Re: Mini-Mod 2021!
I've added a buffer stage to the MiniMod circuit to increase the frequency stability.
I did consider this in the original design way back in 2011. At the time I didn't think it worth the effort, and generally if you're listening on a vintage set with a fairly wide IF bandwidth it's not really necessary. But on transistor sets which often have narrower IF bandwidths, the MiniMod's slight FMing is sometimes noticeable. (The FMing is caused by the modulated signal in the output transistor varying the VFO load.) The addition of Q2 buffer isolates the VFO from the output transistor.
I'm still using the BC109 because I have plenty, but the BC548 is one of many suitable equivalents.
The suggestion of adding C14 was made by Tony Nailer (spectrumcomms) who has been selling the coils and semiconductors for the project on eBay for about a year. Some constructors added an LED to the circuit, which I have also included now that low current types are so common.
There are many ways of getting more power out of such a simple circuit, but apart from the increased battery drain, matching the output to a tiny and inefficient transmitting antenna starts to get a bit complicated. Then there's the harmonic filtering to be addressed. The output tuned circuit of the MiniMod takes care of that. The Q of the tuned circuits give maximum output in the approximate frequency range 1000 kHz (300 metres) plus or minus 50 kHz or so. But this depends upon the exact values of C1 and C13.
There are also much better ways of generating the signal than an L/C VFO. The ProgRock programmable crystal kit (from QRP Labs) and a PLL synthesizer (PCB and crystal available from medtechnica on eBay) are both viable alternatives, but again, probably a sledgehammer to crack a nut for a simple battery-operated device. However, they'd be ideal for something more substantial...
The PCB layout is included which anyone is free for anyone to use/amend. I designed the original layout in Express.pcb which doesn't generate gerber files. I only use Express.pcb to generate a drawing which I use to hand wire the 'tracks' and use single-sided PCB using the copper as the ground plane. A ground plane is not really necessary, it's just easier in my construction method. Square crosses are drilled and soldered to ground plane; round crosses just soldered to ground plane.
Since it is not possible to solder grounded connections of the 90uH inductors directly to the ground plane, C5's earthed side is soldered to one side of L1 to earth and one side of the unused winding of that inductor to earth. C10 does the same for the earthed side of L2.
The linearity of the modulation envelope is visible in the photos.
I made at least two dozen of the initial design a few years ago, and I think many more have been made reading the various MiniMod threads on this forum. I don't intend to build any more.
There are a number of ready-made low-power ''pantry' transmitters already available on eBay and elsewhere, most of which give good performance.
Note: I have no link with any of the vendors mentioned above.
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