A couple of years ago one of the Hackaday Prize finalists was a project to take highschoolers through building a direct conversion radio receiver for the 40 metre amateur band. It was originated by the SolderSmoke podcast, and we’re pleased to see that they’ve recently put up an overview video taking the viewer through the whole project in detail.
It’s a modular design, with all the constituent building blocks broken out into separate boards on which the circuitry is built Manhattan style. Direct conversion receivers are pretty simple, so that leaves us with only four modules for oscillator, bandpass filter, mixer, and audio amplifier. We particularly like that it’s permeability tuned using a brass screw and an inductor, to make up for the once-ubiquitous variable capacitors now being largely a thing of the past.
A point that resonated was that most radio amateurs never make something like this. Arguments can be made about off-the-shelf rigs and chequebook amateurs, but we’d like to suggest that everyone can benefit from a feel for analogue circuitry even if they rarely have a need for a little receiver like this one. We like this radio, and we hope you will too after seeing the video below the break.
Need reminding? See the Hackaday.io project page, and the Hackaday Prize finalists from that year.
That variable inductor is not “permeability tuned” by the brass. The permeability of brass is pretty much the same as air, and changing the amount of brass inside the coil won’t change the permeability of the space inside the coil to any significant degree.
If the screw were ferrite, the permeability would increase and the frequency would decrease as more material is inserted in the coil.
The brass screw behaves the opposite: adding more metal in the coil decreases the inductance and raises the frequency.
The key is that brass is nonmagnetic and is a conductor: it acts like a shorted winding in the air-core transformer, and that is what is decreasing the inductance.
Spot on.
G7VFY
A good cheap way to make a PTO is by using a ferrite rod (AM RX antenna core), wind your coil around it over a piece of thick paper (like a note card) and just insert to get the desired L. First ATU I made used it with a polyvaricon. Simple quick old school hack.
There’s an ugly sort of beauty to manhattan-style circuit boards. It doesn’t look pretty, but it doesn’t look wrong either.