25 July 2020

Independent glow from a Thyratron tube

Thyratron Glow!
This is a self follow-up of a 2017 experiment with a low-pressure gas-filled vacuum tube, a Raytheon Thyratron 2051. [Well, technically speaking this is not anymore a vacuum tube!]

While we know that the gas inside can be excited with an HVAC source through the glass, that seemed impractical when I wanted to show the lighted tube as if it was a (purple) bulb. So I set up a little test.

I fed the HVAC to the cathode. Touching the glass with my hand lights up the gas. Cool. No shock whatsoever.

Then I tried to close the HVAC circuit to the "cold" pole of the generator through a 10 Mohm resistor: now the gas lights up on its own. That's what I was looking for!

Replaced the resistor with 1 Mohm and there is a slight increase in brightness and, if the glass is touched, the glow spreads and increases.

Last improvement: heat up the filament/heater (I used 5.5 Vdc). This brings the glow to another level which is well noticeable in mid-darkness. The tube warms up too, good for the dark, cold Winter season.

I have seen glow forming at the top of the tube just once, probably while the filament was cooling down. I will experiment more to get a repeatable glow on the top too.

Apologies for the lousy picture. Too much light and you don't see the glow. Wrong white balance and the glow turns into blue (NO!). You need to get one of these if you want to see it, or wait that I set up a better photographic studio (evil grin ... this could lead to a challenge to a colleague who is into photography...)

23 July 2020

Lagomarsino Totalia LD-122

Totalia LD-122 calculator.
According to the date I typed on the keypad, I have owned this Lagomarsino Totalia LD-122 calculator for one year and half. 

I got this calculator in good conditions, just needing external cleaning. It has a Panaplex display, which makes it interesting for me because it glows Neon orange.

The power cord has only live and neutral but at least it is still commercially available (the one that looks like the "infinite" math symbol) so the calculator can be powered up with all the proper care.

Side view with power switch.

The label says "Made in Italy". Really?

LD-122 back view with AC socket.

If I had room on the desk I could use the dust cover between each use.
Dust cover in place!

I should take pictures of the inside to document the technology and study if it can be somehow interfaced with a microcontroller to build the Nth clock...

11 July 2020

My current LoRA "DX" is 64 km

After few experiments with spread factor and the GP antenna, thanks to TTN Mapper service I can visualize which gateways have received my LoRA signal on 868 MHz.

On the South facing balcony the signal travels 64 km at SF7 BW125 and the RSSI is still respectable. The strongest link at SNR = 8 dB is "just" 48 km long. Both of these long distance paths are in line of sight or very close to it. A total of 5 gateways received my transmissions.

When I moved the transmitter on the North facing balcony I hit 3 new gateways, two of which are available on TTN Mapper. The maximum distance here is about 38 km, but there is absolutely no line of sight path between us.

That makes a total of 7 (8) TTN LoRAWAN gateways that can receive and relay to the cloud my LoRA messages

10 July 2020

How to calculate LoRa GP antenna

Say you want to try a GP antenna on your LoRa module. How long should it be? A Ground-Plane antenna consist of 1 radiator and a number of elements mimicking an elevated ground plane. All of these pieces of electrically conductive material (preferably copper wire) should be long one-quarter of wavelength.
Since the wavelength depends on frequency, you need to apply the formula:

element length [m] = 300 / 4 / frequency [MHz]

This means that each element will be:

 Lora Frequency Band [MHz]
GP element lenght [cm]
 868 8.6
 915 8,0

Don't be too picky about millimetric precision of your cuts.

How many elements should you use for the ground plane? In my experiments I used two, 180 degrees apart, because that's the amount of wire I had within hand's reach. If you can use 4, each spaced 90 degrees. With reference to the horizontal plane, ground plane elements should be bent 45 degrees downwards. Again, don't be picky with the angle, but try to be consistent with all elements.

09 July 2020

LoRA GP antenna

Now that I can map my LoRA signal, I can play with RF and try to get the most out of it. The most straightforward way is to use a better antenna, and probably the only chance to increase range/coverage.

SX1276 with a directly
attached GP antenna.

To keep things simple and straightforward I replaced the helically wound antenna with a ground-plane directly at the SX1276 board. While this is not a real-life situation, it works well for side-by-side comparison. Also I should have tuned the GP to resonance, which requires a connector and/or some coax.

The signal strength as measured by far away TTN gateways is some 7 dB better. If I understand correctly LoRA terms, 7 dB means you can reduce the spread factor by two and achieve the same signal quality. Or quadruple the bandwidth.  And you LoRA experts know the role of SF and BW on airtime vs bps vs whatever_LoRA.

The expert eye should note that the helical antenna was used without an explicit ground plane, so this experiment is probably not meaningful at all. I need to go back to the bench and re-install the coil with the same ground plane.


07 July 2020

Simulating a LoRA node GPS position on TTN

A simple way to see your LoRAWAN node or gateway coverage is to use the TTN Mapper service.

In order to get my node visible on TTN Mapper I had to enable the Mapper Integration within my Application and output notes's position. TTN Mapper documentation says that either the node sends its position in the payload or you can "pair" it with a smartphone app and keep both close to each other (ie the same vehicle, the same bag, ...).

OTOH if the node sends its position it must be decoded into a JSON through some code written in the web console, Decoder section under Payload Formats. So, why not write a static Decoder function that returns the required variables?

You need to produce latitude and longitude. Optionally altitude and hdop. If a node isn't moving, that's quite easy! Recover the coordinates in your preferred way (smartphone GPS app, GPS tracker, a web mapping service, ...) and fill in your code like this:

function Decoder(bytes, port) {

  var decoded = {};

  // your node's actual coordinates
  decoded.latitude = X;  // a value like 10.456 (this is North-South or equator)
  decoded.longitude = Y; // a value like 80.123 (this is East-West of Greenwich)
  // your node's height above ground (or is it m.a.s.l.?)
  decoded.altitude = Z;
  // your position accuracy, 5 is fine
  decoded.hdop = 5;

  return decoded;

Once the Decoder function is saved, all further transmissions within your TTN Application will carry those variables (added cloud-side once the data is received through a gateway).

Can you see a cheating danger in here? I do. But fortunately there isn't a LoRA distance competition, yet.

Let's get my signal as far away as possible, as strong as possible!

03 July 2020

RaspberryPI and LoRAWAN with SX1276

I had laying around a SX1276 LoRA module for some months and following an IoT discussion at work I thought it would be the right time to play with it.

Instead of wiring an Arduino board with level converters (or use a 3v3 Arduino Nano, which I have), I opted to use an old, single-core, 512 MB RAM, Raspberry Pi B+: wiring is straightforward and I can debug+control it remotely over SSH.

Not that I have meaningful data to transmit. I am interested in the RF part of the whole thing since 868 MHz lies between 70 and 23 cm HAM bands and everyone writes wonders about the LoRA modulation scheme. Because of that I chose the software that offered the fastest way to get a signal on the air with the support of TTN. I used arduino-lmic-rpi with bcm2835-1.64 driver. The RPi distro is based on Debian 8 with kernel 4.9.35, don't ask me its name.

The TTN part is well documented: you need to create an Application and register your device on their portal. Then get Device EUI, Application EUI and App Key which have to be inserted into the .cpp file. Pay attention that DEUI and AEUI need to be reversed, so LSB first. Compile and run.

If everything is correct and your LoRA node can reach a gateway nearby, your transmissions will begin to appear on the web console. You need a TTN gateway in your area and it must be able to receive your transmission!!

Performance? My signal is being received by 4 gateways and a couple of them seem to be quite far away (but in line of sight).

Nothing new so far, I have only replicated someone else's work.