understand when drawn as an autotransformer.

Accordingly the maths says;

n = Np / Ns
Z’ = (n)^2 x Zload

So lets see what happens;

Np = 3, Ns = 2, n = 3/2 = 1.5

So with two antennas in parallel;

Z’ = (1.5)^2 x 25 = ~65.3 ohms

With three antennas in parallel;

Z’ = (1.5)^2 x 16.7 = ~37.6 ohms

This small impedance mismatch (~14 ohms) either side of 50 ohms will not cause many radios much difficultly. It does however tell us that we must ensure that our antennas are resonant (50+j0) since we have ignored the imaginary impedance for simplicity.  What is also not explained is you need to keep the feedlines to each antenna from the stackmatch as close to the same length as you can or there be more trouble with impedance transformations.

There are plenty of designs on the internet and products available from suppliers but their cost are quite simply prohibitive, especially if you want 6 or more of these units to achieve your desired switching arrangement (*gasp*) at a contest.

So here starts yet another project to design and build my own low cost 3 to 1 Stackmatch.   It certainly has not taken long to come up with a workable 3D model.  It kind of helps that I design products like this every day as an Design Engineer.  I’ve also learnt a trick or two about lowering cost in my time.

However in talking about this project over a coffee with an old work colleague/Boss he reminded me about the perils of using Toroid cores for broadband transformers.  He suggested I should instead use a multi-aperture “binocular” core or “pig nose” core as I seen them listed on eBay (LOL).  These binocular cores also have the added advantage of being smaller than a FT240-61 toroid core that everyone seems fond of.  You can see the grey block in the picture below that represents this core, it’s tiny compared to the equivalent toroid.

So that begs the question, just how good are they ?   Well there is nothing like buying a core and winding one to find out (click).

Tower Project – Hexbeam is Up

So having finally erected the tower I could finally set about getting my K4KIO Hexbeam in the air.   Having spent quite a bit of time rebuilding the Hexbeam it was time to reassemble it in the back yard.

From there we had to man handle it into position.  To get the antenna on the pole we simply released the two front cords that hold the arms together, it makes the antenna a little wobbly for a bit but you’ve just got to be careful.

Again having the tower within 300mm of the shed makes for a rock solid platform from which to work.  I will be adding some perforated steel walk way to the top of the shed in the coming months that will give me a level platform on which to stand.  These shed roofs are slippery when wet, safety first.

Time to send it up

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In all it’s glory !

Ok so all that is left now is to get inside and start calling…. HALLO CQ !

Tower Project – The Erection

Yes it’s time to now erect my tower, what did you think the tittle was about ?

Having waited the mandatory 28 days for the concrete to cure we could finally set about assembling the lower sections of the tower.

I didn’t like the way in which the original NBS mast upper pulley system worked, it projects out at a weird angle and sits uncovered.   With the new shed behind I had space and a few friends to help me weld up a new one.

The winch that came with the kit was OK but I’m paranoid about what brands of gear I use for lifting.  The entire weight of the antenna, trolley and feed lines sits on the wire.   So I purchased my own braked winch, new cable and rated lifting gear.  I’m not sure what I was supplied was rated or not,there were no marks to be found.  I’ve spec’d everything at 400kg WLL which is overkill, but at least I know it will not come down due to failure.

Basically if you’re not happy to personally ride the hook in a harness to the top of your tower with the lifting gear supplied then perhaps you shouldn’t use it, think about that a little.

Now that the base was in place I could load the RHS section and stand the tower up.  The RHS section is not light, so I’m glad I had a few friends over to help.   I was nearly out done by my own cleverness with where I’d placed the tower.  At the end of the shed there is a 600mm drop off, which meant getting the pipe into the base section above was tricky since you were nearly 2.5m off the ground on a ladder wrestling with a pole.

However once the pole is in the lower section it’s a simple case of lifting it into place using the hydraulic ram.  This is where the beauty of this tower system lies.  Once the tower is vertical the trolley is lowered down and the ram removed and stored in doors.

Here you can see the Rotator and thrust bearing being installed.  I think I’ve got the height of the shed just right being able to sit on the roof and working on the pole, couldn’t have been any easier.

Now it’s time to finally get that Hexbeam back in the air.

Hexbeam Rebuild

In January of this year I had to remove my prized K4KIO Hexbeam and portable mast to make way for my new shed being installed in the back yard.  The Hexbeam had only been up on the temporary mast for two and a half years while I got around to building a new shed and tower to support it, you can’t rush these things you see.  Anyway here’s a few photos of what it all looked like when I put it up back in June of 2014.

Once the Hexbeam was down it was apparent that the fibreglass poles used in the Hexbeam were not covered with enough sunscreen protection and the Aussie sun had given them a serious case of sunburn.

Basically the top of the poles had perished to the point where the glass fibres were exposed.  The pole at the top of the image shows the fibres, the poles below are brand new.  The difference is easily seen.  I’ve seen references on various antenna forums saying “paint these poles to make them last longer” but I’ve never as yet found anyone explaining what happens if you don’t, so I guess the above tells the story.

So there is no way I could put them back up like that (they are a health hazard), so the question was how to fix it.  Basically I need to cover the fibres again and prevent any further damage to the remaining epoxy and fibreglass.  The poles have not lost any structural rigidity or strength (yet) so the decision was made to paint them with a good quality exterior automotive acrylic paint.  Acrylic paint is designed to go over fibreglass (its used on cars after all) and remain slightly pliable meaning it wont crack when the poles are bent.  I was considering marine varnish, but experiments with some fibreglass tent poles and marine varnish failed miserably when bent.

So I then sanded the poles every so lightly with 180 grit paper by hand to remove the worst of the glass fibres that were exposed.  Of course don’t neglect to wear a suitable mask, eye coverings, gloves and do this outside !!!  You definitely don’t want to be inhaling the small fibres into your lungs that is for certain.   These glass fibres itch like crazy if they get onto unprotected skin, you have been warned !!!

The result of sanding and painting the poles can be seen in the next image.  I think you’ll agree they look much better, I’ve used a slightly darker grey paint than before so I could see it while spraying but the finish should stand up to a few more years of the Aussie sun, I’ll be keeping my eye on it for sure.

The original Hexbeam was supplied with a small flange under the base plate that would accept a 32mm pole.  I was using a short length of water pipe in my temporary install and you’ll see this small pipe mounted straight into the rotator on my previous mast.  On my new tower it has a separate carriage that holds the rotator and thrust bearing, so I’m planning on using a piece of T6 scaffold pipe which is 47mm in diameter.  Thankfully Leo K4KIO had a larger flange to suit, so I ordered one when one of my fellow club members purchased their Hexbeam a few months after me.

Now it turns out that mounting the new flange required some base plate modification, since the new flange is about 50% bigger than the original.

So that meant the smaller flange bolts would interfere with the larger flange.   So to get around this I’ve basically countersunk the original holes that held the flange that supports the centre post to accept a M8 15mm CSK 316 stainless bolt from underneath.

I’ve then drilled the base plate to accept the new larger flange, however I’ve had to rotate this new flange by 90 degrees to allow the bolts to clear the smaller flange on top.  The image above shows the Hexbeam plate from the top, you can see the four new bolts that hold the bottom flange and how the bottom flange has been rotated to clear the top flange.   I’ll need to pay a little more attention to the fibreglass poles as they are bolted down again to make sure that the last u-bolt holds the tube against plate securely and wont slip, a bit fiddly but manageable.

From underneath you notice that you can’t see the top flange bolts.  These having been countersunk are hidden completely under the new flange.  It took me a while to work out how I was going to do this neatly.   The countersunk bolts are 0.3mm or so proud of the surface which means when large flange bolts are done up tight, they lock the smaller flange bolts into place.

It’s handy if you have a drill press to work with and a 19mm rosehead countersinking bit.  I also drilled the holes for the pin that will go through the top of the 47mm scaffold pipe, you can’t quite see it in these photos but it will stop the Hexbeam from turning on the pole, I’m certainly not going to rely on the grub screws to stop things from turning.  This antenna is 7.3m across which can generate quite the torque on the mast.

So the good news is that just leaves me with replacing the pop rivets that hold the centre post into the top flange, it seems over time all of these pop rivets have worked themselves loose.  Then it’s time to put the Hexbeam back together and onto the tower, which will be another blog post shortly.  I will also get to writing up something about the shed that I mentioned at the very beginning of this post at some point too.

UNC vs BSW

I think today I received another  great lesson in why it’s “good to grow up in a Metric world”.

I dropped into our local rigging store A. Nobles and Sons to pick up a Collared eye bolt for my tower, however they were out of stock of M12 collared eye bolts.  Ok no problem there’s a 1/2″-UNC option that I can just substitute.  I would have needed to purchase a M12 nut regardless, no harm done.

So off to the local bolt and nut store United Fastners just around the corner and pick up a suitable 1/2″-UNC nyloc nut.  However sitting down at my desk at work I try to dry assemble the eye bolt and nut and they will turn 1-2 turns and then bind.

So out with the thread gauge, nut 12-tpi check, eye bolt 13-tpi, Umm hang on a second I smell a dirty rat.  Turn eye bolt over and read from the bottom…

*&^%$# that 1/2″ thread it’s a Whitworth (BSW) not UNC !!!

Anyway snot-a-gram sent to Nobles, we wait to see what fun that will create.  However once again I find that there are two thread pitches being used for the same imperial sized bolt which are not visually different enough to raise suspicion.   I’m sure that the old timers that grew up with this UNC/BSW thing would be having a good chuckle.

Serves me right for not dry fitting them in United Fastners and finding the obvious mistake… but this variation between tpi in imperial is quite simply at times unbelievable, this is the second time I’ve struck this issue this past year.  don’t get me started on 1/4 & 1/2 tapered pipe threads, you’ll see the fun I had there with the Hilux Turbo EGT sensor.

I’m lucky that I work for a business that has been around for more than 50 years… So at morning tea time I droped into visit the onsite Fitter and Turner, there I find we have surplus 1/2-BSW nuts that have been quickly turned down to half-nuts so I can lock them together and stop the collared eye bolt from undoing.

So now all I have to do is assemble everything, watch this space.

HAB Diplexer

This past week after work I’ve been working on a RF diplexer and filtering arrangement for a HAB Crossband repeater we’d like to fly on this coming weekend.  So after a bit of simulating and gnashing of teeth I came up with the following arrangement.

HAB Diplexer Circuit

The Crossband repeater will RX on 2m and TX on 70cm using a single HT 2m/70cm antenna suspended under the payload.  Being worried about interference from high power VHF services on the input to this repeater and high harmonic content on the transmitter I decided to use BPF arrangements.   We’re only using Dorji modules on the repeater to keep the weight down, so too much RF at their front end would cause them to freak out a little when suspected 35km above the earth with a radio horizon of 800km or more in all directions.  These modules are also known for having a fairly rich harmonic output on the TX.

In the past I’ve build a few of the filters designed by Tasa YU1LM, so I’ve borrowed his high performance 2m BPF design, you can find the original article on this page (click).  Tasa’s designs are excellent and easily reproducible, so many thanks to Tasa for sharing his knowledge !

The remaining circuits in the arrangement above are standard HPF, LPF and manufacturer reference designs which were optimised for various characteristics.

Having completed the theoretical part it was time to build it.

The board was quickly roughed out in a CAD package and thanks to Mark VK5QI a routed board produced as the first prototype.   Routed boards require you to pick off the large expose sections of copper on the board, so 30 minutes with a scalpel under the microscope and I had a board that was looking the business.

So then all that was left was to populate it, which is easier said than done.  The worst part was the ground plane stitching which was done with a combination of 0.7mm TCW and 0.35mm wire-wrap wire.  However I’m pleased with the result.

Now the inductance of the coils in the 2m filter were initially too high, so a turn was removed after this photo was taken.   The response of the 2m filter (with 5T coils) after a few minor tweaks pulling and pushing coils together can be seen below;

Not too shabby.  The Insertion loss is a little higher and the bandwidth a little wider than I had simulated, but all in all that is the beginnings of a quite reasonable 2m BPF.  The two frequencies you can see in the marker table are the RX and TX frequencies of the cross band repeater respectively.  So the filter is giving a isolation between ports of greater than 55dB which should keep things going well.

However looking at the 70cm port told a different story.

The TX harmonic filter had a terrible response with the cut off frequency below 400MHz, so that was killing any chance of the diplexer working satisfactorily.   So I temporarily bypassed the harmonic filter so I could see what the 70cm HPF was doing, the plot of that is below.

The insertion loss is still too high, but moving the cut off frequency 40MHz higher should restore the insertion loss figures I’m aiming for (<1.5dB), so I’ll pull out the simulations and see what i need to to change.   Looking at the response it’s quite apparent that parasitic capacitance and inductance has crept into the design via the PCB, so with luck we can tweak this back to where we need to.  I’m not lucky enough to have the tools that allow me to simulate the effect of the PCB design as well, sigh.

Anyway we have the makings of a 2m/70cm HAB Crossband Repeater Diplexer, now if only i had another week in which to play with it and perfect the design (grin*), launch will be in two days time nothing like a bit of pressure.

Tower Project – Foundation

Now the entire reason for building a new shed was so I could plant a 10m tower by the front door.  It wasn’t just about getting some more space in which to store stuff you see.

I’d been without a HF beam for the past 3 months while the shed was being built, so having the tower go up at the same time was the cherry on the cake.

I’d purchased a second hand uninstalled NBS Antenna Tilt mast that should do the business of holding up my Hexbeam.  It has also been sitting languishing this past 15 months in a corner of my yard.

So during the shed construction, prior to the concrete floor being poured I had a local contractor come in and excavate the foundation.

The ground as I’ve explained previously is hard to dig.  The clay holds what look like river pebbles that are smooth, when you get clear of those you find layers of limestone.

This should not come as a surprise since the Torrens River is not that far away at the bottom of the hill over my rear boundary.

I’m thankful that we managed to achieve a depth of just over 1200mm.   That meant I had to modify my foundation design a little to ensure I got enough weight into the hole to overcome any uplift from the tower and antenna at a later date.

The hole is over 900mm wide at the top and ratchets down to 450mm at the bottom.  You can quite clearly see the soil profile.  This hole took just shy of 4 hours to excavate with a 40hp Bobcat with auger, I’m glad I didn’t have to dig this by hand.

With the hole not being quite deep enough I modified the base to include some addtional steel reinforcing.  This meant I could then grab hold of the concrete at a greater distance from the pole.

Additional steel was later added and wired to the horizontals before being encased in concrete.  To get the mast base in the right place I had to offset the steel to ensure nothing protruded from the concrete block above a depth of 600m.

After the concrete had been poured they even finished off the pad to make it look neat.  There was also a layer of mesh added in the top box to give it that little extra strength and a fall of 30mm from the centre to the edges to ensure water does not sit where it is not wanted.

From here I’ve got to wait 28 days for the concrete to harden to 80% strength before I put the tower up.  This is going to be murder.

Cellular Modem & GPS

Well it turns out that the HP un2420 cellular modem has an inbuilt GPS that can be turned on and off..   So a bit more google foo and I found all that was necessary is to echo a command into the third enumerated serial port like so;

#$ echo "\$GPS_START" > /dev/ttyUSB2

Then we can just point our favorite GPS monitor program at /dev/ttyUSB2 and we should see NMEA data.  I used gpsmon to test mine.  I was amazed to watch this working on my bench inside the shack.

Now my Wenet ground station can easily go portable with Cellular data and GPS position information and no USB 3G dongles required.

HP Cellular Modem for HP 5320m

After pulling the bottom cover off my second-hand HP Probook 5320m a couple of times I was pleased to discover a PCI-E slot for a HP un2420 mobile broadband modem.  It appears that the Cellular antennas are already built within the lid (or screen) of the laptop.   A light bit of reading also suggested that these modules were capable of decoding GPS signals too.

I just had to try one.

So for the princely sum of $35 I procured a HP un2420 Gobi 2000 module from a local eBay seller.  There are suppliers on eBay that are selling these HP Gobi 2000 modems out of China for $11, which is an absolute bargain. YMMV.  I didn’t want to wait until after Christmas to try one so I purchased one at the higher price.

Now of course I want this to work in my shiny new Ubuntu 16.04 LTS laptop.  Oh well I’ve always welcomed a challenge.   I’ll detail the steps I took to get to a working solution.

First we should check we can see the module;

#$ lsusb
Bus 002 Device 002: ID 8087:0020 Intel Corp. Integrated Rate Matching Hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 004: ID 05c8:0403 Cheng Uei Precision Industry Co., Ltd Webcam
Bus 001 Device 003: ID 03f0:241d Hewlett-Packard Gobi 2000 Wireless Modem
Bus 001 Device 002: ID 8087:0020 Intel Corp. Integrated Rate Matching Hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

I’ve highlight what we’re looking for (in blue) which is the “HP Gobi 2000 wireless modem”.  You can tell that this device is currently un-programmed as the product ID is “241d” and changes to “251d” once the firmware is loaded.   You also need to check that /dev/ttyUSB0 exists and that your user that starts the 3G cellular modem is in group dialout.  This isn’t necessary right now but does simplifies things later.

Now you’re going to require a Microsoft Windows machine, sorry there is no other easy way around this.  Some have managed to pull apart MSI files in wine, I didn’t get that lucky.  The Qualcomm Gobi modems down load the firmware into the modem at power on, once the firmware has been download the modems will work.   I was lucky to have the same laptop with Windows 10 installed, so I simply installed the driver on this second machine and then stole the files I needed on a USB stick.  I had to install the un2420 module in the Windows machine to get the drivers to load.

From within the Windows driver there are two directories from which we need three files;

  • ….\Qualcomm\Images\HP\UMTS\amss.mbn
  • ….\Qualcomm\Images\HP\UMTS\apps.mbn
  • ….\Qualcomm\Images\HP\6\uqcn.mbn

You’ll need to work out where the drivers get placed depending on your version of windows, it will be installed in the program directories there somewhere.  I found them in the Program Files (x86) directory on Windows 10.   The files above are what you require for a Generic UMTS modem (6) that is not vendor network specific or locked.  This worked nicely for my unlocked module and Telstra pre-paid SIM. YMMV.

Now once you have these files on a USB stick you can start work on the Linux machine.

#$ sudo apt-get install gobi-loader

This will get the udev helper we need to load the firmware into the modem.  Once this is installed copy the files from the windows machine into this directory, watching file permissions of course;

#$ sudo cp /mnt/usbstick/* /lib/firmware/gobi

As mentioned we want all three “mbn” files above in that directory.  Now you might like to check that the udev rules for QDL modems are correct in the following file;

#$ less /lib/udev/rules.d/77-mm-qdl-device-blacklist.rules

Your looking to check that the following two lines exist;

#HP un2420 Gobi QDL Device
ATTRS{idVendor}=="03f0", ATTRS{idProduct}=='241d", \ ENV{ID_MM_DEVICE_IGNORE}="1"

These lines will prevent ModemManager in Ubuntu from trying to take control of this device at boot up before the udev helper gobi_loader gets a change to load the firmware.   Now if all is going well reboot the machine and then check the following in a terminal;

#$ lsusb
Bus 002 Device 002: ID 8087:0020 Intel Corp. Integrated Rate Matching Hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 004: ID 05c8:0403 Cheng Uei Precision Industry Co., Ltd Webcam
Bus 001 Device 003: ID 03f0:251d Hewlett-Packard Gobi 2000 Wireless Modem
Bus 001 Device 002: ID 8087:0020 Intel Corp. Integrated Rate Matching Hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

You should notice that the product ID has changed from 241d to 251d, meaning that firmware is now loaded in the 3G modem.  I tried not rebooting this machine which turned out to be a mistake.  ModemManager is a pernickety piece of software that won’t do anything unless everything is 100% right.  I wasted a number of hours chasing my tail trying to get services shutdown and restarting stuff.

You should also see that there are now three tty USB devices attached to the system;

#$ ls /dev/ttyUSB*
/dev/ttyUSB0 /dev/ttyUSB1 /dev/ttyUSB2

If you don’t see these three devices then there is something wrong.  I’d suggest you start with dmesg and searching your way through syslog to find out why what’s gone wrong.  It can take a while.  When you do get things right about 13-15 seconds after the kernel loads you should see the firmware loaded into the modem and the additional USB serial ports being created.   If the firmware doesn’t load you will find error messages in syslog.

Now you should be able to configure the NetworkManager to actually use the Modem.  I simply went into NetworkManager -> Edit Connections and created a mobile broadband connection from scratch.   The APN we want to use for Telstra is “telstra.internet” and the rest is pretty much a vanilla install.

Once you’ve got the connection, done you should be able to connect to the internet via the cellular modem.

Oh and don’t forget to plug your SIM into the slot in the battery case.   Now you can sit back and enjoy Cellular Mobile data without any external USB dongle to snap off, or get left behind.

Yay !