Many thanks to SWLing Post contributor, Frans Goddijn, who shares the following video demonstrating the AMT-MW207: a tiny $25 mediumwave transmitter kit that can be purchased on AliExpress:
To be clear, the AMT-MW207 is not designed to be a whole house AM transmitter like the SSTran and others.
The seller describes it as a device to debug and test AM receivers. Since it only employs a built-in bar antenna, the transmitter would need to be placed in close proximity to the receiver.
Click here to view on AliExpress.
Thanks for the tip, Frans!
(tried to post this a moment ago, disappeared, hopefully not appearing twice now)
an extra video clip:
I just tested mine that I bought a year or so ago. Interestingly, it *does* seem to have a fairly wide audio bandwidth; a quick test showed -3dB @ 16kHz+. Frequency stability isn’t great though; mine drifts a bit too much to be useful as test equipment, except for quick “can this receiver pick up *anything*?” checks, or testing the occasional “Hi-Fi” AM receiver (I’ve got a couple of vintage models with well over 10kHz received audio bandwidth).
FWIW, at the receiver end diode detection can be low distortion; you’ve just got to do it right. Biasing helps; full wave diode & infinite impedance detectors do even better. I don’t even know of any “real” (as opposed to toy or learning-type kit) radios that use diode detectors these days – even a dirt-cheap, $0.01-in-quantity clone TDA-receiver-on-a-chip found in countless no-name AM radios uses a balanced full-wave detector and can achieve sub-0.5% distortion (i.e. comparable to typical FM).
Thankyou Ron for the extra information!
I wonder whether a random wire or longer wire would extend the range to something more useful!
That would be interesting. I could then maybe transmit to all radios in the room at once.
If I place the transmitter next to the GRAHN GS3 antenna I need to attenuate the signal on the GRAHN to a lower ‘pre-stage’ signal strength (or else the red ‘overload’ light goes on) or I can move the transmitter awy from the antenna. So I could probably already feed a few of the antennas with te signal if they are nearby.
Despite the label HiFI AM, it may have a 20 kHz audio range, low distortion and noise, however it is rare for receivers to have an audio range much greater than speech ie up to around 4.5 kHz, diode detection is not low distortion and is always used unless synchronous detection is used, noise is determined by the surroundings particularly switchmode power supplies as now used in lighting and most equipment connected to the mains electricity supply.
Having the transmitter up close to the radio antenna helps push surrounding noise away. I notice the difference when i have no music on (into the transmitter) and hear silence on the radio. If I then switch off the transmitter, a sudden loud noise comes on –from whatever the antenna is picking up instead of the transmitter signal.