AM Morse-Code Transmitter | Hacker News Reader

AM Morse-Code Transmitter

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37 pointstosh17 comments

Not a ham operator, but I'd assume that a transmitter like this will emit a significant amount of noise across the RF spectrum (also, make sure to check your local laws before transmitting in potentially reserved frequency bands, e.g. with the FCC [0], or the Bundesnetzagentur in Germany [1]).

Other than that, looks like a fun little project!

[0]: https://www.fcc.gov/engineering-technology/policy-and-rules-... [1]: https://www.bundesnetzagentur.de/SharedDocs/Downloads/DE/Sac...

I am a ham.

This is a nice, very educational project that a couple of kids could use to communicate between their adjacent bedrooms at night.

It probably provides less rf than a typical noisy LED lightbulb power supply.

The instructions show how to set the frequency between any local AM stations.

If you are worried about neighborhood rfi, listen for it on your AM car radio in the drive and cut back on the length of the one meter antenna until you can’t hear it.

Also, notice how loud the radio frequency interference (RFI) actually is between stations, due to power lines, passing vehicles, LED and fluorescent lighting, many other sources due to modern technology, and lightning and other atmospheric effects. [0]

If someone completes this project, they are more than ready to get a copy of the ARRL General class license manual and go for their ticket. Communicate worldwide with Morse (hams usually call it CW, for “continuous wave”)![1]

[0] https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A...

[1] https://en.m.wikipedia.org/wiki/QRP_operation

Thanks for providing some context! Reading your response makes me realize that my initial assumption was probably way too pessimistic - and that the project serves a nice educational purpose.

On top, sparking interest in tech and ham in kids is always a good thing, a reminder about the possible side effects can always come at a later time :)

You're absolutely correct about RF noise. In the US, the FCC limits the power of all spurious emissions to be at least 43dB below the power of the fundamental frequency. Typically transmitters use a low pass filter (e.g. Buttersworth or Chebyshev) to reduce harmonic content.

Some micro transmitters (at the very low mW level) omit the LPF entirely. Looking at the datasheet for the ATtiny85, the maximum output current is 40mA and maximum output voltage is 5.5v which means this transmitter could theoretically produce an output of 220mW. PWM will reduce this obviously. The harmonics should be below this value, but they should probably add a filter. I've transmitting across the Atlantic Ocean at those power levels using a sub-optimal antenna and WSPR (https://en.wikipedia.org/wiki/WSPR_(amateur_radio_software)) encoding.

If you cut back the antenna length so you can’t hear the fundamental 100 ft away, the harmonics shouldn’t be an issue. But, I’d have to build it to be sure. I might do that!

Here’s some info on suitable low pass filters for $5. You’d have to modify the 600 meter kit for 555 kHz, after reading up on the design at the linked technical pages.

https://qrp-labs.com/lpfkit.html

Note that, for a given too-short antenna length, it will be better at emitting harmonics than the fundamental.

E.g. a 1m antenna is far below resonant for 1MHz AM, but may be pretty good at emitting any 37, 39, 41MHz ... content.

Of course, we expect that content to have less than 1/40th, of the fundamental, both because of Fourier series expansion of a square wave and because the frequency response of the system rolls off (edges off an ATmega aren't perfect, even before you get the reactance of the breadboard in the way.

A primary reason to add a filter, IMO, is to raise the voltage of the fundamental at the antenna, and therefore get a little more RF out on our desired frequencies.

Those are good points. One of the reasons I was thinking of building it was to put it against a tinySA and see what it's putting out.

The thing is, ... anyone who's playing with microcontrollers and longish wires is making similar RF noise.

Efficiency of, and coupling to, this antenna is going to be garbage, even for harmonics. And the breadboard itself provides a fair bit of attenuation of the already-small 10MHz-and-up content.

> the maximum output current is 40mA and maximum output voltage is 5.5v

Looking at the maximum power you're supposed to draw from a pin isn't useful. The output transistors can provide much more: it's just not good for the part.

It'd be better to consider the radiation resistance of the wire and the AC voltage driving it.

Awesome little project! One disclaimer about learning Morse code though -

While that dit-dah tree might be a concise reference, is a horrible learning method. Morse code is learned most efficiently through repetitive exercises in which you learn to "hear" the encoded symbols as characters.

Master CW [1] has a nice little demo that shows how this works. Disclaimer: it's a small app that I built and operate for amateur radio operators and anyone interested in learning CW (Morse code).

[1] - https://www.mastercw.com/demo/

This is definitely the way to go. If you can hear the shape of the symbols, it's much easier to advance in speed. I've used this method a couple times to learn all the letters, though I never seem to maintain my interest in it, and I suck at sending. How do folks practice that side of it? Just get on the air?

If you can, get on the air - it's the most rewarding and fun thing to do. But not everyone is lucky enough to have space for antennas at home.

You may enjoy the competitive side of Morse code: You can practice your code speed and compete against yourself and/or others in a number of ways, for example:

MorseRunner (pileup simulator): http://www.dxatlas.com/MorseRunner/

RufzXP (callsign speed training): https://www.rufzxp.net/

LCWO (callsign, word, plain text training): https://lcwo.net/

Good luck!

Small loop antennas can work well, especially when used with efficient modes like ft8 [0].

You can also get out of the house and get some fresh air using Summits on the Air [1] and Parks on the Air [2].

To learn code, try the Ham Morse app.

[0] https://en.m.wikipedia.org/wiki/WSJT_(amateur_radio_software...

[1] https://www.sota.org.uk/

[2] https://parksontheair.com/

Maintaining interest is hard if you don't utilize your skills on the air, and generally that's how hams practice transmitting as well.

However, I am working on a TX module that will allow hobbyists to practice straight or iambic keying with a keyboard. Sure it's much less sensitive than a proper paddle, but it's accessible and actually quite fun!

That's a neat idea, especially if you could set it up to do practice QSOs. iirc back in the day there were some programs that allowed you to connect a key/paddle to your computer by abusing the serial port.

From what I've read, the Koch method combined with Farnsworth timing is the best way to learn to hear Morse code.

Koch method is about using full-speed dits/dahs from the beginning and gradually adding a couple to the set of characters being trained as you learn to identify them by their sound.

Farnsworth timing is about controlling the overall speed by putting pauses between the characters while training instead of slowing down the dits/dahs of the actual characters.

I don't know the science behind it, but I've spent a few hours training this way. Each set of tones that makes up a character starts to sound like its own word, or phoneme at least. It feels like it's stimulating the subconscious language bits of the brain rather than the logical/conscious ones as if you tried listening and counting the dits/dahs.

Two comments: your app didn’t work on my iPhone, and using the dot/dash notation is generally acknowledged to be inferior to using (e.g. dahdidah) sound patterns. (It looks like you’re using didahdit .-. for “K”, but that’s ”R”. I couldn’t get the following page to do anything, so I couldn’t check further.)

A good app that’s in the iOS store is Ham Morse, which uses the famous Koch method to teach code.

Interesting, but I'm vastly more interested in the LowFER bands than broadcasting alongside FM radio bands.

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