Repairing my Walkabout mk II

Last weekend in the North of England was graced by glorious sunshine and radio rallies on both Saturday and Sunday - perfect conditions to take drives in the country...

Saturday saw the XYL and I take the short run to Bickershaw, for South Lancs Amateur Radio Club's Summer Rally at the Bickershaw Labour Club.

I didn't find much to enthrall me at the event - but I was tempted by a scanner antenna, which looked as if the telescopic section could be just the thing to repair my Walkabout mk II's top section, recently damaged on Aldeburgh beach...

I paid £3 for the donor part - which seemed fair for the complete antenna, even though it was a bit steep for a cheapskate like me who was intending to take it home and tear it to pieces!

On getting back home and stripping off the heat shrink at the base, I confirmed that the telescopic section was indeed exactly the 48 inch length that I needed to replace the broken section of the Walkabout - seen here below the donor scanner antenna...

I removed the loading coil and BNC from the donor and decided I would try to recover the brass base from the broken Walkabout part to save the female 3/8 inch 24 TPI thread...

Unfortunately, in unscrewing the antenna from this base component, the threaded stud from the antenna sheared off and could not be removed - so I drilled it out and tapped the over-sized hole 2BA.

I then made a brass coupling piece with a 2BA male thread at one end and an M2.5 female thread at the other to accept the new telescopic whip from the donor...

A final covering with heat shrink and the new antenna top section is complete - differing only from the original by being slightly less stealthy (in its bright chrome finish) and a little longer when fully extended (which won't hurt tuning at all)...

Not a bad result for £3 - for anybody else wishing to follow in these footsteps, I note that 48 inch telescopic antennas can be had on eBay from Far East sellers for around £3, including shipping.

I haven't had any QSOs on my new antenna - but I have confirmed it tunes up on 40m and the FT817 gives exactly the same indicated SWR and Power Output as before - result!

Sunday offered the LAMFEST Rally at the Elsecar Heritage Centre, so a blast over the backbone to Yorkshire was indicated.

Now this is what I call a Rally location - combining Radio and Railways AND some entertainment for the XYL (gift shops, antiques, vintage clothing shops, cafes etc) all on one site.

We were met by views of a little ex-Coal Board 0-6-0 Saddle Tank plying her trade in front of the car park as soon as we arrived...

I got myself a Sealed Lead Acid brick to enjoy longer visits to future beaches with the repaired antenna. There was even a COMPLETE Walkabout mk II on offer for £25 - a good deal for somebody!

A great weekend.

 ...-.- de m0xpd

Walkabout on the beach

I was fortunate to spend last week on the beach at Aldeburgh, Suffolk. I took my FT817 and Walkabout II Antenna, hoping to work some /p.

Well - I have to report a pretty poor show - I logged only one contact (with Richard, g0iln, in Bexhill, who also was running an FT817, but into a "real" antenna). I was only trying to work G stations - mainly FISTS calls. Perhaps I would have done better to turn my attention to the continent, where I've had better luck with the same portable equipment before.

Nil desperandum  -  band conditions were the least of my problems...

Packing away one day I accidentally bent the telescopic section of the antenna, which promptly snapped...

I thought that would put an end to the week's activities on 40m (the Walkabout II needs the full length of the telescopic section to tune up on 40 metres) but the good folks at MCT Electrical on the High Street kitted me up with an off-cut of garden wire which I could stuff inside the broken antenna and I was back on air.

All their kind efforts were in vain for, try as I might, I could not raise a further contact. I reverted to a different interpretation of "Walkabout", which turned out to be infinitely more fruitful...

Aldeburgh's beach is graced by two conspicuous "Lookout" buildings, once operated by two rival companies (known, as I discovered in the interesting little museum in the Moot Hall, as the "Up-Towners" and the "Down-Towners").

The Southern Lookout is now owned by Caroline Wiseman...

This ecclesiform curio, with its west tower and its east opening to the dawn, lives up to its billing of a "tiny temple of inspiration for artists, poets, writers, performers and thinkers".

It drew me in when I wandered by, because it was playing host last week to a collaboration entitled "The Awe of the Other"...

The collaboration was between Pauline and Matthew Bickerton and Thomas Garnon. Most of their work soars high above the scope of this mundane blog. But Matthew's practice embraces some technologies of real interest and significance to us...

The "Otherness" implied in the "Awe of the Other" title refers (in part) to impromptu conversations between members of the public who choose to sit on two larger-than-life chairs placed some 100 metres apart on the beach, facing the horizon (seen in the images above). Snippets of these conversations have been recorded and assembled into a work which was exhibited on Saturday, 14th June, in the Lookout. Part of this "performance" was delivered through a modified radio.

Matthew took an old domestic radio and fitted a custom media player, which he built around an Arduino-compatible system. The host radio and the new internal media player is seen here...

The tuning mechanism (which originally drove a ganged variable capacitor) has been retained but modified to drive a potentiometer and so generate a voltage, which is sampled to allow the user to select 1-of-n saved sound files (rather than to tune the radio to a station). 

Matthew's custom media player is seen in close-up here...

It uses a Teensy 3.1 (which itself has a ARM processor - similar to that on the Arduino DUE), sitting atop a Teensy Sound Card (which hosts a MicroSD Card for Mass Storage - just visible in the photo) and a digital power amplifier, seen on the blue evaluation board to the left of the photo.

It turns out that Matthew is no stranger to bringing bespoke technology and art together in exciting new ways, as is exemplified in the Guerilla Dance Project.

 Great to see people making beaches MORE beautiful for a change!

 ...-.- de m0xpd

Imprints of Redundancy

I am (as a result of a generous Christmas gift from my daughters) a subscriber to Practical Wireless and had been pleased to see some of Don, g3xtt's "holiday snaps" from the DXpedition to the Austral Islands in the June 2014 number.

The whole DXpedition scene isn't really my cup of tea - but it is part of the rich tapestry of our hobby.

I also am a member of the RSGB - and so today I was surprised to see pretty much the same photo used on the front cover of the June number of RadCom as already had graced this month's PW...

There is enough overlap between the TX6G expedition members, the articles' authors and the editorial teams (g3xtt IS the PW editor!) to have spotted and avoided this gaffe.

It is hard enough for any monthly publication to retain sufficiently fresh content to justify its own existence. When the two most important print periodicals on the UK radio scene start treading on each others toes like this, something is seriously wrong.

These two publications can only coexist in my magazine rack if they are strongly differentiated - any more convergence of the sort demonstrated by the repetition and redundancy seen this month will mean they won't be coming through the m0xpd letterbox next year.

I'll go further - with the obvious extrapolation of the above.

These two publications will only coexist if they are strongly differentiated - any more convergence of the sort demonstrated by the repetition and redundancy seen this month will mean they will not survive as respected print publications for much longer.

...-.- de m0xpd

Sweeping the BITX IF

Having brewed up the little scheme for visualising the magnitude response of filters using RF sweeps generated by the Arduino, I decided to take a look at the crystal filter I made for my SSB rig. That rig started from Farhan's famous "BITX" design and, whilst I have added a number of elaborations, it is still very much - and very proudly - a BITX.

The IF filter had been made from Farhan's schematic - but I paid no heed to the accompanying words and music, specifically making no attempt to match or select crystals. Instead, I just took the first four crystals out of the bag and worked with them...

The results were less than perfect - so I made some adjustments "by ear" - you can see evidence of some of these modifications in the capacitors still hanging off the filter in the photo above. It had been sounding and working FB for the past few months - but now I was in a position to measure it...

I started by disconnecting the filter from the surrounding rig and measuring the IF response according to the scheme described in the previous post. This yielded the response shown below...

OK - it might not win any awards - but it is consistent with the acceptable performance my rig has been turning in over the past few months - and consistent with the BFO frequencies I've been using in my Double DDS Code (as described in the current edition of SPRAT)...

[My rig can be used in either LSB or USB mode, according to the band it is working - the two numbers above are the BFO frequencies for LSB and USB operation, respectively.]

However, the filter isn't intended to be used in some artificial test environment - it is intended to be operated in the very specific loading conditions in the middle of my rig - so I really ought to be measuring its performance there. Before I could do that, I decided some housekeeping was in order...

The IF section of the rig was squeezed into the corner of the PCB (as shown by the dashed red box)...

I took a few moments of redistribute things and give myself a little more elbow room, giving the whole IF section a slightly bigger red box to work in (and opening up the whole dashed blue box pregnant with possibilities for future developments)...

Now, it was easy to measure the response of the entire IF section - not only of the filter, but of the amplifiers on either side of it.

The signal from the DDS module was attenuated through a PI pad (actually a cascade of such sections, embodied in my switched attenuators), to both attenuate the signal and ensure a 50 Ohms source impedance and the amplifier after the filter was loaded by the 50 Ohm termination at the 'scope input, as shown by the overall scheme in this graphic...

Readers familiar with the BITX will recognize the segment of the original system under test.

The response is actually similar to that measured with the filter out-of-circuit (albeit with some gain - associated with the rather random combination of the loss from my attenuator and the gain from the two amplifiers)...

This little sweeper actually seems capable of useful work! Perhaps I should use it to try to refine the passband response of the SSB rig's IF filter - then again, life is quite short and what's 2.5 dB between friends?

...-.- de m0xpd

RF Sweep Generator

Here's some notes about generating RF sweep signals on the Arduino for filter testing - it is really just a digital version of the old Wobbulator concept.

First look was a little disappointing because, whilst I had recently satisfied myself that the DDS modules can have their frequencies changed very quickly, (in fact, I showed that the Arduino Due was capable of changing the frequency of an AD9850 in 9 microseconds ) the humbler Arduinos turned out to be rather slower.  Specifically, I found that a simple loop of code involving just writing a frequency to the DDS and incrementing the frequency value itself took 950 microseconds (Arduino MEGA + Soft SPI interface to AD9850). This was imposing a surprisingly low “sample rate” on my plans for a digital “Wobbulator” before I’d even got started.

I wanted to be able to measure the response of crystal filters – so I needed a filter to measure.

I started off with a Spice model, constructed in the wonderful LTSpice package from the good folks at Linear – wonderful not only ‘cos it is FREE but also ‘cos it now works under a simple graphical interface (not like the bad old days!!).

Here’s the trivial filter 8MHz I threw together in LTSpice...

and here’s its predicted (ideal) response...

I made up a version of the filter on a scrap of PCB material...

I made sure the practical filter was working by driving it with signals from the “Kanga” VFO, which showed me that its passband was actually between 8 and 8.002 MHz.

I was driving the filter directly from the AD9850 DDS module (knowing it to have a 100 Ohm source impedance - which was not too bad a match for the input impedance of the filter) and monitoring the response on the 'scope (with a 50 Ohm termination - see below).

Then I turned my attention to writing the sweep generator code...

I set up two constants, fStart and fEnd, representing the beginning and end of the sweep. Then I simply stepped up a frequency variable, f, in equal increments, fStep, starting from fStart, until I reached fEnd. At each step, I waited T microseconds (plus the extra 950 or so microseconds the system forced me to wait).

Oh yes - I was also making it easy on myself by using my own DDS library's setFrequency() command.

That’s all there was to it...

I added a trigger pulse at the beginning to let my ‘scope know when to start. And a gate pulse to let me know when the measurement was valid (etc).  I could (and did) add any number of other refinements and diagnostic extras – but the bare bones are as described above.

The results are seen below...

What is seen above is the frequency axis horizontally and the magnitude response vertically. It is a photograph of the screen of my digital storage scope on a “single shot” trigger. I have manually set the vertical cursors to correspond to the limits of the gate pulse, corresponding to the start and End frequencies (which, in this case, were 7.995 and 8.005 MHz). 

This might be easier to understand with a little annotation...

Here, I’ve added a frequency scale (just by interpolating between the start and end points) and I’ve drawn in a (linear) magnitude frequency response curve – by joining the dots! You can see that the passband is confirmed to lie between 8 and 8.002 MHz - just as I had established earlier by "conventional" testing.

Being a good boy, I had the filter loaded by a 50 Ohm terminator at the ‘scope input – if you remove that, you get the expected huge increase in voltage levels – but notice how the shape of the filter changes significantly...

It is important that we make measurements of our filters in realistic loading conditions if we want to avoid deluding ourselves!

You saw in my LTSpice model (above) that I used a 50 Ohm source and load resistor with the filter. I made a MATLAB model of the filter (using my own Two Port methods) and looked at the input impedance, which I found closer to 200 Ohms. Sure enough, the response of the "ideal" filter is "cleaner" if it is operated in a 200 Ohm environment - but I didn't get round to that level of detail yet. 

Now I have a viable RF Sweep Generator - let's see what we can do with it.

...-.- de m0xpd 

Short Stay

Just back from an Easter holiday in Pickering...

with its special delights for the preserved railway enthusiast and angler. As before, we stayed at High Mill

but this time my three daughters and their partners were with us - so we had the excuse we needed to stay in the big house. Readers of this blog will remember that High Mill actually forms part of the boundary of the North York Moors Railway.

As usual, after satisfying the XYL's appetite to watch seabirds at the RSPB reserve at Bempton Cliffs, there were flocks of locomotives to be seen and, indeed, heard (the pretty Kitson built 0-6-2T loco that started its working life at Lambton colliery and wears number 29 has a lovely, mellow whistle). 

Having reveled in the grandeur of Sir Nigel Gresley last visit, it was nice to see the little Fowler 3F "Jinty" this time...

There was also a brace of Black Fives marching around - 44871 is seen seen here bringing "The Moors Explorer" into Pickering, taken from the garden of High Mill...

Later in the week, the same loco was to haul "The Yorkshire Coast Express" with me on board all the way to Whitby. It does run in Yorkshire. It does go to the Coast. I didn't notice anything particularly "Express" about it, though!

I took this little video of High Mill from the first carriage behind 44871 as we pulled out of Pickering - you can see what a great place it is, with its long garden and rooms offering outstanding views of the railway...

After pulling past the trout lake and the (now defunct) trout farm, the train soon reaches New Bridge, were the driver swaps single line working tokens with the signal man who comes down from his box to make this exchange. The levers at the New Bridge Signal Box control all the action at the Pickering end of the line...

Talking of signals (how about THAT for a link!) I took no radio gear this time, remembering the QRM I experienced during my last visit. 

High Mill owner, Mike, who had explained how both he and his wife Cheryl had enjoyed careers as Radio Ops, remembered my radio interests and dug out some photos of operator desks at GKA and GKZ, which I copied. Some parts of these copies are reproduced in the montage below...

The colour image (top left) shows Portishead, GKA, on the day of its closure (30/04/00). Mike visited on that day in his rather colourful cycling gear - so I've cropped the image to respect his privacy!

Much of my "train spotting" in Pickering is conducted from the viewpoint of the previously mentioned trout lake - where I spend a couple of hours every morning before breakfast and every early evening. This Easter I landed a total of 233 trout - the best ran to something around 5 pounds - nowhere near as big as the leviathans I have captured on previous visits, but great fun none the less and a great total given the time of year.

Another great time in Pickering - thanks to Paula at the trout lake, thanks to the NYMR and thanks to the hospitality of Mike and Cheryl Potter of the wonderful High Mill, Pickering.

All too Short a Stay!

...-.- de m0xpd

Arduino and the Si570

I've been wanting to experiment with replacing my little AD9850 DDS modules with the Si570 device. Not out of any sense of dissatisfaction with the former - more out of a spirit of adventure and in preparation for future experiments with extended bandwidth. The big problem with the Si570 - apart from cost - is the package.

I decided to solve the problem by mounting the awkward package on one of my little plug in modules, which start with a small printed circuit board...

Here are a few of the "carrier" PCBs, knocked up yesterday in my "back yard fab", along with an Si570

I got out my little USB microscope and did the best I could with my soldering iron, failing eyesight and shaking hands- here's a close-up shot from the microscope part-way through assembly...

Once the chip was on the carrier, I added some header pins to serve as "legs" for the module, which acts as an 8 pin DIL "wide" (i.e. 0.6 inch) package, which is infinitely easier for me to handle (and, indeed, see) than the original bare device...

Now - I just need to program the thing.

I have driven an Si570 before from a PIC (in my original multi-mode beacon) - that was a pretty brute-force piece of coding. This time I wanted something a little more elegant, so I looked around on the 'net.

The first fruit of that search was the ArduinoSi570 Library , by Gabriel Fournier.

I downloaded that library, but could not get it to compile, either with the provided example sketch or with my own simplified examples. Gabriel quickly and graciously answered an email asking for help - but his suggestion was that I should find another library! So I searched some more...

Fortunately, that search soon reminded me of Farhan's new Minima transceiver, and I found the "Radiono" code for it, which includes library resources for controlling the Si570. I downloaded that library and soon had modified the code (which is intended for the ATmega328P) to run on an Arduino MEGA, which allowed me to confirm that my new Si570 module was working correctly...

My real interest was in getting the new Si570 module running under the control of the Arduino DUE and - I'm ashamed to say - I still haven't succeeded in that goal.

My modified program now compiles and I can see the I2C activity between the DUE and the Si570, but they are not yet successfully communicating.

You can see how much I'm struggling by the presence of an empty coffee cup in the frame below - things are obviously bad...

I guess it won't be long before we have Si570-derived RF for the Arduino SDR - but not today!

...-.- de m0xpd