Nordintown Blog

Rubbish musings, projects and hackerings from Nordin

Building a Low-pass filter (LPF) for WPSR Tx ( e.g. a Raspberry Pi)

Nov 202015

I found a nice set of guidelines about how to build LPF board for your transmitter. Only if your radio license lets you, I recommend having a go with one of these and a Raspberry pi [0]. 

For the low-pass filter, I am following a PDF guide [1] authored by Revd. George Dobbs (G3RJV).  The kit list is as follows:

  • 3 toroid cores (type FT37-43)
  • 1m enamelled copper wire (AWG tba)
  • 1 Choc box electrical junction box 
  • 1 case mounted BNC connector (male)
  • 2m 50ohm Coax
  • Capacitors (varied) 
  • Stripboard
  • Solder + iron 
  • An online toroid calculator [2] (if I must, and even then only to check)

We have to calculate the windings from the core size, material and desired inductance. Our design calls for 3 coils and 4 caps.

W3NQN's 7 Element standard Value Capacitor low pass filters


Calculating number of N turns required on a Toroid for a given inductance:

N = 10 x SQUARE-ROOT ( L / L10)

N = Number turns.
L = Required inductance (uH).
L10 = Inductance at 10 Turns.

This design can be soldered onto strip board along with the antenna connectors of your choice. For the enclosure, I can recommend a choc-box connector housing from Screwfix (UK)[3] I have a few of these ready for projects and they work very nicely.  If you are very lazy, you can pick up complete LPF filters from [4]. They have a good selection and ship anyplace (also check-out their other kits) 



Camp oatcake hacking

Aug 242015

The traditional oatcake is a very simple thing to make (perhaps the most simple) and as a result, is a great food to explore cooking on the campsite.

Tellingly, the food has it origins in the kitbags of the Scottish infantry where the food evolved as a quick means to cook a highly available food stuff into a meal. Medium ground oatmeal was bagged and packed for long marches, then mixed with fat and water, balled up and flattened onto a standard-issue metal plate. The plate was then held over a flame and cooked as if over a griddle, flipping when the edges brown. This makes a simple meal for the squaddie on the go and as a food recipe, it offers us a very nice platform to experiment with flavours.


1 cup of oatmeal (medium grade)

1 table spoon of butter, melted

1 half cup of water or milk (or 1 egg white if course meal is used)

Pinch of salt and/or sugar


Mix the oatmeal, salt, fat and water/milk/egg binding agent in a bowl until you are able to make balls of mixture that flatten without breaking apart.

Ball up golf ball sized portions of mixture and flatten out onto a greased pan. Hold the pan over a camp fire/stove until the edges just start to brown. Flip and cook on the reverse side.

Place on a tray to cool.

Some might argue that oatcakes are not the most flavoursome food on their own, tasting much like thinly-salted porridge biscuits. The easiest way to change the flavour of an oatcake is to replace the added salt with something else e.g. additional sugar, spices or herbs. You can also swap the fat for something more flavoured, such as coconut oil. Store brought essance flavourings are easy to throw in and certain wild combinations produce interesting and cheeky results. We experimented with a number of them:

  1. Vanilla and cinnamon 
  2. Citrus essences, such as lemon/lime with added sugar.
  3. Rose water
  4. Peppermint and bitter orange. 

Dried and powered foodstuffs such as seaweed (good for unami) are also interesting options. Just add more or less water/milk to help it all bind.

Happy food hacking!

Space radio: Update 2 - Space TV

Apr 092015

I am not feeling great at all today and so I took a rare sick day from work and oh look, radio.

I needed to avoid spreading the dreaded lurgie and also to recoop from 2-3 nights of horrible sleep. Rather than sitting back and letting Netflix wash over me, I have resolved to finishing my quadrifilar helix (QFH) antenna project in between coughs. A QFH antenna is a 4 part (hence quad) helical receiving antenna design well suited to the capture of APT (Automatic picture transmission) images from orbiting satellites. I want to make something that I can modify for use with both NOAA/ISS APT/SSTV transmissions, which have their downlinks set to 137.500ish and 145.800Mhz respectively.  

John Coppens (ON6JC/LW3HAZ)[2] puts it better than me:

"The QFH is an excellent antenna for satellites, as it receives from 'the entire northen hemisphere', from horizon to horizon, with all the sky in between. Such an antenna cannot have any significant 'gain', as it doesn't have directivity. Such an antenna is not useful for hot spots, as you probably won't have any clients 'in the sky'."

This design [1] looks to be a winner and so I have brought all my parts and tooling together in my study to hammer out something that I can leave connected to an RTL-SDR/Raspi in my loft. I'll take pictures as I go and aim to get it ready for the weekend ISS passes. To achieve this, I need to:

  1. Check the calculations for the two cable loop lengths, spur lengths, height and distance apart using the target wavelength and core thickness [3]
  2. Mark, drill and cut upon my large and small spur pipes using the measurements obtained; and
  3. Solder the wires of correct length to variboard in the correct way.

All doable, so lets go.

Given the wire I am using (1mm wire stripped from a 3 core electrical cable), the calculations turn out to be: (145.800Mhz137.500Mhz)

Large cable loop: 2223 mm / 2357.1 mm (Brown wire)

  • Antenna height (H1) = 672.9 mm 713.4 mm
  • Internal diameter (Di1) = 295 mm312.9 mm
  • Horizontal separator (D1) = 296 mm 313.9 mm
  • Compensated horiz. separation (Dc1) = 290 mm307.9 mm

Small cable loop: 2112.5 mm / 2240 mm  (Blue wire)

  • Antenna height (H2) =  639.5 mm / 678 mm
  • Internal diameter (Di2) = 280.3 mm / 297.3 mm
  • Horizontal separator (D2) =  281.3 mm / 298.3 mm
  • Compensated horiz. separation (Dc2) = 275.3 mm / 292.3 mm

Diag 1 shows where measurements map to on the design.

Diagram 1, Copyright © 2015 John Coppens [3]

The source website also includes a very handy template that is generated from diameters of the larger and small tubes given. [Diag. 2].


Diagram 2, Copyright © 2015 John Coppens [3]

I didn't use this as I am drilling holes for both NOAA and  ISS SSTV frequencies. For now I am wiring for 137.500Mhz (NOAA) since I can test it today. All going well, tomorrow I'll cut lengths for the 145.800Mhz ISS SSTV and swap out the cables, moving the separator rods as needed.

I am using 10mm 25mm PVC piping for the horizontal post and old 12mm fibreglass tent poles for the separator rods.  Let the cutting commence.


Not bad. Not great, but not bad. Next up is the wiring. I used polymorphic thermoplastic to neatly bind the wire to the middle separator rods. The wire can be easily adjusted to make the rounded form required whilst holding firm.


The polarisation does matter here, so with the smaller loop (green) running north-south, the wire is twisting counter-clockwise. Soldering next [diag 3].  


Diagram 3, Copyright © 2015 Akos Czermann [4]

A little clumsy, but eventually sorted.


Having made the basic structure and securely wired it, I need to place it somewhere with a good view of the horizon to test against my store-bought Sky Scanner Rx antenna.

The plan is to hang it in the loft and wire it into a Raspberry PI running RTL-TCP via an RTL-SDR dongle.

Results will follow.





Quadcopter Drone Army - Update 3

Apr 072015

I am keen to complete this project, and so I am just about to purchase the following:

  • 15x N-channel MOSFET Transistor, 4.2 A, 20 V, 3-Pin Micro3 (IRLML2502PBF )
  • 2x Hot Swap Controller Bidirectional i2c level converter (LTC4301LCMS8)
  • 5x CD74AC04M, CMOS Hex Inverter 24mA 1.5 → 5.5 V 14-Pin SOIC
  • 10x HC-49 Crystal 25MHz, ±20ppm, 2-Pin

The FETS are to fix one of the quads that has blown it's cheap versions (a known problem, sadly [1])

The I2C level converter is to convert the wiimote logic level to something I can experiment with on an Ardunio Nano v3.

The Hex inverter is used around the crystal along with caps to stabilise the clock signal for the Pixart camera

The 25Mhz clock provides the clock needed to drive the pixart IR camera's onboard IC.

These are mostly SMD components, so I get to gain more experience over dealing with these guys.  I have used variboard with SMD in the past so theres is that, but I might take the opportunity to make a batch of useful 8pin/14pin/16pin smd breakout boards for the space [2].


Project Intro: Space Radio

Mar 212015

Today I drove my small car in an after-work dash to a quiet low cliffside car park in-between the harbour lighthouses.

There I sat intently listening to radio static blasting from my mac. A freshman radio enthusiast, I was planning to receive magic space images from orbit via my magnetic roof antenna plugged into a cheapo SDR radio receiver. The long, drawn sputnik-sounding 'weep weep' with superimposed donkey-like short 'clip clop' of the NOAA Automatic Picture Transmission (APT) signal was my paydirt. After its capture, the audio signal was going to be processed via a simple toolchain to become a beautiful, fully-formed live weather photo from orbit. This exciting outcome was not a given however, as I had found at my last two attempts. I was pretty sure this run would be a success and not a waste of hours. I had brought sandwiches and a banana just in case.

I'd heard about the magic NOAA weather broadcasts from hackerspace radio folk and it seemed like a fun way to use the RTL-SDR and Sky Scanner Rx antenna combo I'd bought on a whim. A number of weeks ago, space members hosted a radio weekend at Aberdeen Uni where folks erected their roof dipoles and made contacts over HF. Making use of the roof also, I played with receiving the NOAA signals, sprinting to the chilly roof when the sats passed overhead. Orbiting National Oceanic and Atmospheric Administration (NOAA) satellites circle pole-to-pole 540 miles above the earth. NOAA have a few sats up there and the three main APT players - NOAA15, NOAA18 and NOAA19 - pass overhead a few times a day. NOAA15 and 18 have their downlinks as 137.6200Mhz and 137.9125Mhz respectively and seem to be the strongest signals for me.

To track sat passover times, I found gpredict [1] to be a great open source tool that can be installed cross-platform. To capture the actual audio via an RTL-SDR once I had a sat overhead, I used GQRX [2] for the mac set to narrow FM, but SDR# does a great job on a windows machine. On the mac, outgoing audio can be directed to an input device via the virtual soundflower device and recorded via Audacity set to a sample rate of 11025hz. On a non-mac, you’ll probably have a ‘stereo-mix’ device in place already. The resulting wav file then needs to be imported into the very cool WxtoImg tool. It does a one-click job of automatically transforming the audio into imagery, even if the signal is incomplete. This was the audio [3] and resulting image [4] of my first attempt. Not great, but a good start. At 38secs the audio is perfect, but it does not last. I sought a clearer, more rural sky, so a few weeks later I drove to the harbour coastal car parks and took a shot there, but the cloudy day seemed to prevent a strong signal. I got the ‘weeps’ but not the‘clops’ :(

So back to the present: there I was in my third attempt, waiting in my little car in a carpark with worlds smallest entry gate (seriously, my Corsa barely fitted past). After waiting a spell, I started to hear shadows of that magic sound. I had run my mac battery down by this point and I did not want it to die at this crucial stage. I plugged in my car charger for more juice. Not long after, I COMPLETELY lost the signal. Having not started the audio capture I was pretty pissed when no tweaking about the freq made any change. The sat was still in full view but I was getting nothing with 3mins till LOS. Infact the entire noise floor had raisen up (that should have been a clue). I checked the usb/coax connectors and roof aerial but no change. The sat passed below the horizon just as I realised that I had started the car engine to make sure my charging laptop and phone would not kill my car battery. Stopping the car brought the noise floor back to normal. DAMIT DAMIT!! Thats how I learnt that using cheap radios from inside a car sometimes work better with the engine and chargers off. Feel free to write that down if you are a radio dumbass like me. On the plus, I guess I forever learnt something new about SDR radio and found a nice radio spot, so not a waste of hours at all.

If *you* want to have the same (or hopefully more successful) raw audio-to-imagery experience, you should hurry. The APT sats are getting pretty old and cannot hold orbit forever. NOAA APT transmissions are scheduled to die from 2017 [5] and will be replaced with something digital. Boo hiss boo. If you do have a go though, for gods sake make sure you bring sandwiches. It seams brains are optional.



Project Intro: Dirty Great Screen

Feb 112015

57north are hosting a hacker camp, CampGND, in the raw scottish wilderness; well wilderness of sorts.. we may have poor toilet facilities, which is raw for some. Amongst the not-so-light drinking and poorly conceived meals, I shall be building a DIY portable projector screen as my camp project.

The Aim:
A weekend to build/test a car-housed foldaway outdoor cinema, complete with ambient edge lighting. T

Things I'll be hacking:

  • an ancient but functional InFocus LitePro 620 projector/sound amp, mounted to point out my car window, with power lead to the genny.
  • Raspi + OpenElec + boblightd + ardunio nano + PC power supply
  • 2 to 4 WS2812B RGB LED strips (assuming they arrive)
  • a number of things to try as cheap high-gain front and rear projection screen material
  • old tent poles, strap material, eyelets and anything else that might be useful to hold up the screen and keep tension.

Project Milestones
MS1) Build the screen, attach first material, test projector, Movie 1
MS2) Add ambient lights and control, Movie 2

I'd have 2 attempts, one each evening, to watch the original Wargames or 5th Element, then Kungpow/Kung Fu Hustle. If it craps out, then Techno DiSCO!

Call for parts
Hmm any old tent poles from dead dome tents you have will come in very handy. An oldschool Power-to-screen PC supply might be cool also