Problem with AM transmitter

Hello:
Im trying to build the following AM transmitter:
https://forum.arduino.cc/index.php?topic=88422.0
The only change I have made to the code is that I need to press a button to make the square work.
When I do so, I can probe the output with the scope and I can see the square wave pulses of 800 KHz no problem. Here is the tricky part: after I added an antenna of about a meter long I can not receive anything in a radio I have almost touching the wire (AM and the 800 KHz). But the weird thing is that if I put the radio on FM and sintonize to the lower frequency, I can clearly hear it, and event probe it with my scope! Thing is, that frequency is 88 MHz, why does this not work with AM at his right frequency but can be heard at such high freq? I though of harmonics taking play here, but wouldn t still be here a way too high frequency?
The last thing I wanna mention is that the 800khz band is already silence, and the transmitter cant change that.
Is that because it only can create silence?
Any help would be appreciated.

Digital electronics puts out harmonics well into the microwave region. Post your code because clearly its
not doing what you think...

The silence on an AM radio means there's a strong unmodulated signal.

You won't even need an antenna wire once you have modulation working, radios are extremely sensitive...

MarkT:
Digital electronics puts out harmonics...

As MarkT suggests, RF is a complicated beast. If you're interested in radio it's well worth looking in to an amateur radio licence.

Thanks for the replies!
Funny thing is, the problem was in the RADIO. I had tinkered with it a bit in the past and had changed the trimmer values. I tried with a new one and it worked. Furthermore, thanks to the internet I have been to readjust them and now works. Homewer, the question remains, de can I listen it in ALL fm frequencies?
PS this is just for and arduino remote control of around 10 m. Sadly, right now I am not that much into RF but thanks forma the idea. I will check if there is something similar in Spain.

Another thing: when I touch the cable it works up to 5 m, but when I let go it doesnt arrive to 1 m. When I put an antena, it goes up to 2 m. How can I achive the same that with putting my hand?

Is you antenna tuned to the operating frequency. At 800KHz you'd be looking at around several hundred feet !!

Irrelevant, the signal will plenty strong enough just from the PCB traces I suspect - radios pick up signals
measured in nanovolts and microvolts.

Another thing: when I touch the cable it works up to 5 m, but when I let go it doesnt arrive to 1 m. When I put an antena, it goes up to 2 m. How can I achive the same that with putting my hand?

Hint, you are not an electrical insulator...

Yes I also checked my conductivity with 220 V recently... :frowning:
By antenna I'm using the standard retractable one I took from an old radio. How can I improve the range ( some sort of amplifier and the emitter)?

Atomillo:
Yes I also checked my conductivity with 220 V recently... :frowning:
By antenna I'm using the standard retractable one I took from an old radio. How can I improve the range ( some sort of amplifier and the emitter)?

More wire! Scotty. I need more wire!!!!

Paul

Atomillo:
How can I improve the range ( some sort of amplifier and the emitter)?

You said you wanted to use it as a remote control link.

In all honesty, you're going to really struggle to get a reliable "remote control" link at 800khz using such a simple rig. RF is way more complicated than just amplifying it as if it were an audio signal, and as has been mentioned a half wave dipole antenna would be over 500ft, which the arduino data pin couldn't drive anyway.

It's a very interesting experiment which you may well get to work under "lab" conditions, and I'm going to have a go myself (using an ESP8266 for a higher frequency), but if ultimately what you want is a cheap wireless connection, then the 433mhz modules are super cheap and relatively reliable
e.g. random shop http://www.robotshop.com/en/seeedstudio-433mhz-low-cost-transmitter-receiver-pair.html or ebay has them even cheaper.

You could try wrapping a few hundred turns of enamelled covered copper wire around a length of ferrite rod to act as your antenna (aerial)

The AM broadcast band is technically very poorly suited for your stated application. It's also vexatious for anyone that is trying to listen to a radio.

That posting you linked to produces a signal on 800 KHz, but it is NOT an AM (amplitude modulation) radio. You may hear it's noise on an AM radio, but in no way can you "amplitude modulate" the thing.

Paul

Ahhhh. So its not really AM. That explains pretty much anything. I was aware of those modules, but I wanted to try and see if I was able to come up with something that would work. As a way to end the experiment, when I arrive home will try to use the ferrite rod and then guess I will justo buy the modules. Thank you for all the help!

Ahhhh. So its not really AM. That explains pretty much anything.

Well the name amplitude modulation should have spoken for itself...

Somewhat less legally dubious too.

Crude OOK isn't hard, an AM radio will hear that!

MarkT:
Well the name amplitude modulation should have spoken for itself...

Somewhat less legally dubious too.

Crude OOK isn't hard, an AM radio will hear that!

Post #16 from the thread linked in the original post gives code for OOK (on/off keying) with a 625 Hz square wave which would produce that tone (and odd harmonics) at the AM radio receiver. It could be reasonably argued that OOK is a subclass of Amplitude Modulation, that is AM with 100% modulation via a digital sequence.

Regardless, using Arduino directly as the transmitter is more a curiosity and perhaps an interesting demonstration than it is an practical approach to inter-processor communications.

Perhaps better described as a crude CW transmission (Continuous Wave)

Atomillo:
. . . But the weird thing is that if I put the radio on FM and sintonize to the lower frequency, I can clearly hear it, and event probe it with my scope! Thing is, that frequency is 88 MHz, why does this not work with AM at his right frequency but can be heard at such high freq? I though of harmonics taking play here, but wouldn t still be here a way too high frequency? . . .

I played a bit with this and got a similar result, that is, the tone shows up in the broadcast FM band. The reason, as others have noted is that the square wave output from the Arduino produces odd harmonics of the fundamental frequency, that is, 800 kHz fundamental, 2400 kHz 3rd harmonic, 4000 kHz 5th harmonic, and so forth. Thus 88.8 MHz in the broadcast FM band is the 111th harmonic. The signal will also be found at 90.4, 92.0, . . ., every 1.6 MHz.

From signal processing theory, the voltage level of square wave harmonics go as 1/n for the nth harmonice. Thus 88.8 MHz from an ideal square wave has 1/111 the voltage level of the fundamental or 20*log10(111) ~= -40 dB power. On the other hand a 1 meter wire is resonant (1/4 wavelength) at about 75 MHz so the antenna is much much more efficient near the FM broadcast band.

It is because of the likelyhood of this sort of unintended radiation that using unfiltered square wave oscillators as transmitters is strongly discouraged.

It is because of the likelyhood of this sort of unintended radiation that using unfiltered square wave oscillators as transmitters is strongly discouraged.

This is termed "EMC compliance" when taking a product to market, and can be a right headache as
digital circuitry is all too readily able to broadcast across VHF/UHV/microwave bands from unintended
aerials on the PCB and cabling. You often see ugly ferrite suppressors on the ends of cables which act
to prevent the cable being an antenna.