Amateur Radio


Back in July 2017 I wrote a post on here in which I gave a rough sketch of a combination transceiver/computer that would allow me to take a single unit, antenna kit and power, and work digital modes portably, with a minimum amount of baggage. Like one might do with the Mountain Topper range of CW transceivers, but capable of operating with digital modes.

When I wrote the article, I was into JT65 and JT9. Now of course, FT8 is the mode du jour.

The DDS of choice was the ‘el cheapo’ AD9850/AD9851 boards that are available on eBay: now I’d go for the Si5351 DDS boards, with a module available from Adafruit, and also available in an ‘el cheapo’ variant! This DDS creates fewer harmonics.

I’m still very much a beginner at RF design: that is still the major risk to the project, as is the absence of copious amounts of spare time in which to work on it!

However, one risk I’d identified – making an Arduino present itself as a sound card + multiple serial devices – seems to be reducible. LUFA (Lightweight USB Framework for AVRs is a “an open-source complete USB stack for the USB-enabled Atmel AVR8 and (some of the) AVR32 microcontroller series, released under the permissive MIT License”. It has examples of Audio In/Out and Serial devices. I’m hoping it can also provide a composite device that allows the single audio I/O channel, and two serial ports (diagnostic and CAT control).

So the next action on this project is to make an Arduino Micro look like a sound card with two serial ports. It’ll be a loopback device, so whatever sound you play at it (i.e. when transmitting) will be played back to you when receiving; whatever you send on serial port A will be echoed back to you with a ‘DIAG’ prepended to it; similarly with port B as the ‘CAT’ port.

Still unknown: SSB transceiver design that’s buildable by a beginner, and that can be connected into a ADC / DAC pair. How many bits of audio do I need to sample, at what rate?

This may well require a microcontroller that’s a bit more powerful than my usual Arduino Micro – possibly one from the Teensy range.

… to be continued…

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This is part 3 of my series on building a cheap loop antenna. If you haven’t read part two, it’s here.

It has been a while since I did anything on this project, but I intend to finish it this year, so here goes! One of my other goals this year is to publish at least one article per month – so this is a short update.

The topic of loops and their capacitors came up at a recent meeting of the West Kent Amateur Radio Society, with some very constructive feedback on my capacitor build, which I’ll relate here. Thanks to Chris G6HTH, Robert, Richard, Barclay, Bob, and Alex.

The main problem Chris pointed out is that I’ll have an unacceptably high resistance by connecting the loop (RG213 braid, copper) to the stainless steel/aluminium vanes of the capacitor. Soldering such dissimilar metals will be ineffective (or impossible). Brazing is advised (I know nothing about this, so research required).

Also, why not construct the capacitor vanes/rods from copper? I had not considered this – all the designs I’d seen on t’Internet were aluminium, so naturally I chose that. Looking closely at proper transmitting variables, the vanes are brazed to their supports, not bolted, and both are the same material.

I was also considering – based on the project I was following: A Simple RG213 Loop by Mario G8ODE – on using RG213 (the outer braid) connected to the box housing the capacitor via PL259/SO239 connectors. Inside the box, these SO239 connectors would connect to the vanes of the capacitor with very thick wire, soldered to ring terminals bolted under the nuts.

The advantage of this scheme would be that I could switch the 1m loop for a longer version as needed. The disadvantage is that it introduces much resistive loss, and for loops, minimising this is critical to increase efficiency.

A loop of RG213 is also attractive since I also have some, and can work with it. Using a solid copper pipe loop would be better, but I don’t know how to bend copper effectively. I have seen schemes for bending it using rollers fixed to a Black & Decker Workbench. I could also fill pipe with sand, or get a spring and try bending a length… but the problem was solved very nicely when Chris gave me a 1m diameter loop of pipe, pre-bent at the next WKARS meeting. Thank you, Chris!

After some discussion, a method of attaching this to the capacitor has been suggested: two strips of copper plate, of similar thickness to the vanes (~1mm), bolted across the two terminals of each side of the capacitor. Bend the plate along its length into a right angle with the horizontal half bolted to the capacitor, and the vertical half connected to the loop. To connect the vertical half to the loop, drill a hole in the vertical half through which the loop end can pass, and braze the loop to the right angle.

So I’ll need a small sheet of copper with which to make the two loop-capacitor right-angle sections.

Item Cost each Total cost
Copper Sheet Plate 0.7mm Thick – Various Sizes, Copper, 100mm x 100mm £4.00 £4.00
£4.00

 
We also discussed how I’d mount the capacitor and loop – I was thinking of putting the capacitor in a large plastic box, but the loop isn’t going outside in wet weather, only in the loft, or outside on pleasant warm days operating /portable. If I did encase the capacitor, I’d have to get the loop ends out of the box, through holes. Chris raised a valid point on this – if the loop touches the side of the insulating box, any moisture on the surface of the box between the ends would short the loop out. So, no need for the box – I’ll attach the capacitor to a board that’s fixed to the frame. I will place a “Danger! High Voltage!” sticker near the capacitor though!

The overall frame will be made of thin wood, in the shape of a cross. The loop will be attached to the top, left and right extremities of the cross by plastic clips – usually used for fixing radiator heating pipes to walls.

The coupling loop will be fixed to the upper section of the cross; the capacitor to the lower, mounted on a board, where I’ll possibly place a reduction drive and stepper motor.

I read the Feb 2019 issue of Practical Wireless with interest: there’s an article by Hamish Storie, MM0GW0, relating his experience building a variable capacitor – he built his with thin brass plates and fixings. It’s not a butterfly capacitor like mine, but he does point out some aspects of engineering that I’ve learned the hard way: use a metal nibbler to cut the vanes out, rather than shears, as shears will deform the metal.

I’ll continue next month, hopefully, with pictures!

Ah, the optimism of the 1st January!

As I reflected on 2018, it became apparent that ‘starting, not finishing’ is a big problem, chez M0CUV. My muse bestows plenty of interesting ideas, but some of them are a bit ambitious. I start, then things grind to a halt. This, coupled with chronic procrastination means a lot of churn, and a feeling of dissatisfaction, angst, and despair at not being able to find the time to do all this – or to prioritise better. A look back through the log shows a big gap of radio silence from June to October, a page of digital mode contacts, and not a single CW QSO throughout the whole year. On the software side, I hung up my own P2P stack after a baffling test failure wore me down. I do want to return to that.. However, I spent most of my hobby development time working on the Parachute project, which despite being really tricky, is going well. I never thought writing an assembler would be this hard. The devil is in the details.

So, after giving myself a good slap for a lacklustre radio year, 2019’s going to be goal-driven. Hopefully these goals are SMART (Specific/Stretching, Measurable/Meaningful, Agreed-upon/Achievable, Realistic/Rewarding and Time-based/Trackable). There are quite a few, and only the tech/radio-related ones are blogged about here. I’ve been using the Getting Things Done method for a while, and it stresses the importance of defining the Next Action on your activities..

So…

  • 1 blog post/month, at least. Progress updates on projects, etc.
  • 1 CW QSO/month, or 12 in the whole year. This’ll probably be really tough.
  • 1 QSLable QSO/month, or 12 in the whole year, any mode/band. FT8 makes this much easier!
  • Try to contact each month’s special callsign for the Bulgarian Saints award from Bulgarian Club Blagovestnik. I’ve already bagged January’s, LZ1354PM via SSB on 1st Jan 🙂
  • Take the next step on the magnetic loop project: build the frame, house the capacitor. I bought some wood and a food container for this yesterday.
  • Box up the 30m QCX transceiver properly – then use it.
  • Keep up with magazines as they arrive rather than building a pile of them.
  • Fix the current bizzare bug in the Transputer assembler – then ship the first release on Windows, macos and Ubuntu/Raspbian
  • Convert the Parachute Node server to use the IServer protocol – then write the IO code for eForth.
  • Build a web application with elm. I’m thinking of a web-front-end to WSJT-X, to allow me to operate remotely.

Let’s see how I get on…!

It’s fun to take a minimum of equipment out to a high place, lob a half-wave long wire antenna up into a tree, or support it with a fibreglass pole, and off you go – but such a long wire typically has a high (2.5kΩ to 3kΩ) impedance. To make it work, this needs transforming down to match the 50Ω output of a typical transceiver. This is done using a 49:1 autotransformer, built on a small toroid.

This post details my construction of the end-fed half-wave autotransformer and antenna for the 10m, 20m and 40m bands.

The antenna is very effective, can be erected in a variety of ways, exhibits low noise, doesn’t cause any RFI in the shack (use a choke balun in addition), places the feedpoint very near to your transceiver and it’s pretty easy to build.

Acknowledgements

There are many other articles out there on this subject:

There are also several other pictures of builds I’ve used to assist in this project, particularly from Ian MW0IAN and Michael G0POT. Thanks!

Introduction

I hope to add some detail and pictures for the terrified constructor in this article, but the above are my source materials. This antenna and its autotransformer are a great project for those new to construction – the toroid winding isn’t hard.

One problem I find in many projects is that they go from diagram to finished construction very quickly with insufficient detail. It’s a bit like “How to Draw an Owl”….

How To Draw An Owl

So I wanted to document what I’d done, in the hope it might be useful to others, especially if you’ve not wound toroids before. The key point is that the count of ‘turns’ relates to the number of times the wire goes through the inside of the toroid.

Parts and materials

I’m building this autotransformer on a FT-140-43 toroid, purchased as a pack of two on eBay from Spectrum Communications, for £9.50 incl P&P.

I first wrapped this in plumber’s PTFE joint sealing tape.

I’m using Maplin 0.9mm 20SWG enamelled copper wire, code YN82D. £8.49 for a small reel (the picture on their page is misleading; it’s not as small as indicated). The source articles used 1.0mm wire; this will allow higher power to be transmitted – I’m mostly QRP, and without calculating it, 0.9mm will be fine for this.

For connectors, I’m using a BNC socket, £2.49 for Maplin’s code HH18U, and a small terminal post, again £2.49 for Maplin’s code FD69A. A small black plastic box, for £3.39 code KC91Y houses the completed autotransformer. This box is a bit ‘tight’, but it does all fit. You’ll also need a 100pF 3kV ceramic capacitor, available for £0.131 each (min order, pack of 10) from RS Components.

Building the Autotransformer

Wire lengths for the toroid winding given here come from reading other articles which describe builds on the larger FT-240-43 toroid. After I’d wound on the smaller toroid as shown here, I had some waste wire that required trimming – you could start with shorter pieces of wire, and I’ve estimated possible lengths for this [in square brackets]. However please note I haven’t tested those lengths.

Measure two lengths of wire, 1.0m, and 22cm. [For the FT-140-43 toroid, and a small box as given above, you could probably use lengths of 80cm and 18cm.] Strip the enamel from the ends for about 2-3mm, clean with sandpaper and tin with solder. Solder the two right-hand ends together.

Solder the right-hand ends together

Holding the soldered end in pliers, twist the two lengths tightly together for 13cm [for the FT-140-43, 9cm may suffice], to form the bifilar winding. ‘Bifilar’ meaning ‘two filaments’, I presume. The remaining length of the shorter piece of wire (at the end of the bifilar winding) being about 6-7cm long. Let’s call this length ‘the tail’.

Now to wind the bifilar part onto the toroid.

Pass the bifilar part through the toroid from the back, so that the ‘tail’ passes under the bottom edge, and the start of the bifilar part is entirely running through the inner of the toroid.

Starting the Windings

Wind the bifilar part back round the top and outer edge of the toroid, and back through the hole. Bend it again over the top. You should have two bifilar passes through the toroid. The end of the bifilar winding (that’s the soldered-together end) will be connected to the outer/shield/chassis connector of the BNC socket. The end of the tail will be connected to the inner/centre connector of the BNC. I’ll get back to the connections later.

The Bifilar Winding

Now, there’s the rest of the toroid to wind.

Wind the long single strand of wire through the toroid so that you have six passes on the inside.

The first part of the rest of the winding

Then pass the wire over the top, and over to the other side, going under the far edge. Loop over the top, and go back through the toroid so that on the far side, you have seven passes, with the remaining end (‘the antenna end’) going under and out.

Ensure that the windings are spaced as evenly as you can.

The final winding

Now tin the connections of the BNC socket and terminal post, and fit them in the box.

Carefully trim the three wires coming from the toroid to fit; scrape off enamel (might need to unwind the end of the bifilar winding a couple of mm to do this properly), clean and tin the ends.

The toroid, trimmed, with box

Solder the bifilar winding to the outer of the BNC; the ‘tail’ at the end of the bifilar winding to the centre pin of the BNC; the ‘antenna end’ to the terminal post.

Solder a 100pF 3kV ceramic capacitor between outer and centre connections of the BNC. Use a hot glue gun to fix the toroid to the inside of the box (not shown).

The assembled autotransformer

Put the top on the box, and there you have it.

The completed autotransformer

Building the antenna

There’s a long antenna wire attached to the autotransformer terminal post, followed by a coil and a further bit of wire. I used fairly rugged plastic-insulated wire; the outer diameter is 2mm, bought years ago on a 100m reel, probably from Maplin.

-----+                                                           
auto |         Length L1           Coil in uH    Length L2  (fibre-
xfor-|=----------------------------|\\\\\\\\\|------------|  glass
mer  |                                                    |   pole)
-----+                                                    |

For 80m+40m+20m+15m+10m, L1 is 20.35m, the coil impedance is 110μH, and L2 is 2.39m.

For 40m+20m+10m, L1 is 10.1m, the coil is 34μH, and L2 is 1.85m. I built this variant.

These lengths are taken from the diagrams in the PD7MAA article. Also see PA3FRP’s notes.

I had a length of plastic tube with an outer diameter of 15mm; I wound 141 turns of the Maplin 0.9mm 20SWG copper wire on this, and measured 34μH. This was very much a trial and error exercise; I had no idea how many turns it would take, so the initial length was extended by soldering more on, several times.

Testing

I attached the far end of the wire to the top of my 7m SOTABeams fibreglass pole, and fed the wire out so that the autotransformer was on the ground – imagine a right-angled triangle with a bowed hypotenuse; I also lifted the autotransformer up onto a small table.

Measurements with an antenna analyser show very favourable SWR readings for the three bands I’m interested in:

  • At 7.305MHz, SWR is 1.21:1
  • At 14.318MHz, SWR is 1.14:1
  • At 27.780MHz, SWR is 1.15:1

With a bit of adjustment, and appropriate positioning – it does seem sensitive to its location above ground (this is explained in the test article by John Huggins) – I think it’d be fine to transmit into this. I’d ideally like to tune it a little more, so I can use it on the exact frequencies I frequent without an ATU.

I sometimes want to operate using the JT65/9 modes from local hills, as my home QTH is close to sea-level. However, doing so is not so easy, due to the quantity of kit I’d have to take:

  • Battery
  • Laptop
  • Phone (for Internet access and NTP sync of laptop)
  • HF Transceiver
  • USB external sound card / rig interface
  • Antenna and associated matching unit
  • Antenna mast/fibreglass pole/guy lines

This does not all fit in a single rucksack, as a CW QRP system might. So, I’m thinking about what a self-contained JT-modes transceiver might look like, so that I just take:

  • Battery
  • Self-contained JT-modes transceiver
  • Phone
  • Antenna and associated matching unit
  • Antenna mast/fibreglass pole/guy lines

So what would be in the box? I think this could be developed in two phases.

  • Phase 1: Merge the transceiver and USB interface.
  • Phase 2: Add the computer.

So for phase 1:

  • Arduino Micro (my board of choice) presenting multiple devices via its USB connector: a 16 bit 8000kHz mono sound card; a serial interface for CAT commands (to change operating frequency); a serial interface for diagnostics/control via a terminal emulator.
  • The Arduino would be connected to a 16-bit digital-to-analogue converter, which, when transmitting, would provide the analogue signal to the transmitter (as sent from the main computer).
  • Also, an analogue-to-digital converter which passes the received audio in receive mode back through the USB sound card to the main computer.
  • The USB CAT interface would control the output frequency of a direct digital synthesis (DDS) unit, for precise generation of the correct frequency for the JT-modes section of the band.
  • Transmitter/PA and receiver system suitable for the transmission and reception of JT-mode signals.
  • Possibly monitoring of SWR forward/reverse measurement, and PA transistor temperature; triggering of a cooling fan.

This phase 1 system could be connected to my existing OSX laptop and appear as a CAT-controllable USB sound card, with decode/generation of the JT-mode signals being done by the ‘proper’ WSJT-X software.

Phase 2 could extend phase 1 by adding:

  • A touch screen, connected to…
  • A Raspberry Pi Zero W, running a cut-down/embedded Linux, a customised version of WSJT-X optimised for the touch screen, and WiFi/Bluetooth connection to an optional Bluetooth keyboard and phone (for NTP sync). USB connection to the phase 1 ‘sound card/CAT’ transceiver.

Risks/things I don’t yet know:

  • How to make an Arduino provide multiple devices; It provides a single serial interface ‘out of the box’ – I require two discrete ones, and a sound card.
  • How to provide a USB sound card interface.
  • The design of the transceiver capable of digital modes, in much more detail than a basic SSB transceiver block diagram.
  • How easy it’ll be to customise WSJT-X for the phase 2 system.

So to address the first two, it should be possible to provide the two serial interfaces wired ‘back to back’, so that anything you send on one, gets received by the other, and vice versa.

The sound card could just send a varying-pitch sine wave (simulating the ‘receiver output’), or send back what it has been receiving (on its ‘microphone’ when ‘transmitting’).

The transceiver itself could be based on existing SSB DDS-driven systems, possibly the BIT-X, or the QRPver (replacing the VFO with DDS). This is the riskiest part of the entire endeavour, as I’m a very amateur amateur, and something of a beginner at RF electronics design.

To be continued….

 

This is part 2 of my series on building a cheap loop antenna. If you haven’t read part one, it’s here. In this second part, I’ll cover the building of the tuning capacitor.

Obligatory Safety Notice

This article describes machinery and tools, workshop practice and construction that if not performed carefully and with the appropriate protective equipment (goggles, etc.) – could lead to serious injury. Please take care. I won’t be held responsible for your accidents!

Tools Required

In addition to keeping the material costs of the antenna as low as possible, it should also be buildable with a minimum of specialist tools. The following were used in constructing the capacitor:

  • Superfine permanent marker (Staedtler Lumocolor S)
  • Compass with universal adapter to take these pens (Staedtler Noris Club 550 01)
  • Metal / plastics drill (Dremel 3000)
  • Drill bit 3.2mm (Dremel)
  • Drill press (Dremel 220 workstation)
  • Thin circular metal file, flat metal file (Stanley warding file set)
  • 8mm spanners for M5 nuts
  • Junior hacksaw
  • Fret/coping saw (Stanley fatmax)
  • Centre punch
  • Two C-clamps for holding things whilst cutting (a vice would be far better)
  • Straight aviation tin snips

Materials

Item Cost each Total cost
9mm thick IKEA Legitim Chopping board £1.00 £1.00
Stainless steel washer M5 100 pack (Toolstation 79167), x2 £0.96 £1.92
Stainless steel Threaded Bar M5 (Toolstation 61139) £1.78 £1.78
Stainless steel Lock Nut M5 100 pack (Toolstation 64158) £1.58 £1.58
Stainless steel Spring Washer M5 100 pack (Toolstation 92679) £0.97 £0.97
0.9mm thick Aluminium (grade 1050) plate sheet, 300mm x 200mm, protective coating on one side £3.80 £3.80
£11.05

Well, there you go, I wanted CHEAP, and you can’t get much better than £11.05!

I chose M5 threaded rod, nuts and washers as it would be mechanically substantial, and not present too much resistance to RF. I considered M3, but it would introduce greater loss, despite being easier to drill for. The Dremel drill I use is excellent, but does not drill holes with a diameter greater than 3.2mm. I thought of drilling this size, then increasing the hole size with a burr, but this did not work out well, so I reverted to filing out to 5mm. Another reason for going with M5 hardware is that Toolstation in Tunbridge Wells stock M5, but not M3, and they’re really quite reasonable as the table above shows.

I initially considered Aluminium Warehouse for the plate, but their shipping cost was £15, so scratch that! I mentioned this to my wife, who suggested Amazon…. which turned out to be an excellent idea, as Hardware Outlet sell it for £3.80 with free postage. Nice.

Design

The diagram below shows the design of the rotor/stator vane plate (six of which are cut from the aluminium sheet), and end plate (two of which are cut from the 9mm thick cutting board). This layout comes from the article Magnetic Loop Antennas, by Tony ON4CEQ. The rotor and stator are cut from a single piece of aluminium with a minimum of tricky cuts. Other articles have separated them with Dremel cutting disks, which I had bought, intending to try – but managed to cut out Tony’s design easily.

Construction

In hindsight, it would have been better to make the end plate a few mm taller, as the rotor is exactly the same diameter as the end plate’s height (8cm), and if not raised, touches the desk as I rotate the finished capacitor.

I cut the end plates out using the above fret/coping saw, along with some extra pieces used to clamp things in my drill press. The holes for the threaded rod, forming the stator arms and rotor were drilled out with the Dremel’s 3.2mm drill bit then filed out to 5mm diameter.

I marked out the aluminium on its protective layer, using the microfine permanent marker and compass as shown. I left a 5mm gap between each plate (see detail below), thinking that the extra metal would prevent the drill from tearing the thin aluminium when drilling or increasing the size of the holes from 3.2mm to 5mm. I need not have done this; increasing the size of the holes was best done with a file, which was tedious. The vane panels could have been laid out directly next to each other; cutting each panel out was easy with the junior hacksaw. Each hole was centre-punched, which made drilling far easier.

Drilling was straightforward, although the Dremel workstation tends to allow the drill to move slightly. Centre-punching is essential.

The stator and rotor are cut from the panel easily; the diagonal cuts toward the centre are easily done with a junior hacksaw; the short cuts near the rotor axle, joining the diagonal cuts are done with the fret saw shown above (which is probably only supposed to cut wood; it handles 0.9mm aluminium well too).

Tin snips are then used to cut the unwanted parts of the rotor blade away, to yield its circular edges.

Burrs are filed off, edges and corners smoothed to remove any sharp points that could a) cut you and b) offer a point of voltage breakdown and arcing.

I then cut four 85mm lengths of M5 threaded rod for the four corner stator attachments, and a single 105mm length for the rotor. I then assembled the frame with the plastic, rod, spring washers, washers and nuts.

I assembled the rotor on the longer length of threaded rod. The washers I’m using to separate the vanes are exactly 1mm thick. Initially I used five of them, to make absolutely sure I’d have the inter-vane gap I’d need, but found the capacitance to be too low. So I rebuilt with four washers between vanes, and achieved a capacitance much closer to that needed.

The stators were assembled onto the frame, and roughly aligned with each other.

The rotor then fits snugly inside the stators and its axle fits in the centre hole. It’s hard to make out the end plate due to the perspective here, but it’s there.

Flat washers are fixed on either side of the rotor/end-plate mountings, in lieu of bearings. The top plastic end plate is then added, and everything tightened up. There’s a certain amount of adjustment of the stators and rotor to ensure that the rotor rotates freely within the stator plates, and that there’s an equal gap between rotor and stators. This adjustment is mostly done via the nuts mounting the stators, but there might also need to be gentle bending of the various plates.

Measurement

After construction, I measured the capacitance, and read 0.006nF (6pF) for completely unmeshed, increasing to 0.043nF (43pF) for completely meshed – close to the upper value I need – 46.311pF from the first article in this series.

Conclusion

There’s a bit more mechanical tweaking to do, but that’s the risky part of the magnetic loop project done. Next time, I’ll assemble the rest of the loop and think about manual/automatic tuning.

Here’s a short clip of the capacitor being tuned and measured…..

Stay tuned!

73 de Matt M0CUV

I’ve been wanting to try a different aerial for 20m for some time. I currently either use a dipole in the loft, or a temporary SOTAbeams linked dipole on a 7m fibreglass mast. The magnetic loop antenna appeals because it’s compact, and if designed/built for efficiency, gives excellent performance.

They’re also expensive. One well-known UK dealer is selling the Ciro Mazzoni baby loop for £1299; another dealer sells the MFJ-1782X for £439; the Chameleon CHA F-Loop is available from a third dealer for £399. All are far, far outside my budget.

It’s not called amateur radio for nothing, and what I describe in this series of articles is truly amateur. I’m going collate my research sources, and try building a magnetic loop antenna (properly called a small transmitting loop antenna) using bits from my junk box, making good use of the things that I find, things that the everyday folk leave behind. I don’t have all the parts, but whatever I need to buy must not break the bank. Similarly, tools: I’ll need some, but they must be cheap 🙂

(more…)

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