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If you want to have a bit of fun and try out some superb new beta software, then get this app installed on your smartphone, and meet me on GB3HY after one of the Wednesday night nets for a little testing and larking about.
The app you need is called Rattlegram, and it's available on Google Play, but is sadly not yet available on iOS.
It's based on COFDM (coded orthogonal frequency-division multiplexing) technology with the following specifications:
- 160 ms long OFDM symbols
- 6.25 Hz per sub-carrier
- 1/8 guard interval
- differentially encoded PSK (phase-shift keying) modulation
- systematic polar codes for forward error correction
...and it works purely on audio files!
You type some text, the app does the OFDM encoding and DAC and then spits the resulting noise out of the speaker. The encoding uses something called an Inverse Fourier Transform, which is way above my level of comprehension.
The sound is then received by the same app on another phone (in 'listening' mode) and then ADC'd and decoded back into text. That may not sound so new, but the technology is (relatively) and especially in terms of amateur radio. In fact it uses basically the same protocol as WiFi, 4G and 5G, so data rates can theoretically get pretty high if you can keep the noise levels down.
Each of the 256 sub-carriers is - yes you read that correctly - just 6.25Hz wide, and to deal with reflected signals, each data packet has a 1/8 guard interval. That equates to a total 180 ms symbol duration including a 20 ms pause between symbols to avoid data 'collisions', but it also includes FEC to add a whopping 50% redundancy and therefore dramatically reduces transmission errors. This is important as there isn't any possibility of re-transmission. FEC works in the same way as for unidirectional digital satellite, cable and terrestrial television transmissions and Rattlegram employs QPSK (Quadrature Phase Shift Keying) modulation.
In testing, I've had this working with good reliability. I started with phone-phone, then tried it with a radio link in the middle on 146.9125MHz, and then I even tried it on GB3HY - receiving on both the input and output frequencies using an Icom and an SDR on a laptop on opposite sides of the house.
If I get a chance, I'll be trying it out on the HF bands tomorrow using the clubs' Flex radio, though I expect the error rates will increase.
This is the easiest 'data mode' technology I've seen that has usable performance. What could be simpler than holding your phone up to your radio?
Data over RF for the smartphone generation. Everybody wins... and this could easily be turned into a PC-based utility cabled directly to a radio to increase throughput.
Enjoy your radio!
Berni M0XYF
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Alex M0TOT has been back in the workshop. In fact I think we can probably call it a 'laboratory' now, as he's experimenting with a whole new toolset.
Interestingly, he's been finding his way around the world of experimental microcontroller boards, and has ended up building himself a really neat little GPS receiver.
I'm going to guess that this may be of interest to other members too, as coincidentally, Alex is the third person in the club to have built himself something like this that I know of - myself and Merv have already constructed similar projects.
Here are the hardware building blocks that Alex used:
It's a 'standard' Arduino Uno (a great universal starting point) which has then been paired with a GP-20U7 GPS module from Sparkfun Electronics. The Uno's are getting relatively expensive now, what with all the global semiconductor shortages, but this project will also work with most of the cheaper Arduinos, as well as similar microcontrollers like the Seeeduino Xiao, which is what I used. The GPS modules are fairly generic too, and as long as you can find a suitable library for them, you're laughing. The basic raw serial read below doesn't even require this, and will happily deliver you all the data you need from Low Earth Orbit. More on manipulating and interpreting that data later on, but here is the basic Sketch which will give you that data:
It's a serial UART read loop and print, that's all, but it gives you instant access to the GPS data. Here is a sample of that data as provided by Alex:
"$GPGGA,165405.000,0405.4230,N,10511.10842,W,1,09,0.91,1584.0,M,21.4,M,1.0,0000*53"
and here's what some of it means when parsed:
$GPGGA: GPS fix data
165405.000: UTC Timestamp - 16:54:05.000
0405.4230,N: Latitude = 40 Degrees + 5.4230/60 = 40.09038 N
10511.10842,W: Longitude = 105 Degrees +11.10842/60 = 105.18514 W
09 = Number of satellites used in calculation
1584.0M: 1584 Meters above sea level (estimated)
Alex is therefore having a lovely time in Inner Mongolia. Yes, of course I checked.
Different GPS modules give you access to various and possibly multiple geo-location satellite systems, including GPS, GLONASS and GALILEO.
Here's what Alex had to say about his latest project:
This is a side of ‘radio’ in which I have recently become interested. The Longitude and Latitude are for my own house in Balcombe but the project will work anywhere. I am not a specialist in ‘C’ Language Programming, so I obtained the Sketch from SparkFun (see web site below). I have a feeling that the original Sketch needed correcting, so I went onto the Arduino Forum, to find some help. The ‘VisualGPSView’ figures are from the web site below. Maybe this is a side of radio in which other members might be interested. I believe that Merv is a bit of an expert in using GPS and Arduino.
GPS Receiver GP-20U7
Connections to Arduino:
RED wire to 3.3 Volt +Ve
BLACK wire to Gnd.
WHITE wire to ~D10 PB2 - The '10' is the Rx pin definition in the SoftwareSerial declaration in the above Sketch. It's easy to get the Rx and Tx round the wrong way!
Sources:
Alex has then used some software called 'Visual GPS View', something I've never used before, but I guess you just feed it the raw GPS stream and some basic configuration data and it does the rest.
Here are some screengrabs:
Mervyn and I have extended the GPS functionality by incorporating a LoRa radio uplink via The Things Network, though that is a whole different world. We use TTN for many projects other than GPS builds, and it's a great way of getting data up onto an application server or into a cloud-based repository from where you can manipulate and view it on any number of different 'dashboards'.
If you fancy having a go at building a GPS gadget, it's very straightforward and is a great educational tool. You just need an Arduino, a GPS module, a computer and a USB cable. The Arduino IDE (programming environment) is free and easy to use. Download and install the IDE if you want to avoid any usage (compiler) limitations.
Berni M0XYF & Alex M0TOT
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Completing the last page of my 32nd RSGB log book recently made me think of the information recorded within its pages and those of its predecessors.
I know that Sean EI7CV and I have completed 2033 weekend contacts over the past 45 years but what about the lunchtime net? What do my records show about that?
32 RSGB logbooks each containing 80 pages, with 26 lines per page. That's 66,560 entries, the first of mine being a top band AM contact with Eric Letts G3RXJ on the 13.10.1967.
My lunchtime net started on 1.9.1979, a 15m SSB contact with HO Townsend (then WA5MLT, later K5CX) when he was living in Norman Oklahoma. My eldest son David had just arrived at Oklahoma University to take his Masters Degree and HO promised to look after the 20yr old during his stay.
HO and I then started a daily sked, which developed into a net including MSARS members and others over the years. When David moved to Pensacola Florida, the net began to include local amateurs in the Serious Hams ARC (SHARC) including Mike N4MAD, and of course Bob, N4XAT who later moved to New Jersey and remains a very active member of the net today.
HO you may remember came over to Louis G5RV’s funeral as an official representative of the ARRL and met many MSARS members on that occasion. I’m still in touch with Anita N5AOK HO’s widow who is now living in Texas.
From 1.9.1979 to date is 43 years plus 2 months and another 10 days for leap years and at 5 contacts a week means that the lunchtime net has, so far, met on 11230 occasions with a few breaks for holidays etc. Think about that when you next log on to 21,345! There have been one or two forays onto 20m at the depth of some particularly bad cycles but in general the net has been on 15m for most of its life and I suppose you could say that we are very well known there.
My log books also contain records of contacts with many other long-term friends. Who remembers Dell Popplewell K4NBN (No Bad News) for example and how many of you have worked a King (JY1)?
Nowadays log keeping is digital and many amateurs don’t even keep them, but perhaps they are missing something in not being able to pick up an old logbook and remember contacts made many years ago.
Ken Gibson G3WYN
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Revisiting a Budget QRP ATU Project
I first built this project in March 2016. Since then ‘hundreds’ must have been sold! However, I recently came across a reference to this particular ATU on a YouTube video, put together by Carol Milazzo KP4MD in July 2021. This video showed that the inductance had been connected incorrectly on the original schematic. The number of windings at certain points on the toroid were wrong. There was also a mistake on the PCB silkscreen (see references below).
So, I used my original PCB, but re-wound the inductance in accordance with the recommended numbers in the YouTube video.
I checked the antenna’s SWR values with an MFJ Analyser (without the ATU) and then tested the ATU and antenna with a miniVNA (The original antenna was a Windom, the present one, as tested, is a vertical). With the switch in the 'Tune' position, the LED on top comes on if the unit does not 'see' 50 Ohms (this is done by a Wheatstone Bridge).
Test Results
| Band (m) | Mid-Frequency (MHz) | SWR(1) | SWR(2) |
| 160 | 1.92 | >3.00 | >3.00 |
| 80 | 3.70 | >3.00 | 1.38/10 |
| 60 | 5.32 | >3.00 | 1.18/8 |
| 40 | 7.10 | >3.00 | 1.94/5 |
| 30 | 10.12 | 1.20 | 1.35/3 |
| 20 | 14.18 | 1.65 | 1.27/2 |
| 17 | 18.10 | 2.40 | 1.27/1 |
| 15 | 21.23 | 3.00 | 2.22/1 |
| 12 | 24.94 | 3.00 | 1.39/1 |
| 10 | 28.95 | 3.00 | 2.39/1 |
| 6 | 51.00 | 1.60 | 1.25/1 |
Notes:
SWR(1) = Results using MFJ Analyser, Model No 249, on antenna only.
SWR(2) = Results using miniVNA and QRP A.T.U. on antenna.
The secondary figure in SWR(2) column is the band switch position.
The antenna on test alongside this ATU was the Thunderpole SE-HF 360 Fibreglass Vertical Antenna.
I have a few spare PCBs if anybody is interested in taking on this small project.
Kind regards
Alex Henderson M0TOT
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Some of you know that I've been experimenting (or maybe that's just 'dabbling') with various SDRs (ostensibly the Kiwi and the RSPdx, but also the HackRF One and Nooelec NESDR SMArTee RTL-SDR) running on a succession of semi-home-made magnetic loop setups. To clarify, what I mean by that is a re-working of the 'loop' element of a couple of off-the-shelf mag-loop pre-amps and Bias-T units placed in various locations around my QTH.
I thought I'd post a brief description, along with a few photos of the antenna I ended up with.
I've been tinkering for several months now, to see what sort of results I can get. I was hoping that the club would invest in a Wellbrook ALA1530 loop, which is the de facto standard if you like, so that I would have a yardstick with which to appraise my results. Happily, one now adorns the rear of Cyprus Hall.
I'm going to build my own Web-connected SDR (again, from off-the-shelf bits) running the same OpenWebRx system that the Kiwi uses, and then pop my homebrew effort on a big stick in Cyprus Road car park so that I can perform a direct comparison.
In all honesty, I don't really need another antenna, but (spoiler alert...) these loops do provide a pretty good HF reception solution in a compact and practical package. I can tell that already from comparisons with my main antenna.
So I started off by buying one of the cheap Chinese MLA30+ loops, via airmail from the source.
It was tiny. In fact, it came complete with Bias-T, USB lead, wire loop and maybe 15m of very thin coax which fed straight into the weatherproof pre-amp box - no connector, and it all came in an A5 padded envelope. I ran it like that for a while, but when I saw the modifications that other people were making to them, I decided to follow suit and snip off the coax (and SMA connector at the Bias-T end) replacing it with a chassis-mount BNC female connector and soldering the remaining short tail of coax to the appropriate points now on the inside of the enclosure. I re-sealed the entry-point with Araldite.
Pushing the 9mm drill through the side of the remarkably well-made enclosure without damaging the circuit board or the large coil you can see above was quite tricky and took quite a lot of concentration. The electronics are very well sealed in whatever goop that is, and that includes the coil and its windings, so I have no concerns around the longevity of the internals.
The big plastic mounting tabs that protrude from the top and bottom of the box had some useful mounting holes. Less usefully, they were only around 4mm diameter - not enough to get the common-or-garden 4.8mm wide cable tie through. They were soon opened out to 5mm.
Without touching on the actual performance of the unit in receiving RF signals, the only other slightly negative thing I have to say is that the attachment bolts for the loop itself (be it the supplied 70cm diameter stainless steel wire loop, or any other loop construction you wish to attach) are too small at 4mm diameter. In fairness, I suppose they're OK for the wire loop, but it wouldn't have hurt (or cost the manufacturer any more) to have fitted 5mm or preferably 6mm button-head Allen bolts. I didn't bother changing them out, as there isn't a lot of room going spare in there, but they could easily have been accommodated pre-assembly and prior to the tags being soldered.
Turning then to the loop itself, I played with a number of options. I started with some very promising composite tubing given to me by Phil G4UDU. It's a sandwich of PE-X/Aluminium/PE-X and on the face of it, it looked perfect. Neat, stiff, lightweight and pre-formed into roughly a 1m coil.
I cut a suitable length and started work on fabricating something to make the connections to the pre-amp attachment points. I started by trying to strip the outer sheathing of PE off the ends of the tube. Nightmare. It was all bonded together like you wouldn't believe. Next, I made some cranked brackets out of 2mm thick aluminium sheet with a 4mm hole at one end and a 14mm hole at the other. I then drilled out the bore of the pipe with a 14mm drill to remove the inner layer of PE and screwed a 14mm stainless steel nut and bolt up into the ends of the loop, securing the aforementioned brackets between the nut and the head of the bolt, leaving a good inch of thread in the tube (ooh err, look at me mixing my imperial and metric units...). That was obviously going to provide a solid connection, except that it didn't. It worked OK at first, but then stopped working. Intermittently infuriating, before being thrown up the garden in a childish rage.
Next I tried some beefy coax with an obscene amount of copper in it, also supplied by Phil, but it too wasn't really lending itself to the connection approach I wanted to use, plus it wanted to form any other shape than a perfect circle. When I started to see various animal-shapes in it, it followed the PE/ALU/PE loop into the herbaceous borders. I will retrieve it at some point, as I'm going to use it for yet another mag-loop in the loft, and I don't care what that looks like.
I'd watched a YouTube video of some guy that had formed a 1m loop out of two lengths of 15mm copper plumbing pipe. Very sensible, except that he had joined them with one of those plumbing connectors pre-filled with solder, and then used other connectors and joining methods to attach wires to go to the pre-amp lugs. Basically, he had taken a really nice, simple concept, and then proceeded to turn it into a right dog's dinner. Spray paint, tape and dribbly solder everywhere. OMG.
He obviously hadn't found the 3m lengths of copper pipe in Wickes DIY store in Burgess Hill. Other DIY emporiums are available.
So the diameter of a circle is 3m of copper pipe divided by π, which equals a 1m diameter mag-loop. Thank you Archimedes.
Time to get baking. Yes, baking. Wet sand is no good for filling copper pipe, so I heaped one of the GLW's baking trays with soggy sand and popped it in the oven at 170°C for 20 minutes. I told the wife that I was making a tray-bake. I don't know what that is, but I can tell you that when the beeper on the oven went, and she went to help herself to a tasty treat, I had to do some pretty fast talking.
I plugged one end of the pipe, filled it with sand, then plugged the other. The sand stops the pipe from deforming and collapsing when you bend it of course. Once I'd wrangled the 1m diameter round kitchen table out into the garden and duct-taped the pipe to the edge, I was easily able to bend the copper around it. It went remarkably smoothly, and no, you can't borrow my table.
After emptying the sand, I bent a couple of tabs on the ends (annealing the copper mid-bend to stop it from fracturing) and drilled a couple of 6mm holes in them. A bit of filing and so on, and we were done. Simplicity is nearly always the best policy.
Why 6mm holes? Ahh, revelations follow.
Not a bad job, if I say so myself.
I made the top mount out of a white 20mm electrical conduit tee with a 13mm wide slot cut in the top to snap the 15mm pipe into. This was in turn held in place by a white BMX handlebar grip, cut down to be a really snug fit in the end of a length of the ubiquitous 40mm waste pipe.
A decent new male BNC compression plug and some Mini-8 coax feeds the signal away and up into the shack. The loop is in a compromised location currently, and testing is therefore also compromised, but I can tell you that it's remarkably good. Proper comparative testing will follow, as hinted at earlier.
Job done then.
Well, not really. Of course I now have the same problem that I had when the KiwiSDR was located here. Namely, a nearby HF transmit antenna spitting out 100W (or 6.1W ERP if you're the ICNIRP police). I started looking at several solutions to this on the various forums, and didn't have much confidence in any of them, until I came across an apparent consensus that the pre-amplifier produced by Cross Country Wireless does what it says in the sales literature i.e. it "provides protection which allows it to be used very close to transmit antennas without damaging the amplifier or the attached receiver".
Now, these units get mixed reviews in the various loop shoot-outs that I've read, but I pulled the trigger at £55.20 plus shipping and I haven't been disappointed thus far. Again, some hard-nosed testing between the Wellbrook, the MLA30+ and the CCW amplifier will follow, with the last two using the exact same copper loop, but that's for another day. No mounting tabs/lugs on this unit you'll notice, but it does have 6mm loop mounting bolts and wingnuts!
If you fancy an unobtrusive Rx antenna for your shack or even your backpack, then a mag-loop may be the answer. You could potentially avoid having to spend £264 on a Wellbrook loop too, if you're not super-fussy.
If you can't wait for my testing results, there are a number of quite objective studies already out there on the Internet - the best of them on YouTube.
For the price (around £35 for the MegaLoop MLA30+) I don't know why you wouldn't have one at least on standby. OK, so it's receive-only, but what a great pair of ears, even when used with a random length of wire draped over a nearby thingamajig. They even work indoors.
M0XYF
