How to test a simple homebrew CW transmitter - can I use an AM radio?

Question

I've built a simple very low power (10-100mW) CW transmitter, and would like to test it over very short distances before working on my amplifier. (For background, please see A first homebrew design: Band, antenna, and ground wave propagation ). I've tested in on an oscilloscope, and it looks great.

I don't own an HW receiver. How can I test my transmitter - even over a few meters?

The frequency is adjustable and can easily go to 1 MHz range. I thought of testing it by listening on my standard AM car radio. But given that CW has a flat line for an envelope, I don't think I'll get anything other than a DC out (?). Is there any way I can use a commercial AM receiver to test my CW transmitter?

If not, is there any other way, short of building or buying a receiver, to test it?

I also thought of testing it via https://websdr.org - it's nearest station is 40 miles from me. But given my current low power, and usage of a random wire antenna, I'm doubtful I could reach it.

I could just continue and build a power amplifier and proper antenna. But my experience is that good engineering always proceeds incrementally. So: How can I test this simple low power CW transmitter without dedicated equipment?

Answer 1

Tune your transmitter to a frequency that's within a few hundred hertz of a local AM broadcast transmitter. When you key your transmitter, it will "beat" with the broadcast carrier and you will hear the tone.

Anyone else nearby who is listening to that station will hear it too, so do this kind of testing very sparingly.

Answer 2

Regarding your car radio: You probably can. You're right, in the stable case, you wouldn't hear anything (There's no DC your receiver would be putting out: in broadcast AM, the signal modulated onto the carrier is ( 1 + audio(t) ), so that there's an unsuppressed carrier to mix down the audio with. You are only transmitting the "1", so your audio(t) == 0; also, any audio system will have a high-pass characteristic that blocks DC). So, the typical difference between a ham radio AM receiver and a car radio is that the car radio expects an unsuppressed carrier and "tune" exactly to that (inherently through homodyne reception methodology, or explicitly, e.g. with a PLL), whereas the amateur radio receiver (might) allow you to tune "off" arbitrarily.

But the moment you switch your carrier on and off, you should be hearing noise; the switching is a transient thing, has bandwidth, and thus might make it through the receive filter. Depends a bit on how smart the car radio is about filtering noise.

And: You have an oscilloscope! So, wind (or use a rod antenna from a broken old radio) a magnetic receiver antenna (pedantically, that's not much of an antenna – you're in reactive near field for as long as you can still clearly see your transmitter, the wavelength of 1 MHz is 300 m) , set your oscilloscope to high input impedance, and see whether you can see the 1 MHz. Might be easiest if your scope has an FFT mode.

Are you frequency-selective with that? No! Will your reception be very weak? Yes! Will you possibly see the switch-mode supply closer to the antenna before you see the transmission? Maybe! Does a 1 MHz carrier that disappears when you turn off the transmitter prove you are generating an alternating magnetic field? Yes! Very much so! Will things get better if you put a variable capacitor in parallel and start tuning your antenna? Likely!

You can of course also add an amplifying stage here; at that point you've really built a receiver yourself.

Answer 3

If you have the resources to build a CW transmitter, you can probably also manage to build a simple regenerative receiver. These receivers are unique because the self-same receiver can "hear" AM, sideband, and CW, requiring only adjustment of the amount of regeneration.

Effectively, the regen will receive AM with little regeneration (amounting to increased RF gain), and will receive both SSB and CW with more regen (regeneration produces RF oscillation in the receiver that beats against the received signal, which will make both SSB and CW audible by detuning by a kilohertz or so).

A regen receiver can require only a dozen or so components: a dual coil, variable capacitor for tuning, variable resistor or capacitor for regen control, RF choke, and your active component (transistor or vacuum tube), plus a suitable power supply (ideally battery, which produce no radio noise of its own).

As a bonus, with suitable antenna and a switching relay, the regen receiver can be used alongside your already-built transmitter to make CW contacts!