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Building a digital VHR AM Aircraft receiver

  1. Dec 29, 2015 #1
    This is a continuation of the archived thread from https://www.physicsforums.com/threads/how-to-make-inductor-coils.570700/. Judging by the list of needed coils, I suspect the author was looking to build VHR AM Aircraft Receiver as described here: http://electronics-diy.com/vhf-fm-aircraft-transmitter-receiver.php. I am also interested in building this device and later interfacing it with a PC using Arduino development board (to set frequency and receive digital mono stream). Mainly I am interested in this project to listen to aircraft communications because I am learning to be a sports pilot as a hobby and also because I am a software engineer and have recently become interested in building hardware that can interface with a computer.

    A few questions that I have:
    • Do coils get sold already wound or do I just need to buy a wire and wind it myself?
    • According to https://www.physicsforums.com/threads/how-to-make-inductor-coils.570700/#post-3727829, I could use http://www.crystalradio.net/cal/indcal2.shtml to calculate the resulting inductance given number of turns, diameter of coil and wire diameter. Unfortunately, after trying to get the 0.15 uH by specifying the input parameters from the first post (1.5 - number of turns, 3/8=0.375 inches - diameter of coil, and various values from the tables for #26 wire), I get resulting induction ~0.04 uH, which is nowhere close to 0.15 uH. Where is my mistake?
    • How can I interface a variable capacitor with Arduino? Is there any way to control capacitance between two inputs on Arduino digitally? Although I do not technically need Arduino or PC to listen to aircraft communications, building a digital device will be a good exercise for me to learn writing drivers and creating digital hardware.
    • I am also not sure which components of the circuit correspond to various parts of the super-regenerative receiver pipeline as shown here: https://en.wikipedia.org/wiki/Superheterodyne_receiver#Design_and_principle_of_operation. From the description it's only clear that the implementation is mostly done by components on the left of the L3, but I have no clue what is needed for which purpose.
    • Does this circuit provide protection against transmitting on the same frequency? I only intend to build a device for listening and would like to avoid interfering with any aircraft communications.
    Also I have never really done any circuitry work before and since I live as an expat in Germany, I wonder where can I buy basic components cheaply here? Or can I order some beginner sets from US or some other country? I come originally from Ukraine, where there are plenty of radio shops and markets, but I don't seem to be able to find them in Germany...
    Last edited: Dec 29, 2015
  2. jcsd
  3. Dec 29, 2015 #2


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    There is only 1 coil for you to wind ... that is L2, a 1.5 turn and about 5mm diameter
    ALL the other coils are small commercial ones that you buy

    there may be a way using a varicap diode instead of a variable cap. how to interface it to an Arduino, I have no idea ... those controllers are not my field

    clarify what you mean

    it is just a receiver

  4. Dec 29, 2015 #3

    jim hardy

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    You might look into a kit like this

    http://www.ramseyelectronics.com/Aircraft-Receiver-Kit-Ramsey-AR1C/dp/B0002NRIO6 [Broken]
    i built one. It works but tuning was a bit on the sensitive side. It could use a fine tune like the one you linked.

    not quite so much fun as etching your own circuit boards ... but convenient
    Last edited by a moderator: May 7, 2017
  5. Dec 29, 2015 #4


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    Yes but he refers to a "super regen" receiver. They can easily hoot and produce loads of interference. I remember, in the old days, people used super regen to produce high gain and high selectivity from inherently low gain devices. That isn't necessary these days (I would have thought) and it's a good thing not to have to tame a potentially unstable circuit. Integrated IF amp circuits with off the shelf ceramic(?) filters are the way to go - if you want a result without too much effort.

    If the idea is to control the receiver with a processor, it may be best to use a synthesised Local Oscillator - again, I believe these are available, although I don't do much actual construction these days.
  6. Dec 29, 2015 #5
    Thanks for the tip, I'll read up on varicap diodes and whether they can be interfaced with Arduino.

    On Wikipedia the Superheterodyne receiver is shown in a diagram as consisting of the following blocks: RF Amplifier, Mixer, Local Oscillator, Filter, IF Amplifier, Demodulator and Audio Amplifier. I understand their function thanks to detailed explanation on the same page, however, I can't map them to individual components in the circuit. The only bit that I understand from the text on that page is that all the components to the left of L3 are radio-related, while everything to the right is audio-related, which makes me assume that all but Audio Amplifier and possibly Demodulator blocks are implemented with just 10 components (Antenna, L1, C1, C2, L2, C3, Q1, C4, C5, L3). Could it be true? How can so few components implement so many blocks? Are some components reused? Or is built in a completely different way than explain on Wikipedia?
  7. Dec 29, 2015 #6
    I can't find it anymore, but I've seen another design where the signal was transmitted from antenna to the circuit using two coils and induction and the author said that this was done to prevent circuitry generate output signal on antenna. I am not sure why induction wouldn't work the other way around (to transmit signal from circuitry back to antenna), but then again, I know very litte more about coils and induction than that it's used to transfer power from my wireless charger to my phone.

    EDIT: Just found another design for Crystal Radio, which also uses two coils and induction. I suspect that I've seen this design before and that it has nothing to do with super-regen receivers.

    Yes, it seems that's the way to go. Most sites that I read suggest to forget about the capacitor and just generate the digital oscillator signal. Initially, I didn't understand it because I thought that that capacitor also tunes the RF amplifier, but later I realized that RF amplifier is not a band filter as in Crystal Radio, but amplifies everything and thus doesn't need to be tuned.
    Last edited: Dec 29, 2015
  8. Dec 29, 2015 #7
    Thanks for the tip. Indeed, I would prefer to build my own circuit, especially considering that I am planning to digitize it later. From the page it's not clear which level of assembly is required. Is it basic screwing and casing or would I actually have to solder things to the board?

    I've also just found a kit for almost exactly the circuit that I'm trying to build (schematics). It's essentially a version 2 of the same circuit and they used to sell a kit for the version 1 until it was discontinued. Since I am visiting family in Ukraine right now, I'll try to get all needed components in a radio shop myself and build it on a bread board, but if I'll fail to do that, I may choose to purchase their kit with all the instructions and a printed circuit board.

    EDIT: Version 2 actually only works with 108 MHz and doesn't have fine tuning and sensitivity. So there are more differences that I saw at first. I'll instead be trying to assemble the version 1.
    Last edited by a moderator: May 7, 2017
  9. Dec 29, 2015 #8
    Another question that just crossed my mind: how can I find out which specific frequency is my receiver tuned to if there is no persistent transmission on the airband (pilots communications are typically short and rare unless listening next to a busy airport)? This is especially difficult considering that the manually wound coil would have an imprecise inductance and the unknown error would have to be compensated with the capacitor tuning.

    I've also tried to look at other bands - the receiver can receive anything between 107 and 135 MHz. The airband is from 108 to 137 MHz. The only remaining bit is 107 MHz to 108MHz, which is reserved for FM Radio. Hopefully I'll be able to find some radio stations in this range, but what if I don't - is there any way to determine if the receiver is receiving at a specific frequency if there is no active transmission?

    EDIT: Given that I am trying to build the super regen receiver, I wonder if I should be able to just measure what does the local oscillator produce...
    Last edited: Dec 29, 2015
  10. Dec 29, 2015 #9

    jim hardy

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    Mine came as an unpopulated circuit board silkscreened where everything goes , a bag of components and instructions.

    Good luck!
    Keep us posted of progress?
  11. Dec 29, 2015 #10
    Thanks. Will post updates here. Hopefully I'll manage to build it before my holiday is over because later I may struggle to find time between my work, learning for my flight theory exam and spending quality time with my gf :-). For now, I've been looking more at the circuit and have realized that here two coils L1 and L2 are actually next to each other, so they do transfer signal using induction. Suddenly with this realization the circuit started to make a lot more sense, but I guess I'll leave it for tomorrow as it's 4am here and I should be seeing a third dream by now :-).
  12. Dec 30, 2015 #11


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