A Mixture of Things

So many projects, such scattered documentation, such great effects of ADHD type behavior. Polyprojecting, if you will, seems an apt description.

Since when does blogger.com only allow me to add files that have already been uploaded to google docs? Ah, only if I allow a potentially unsafe cross-scripting as described by NoScript.

Should I write about an attempt at an antenna-powered transistor radio? 

The circuit attempts to use all the unwanted RF energy incoming on the antenna to power a single transistor amplifier - which is fed the wanted signal from a high-Q LC tank. It worked about as good as any passive radio I've built, though sometimes heterodyne whistles were produced, indicative of actual amplification. The schematic is somewhere in my piles of notes if anyone if interested.

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Perhaps I should write about my attempts at making a simple vacuum triode from scratch?

Or about the promising attempts at building a regenerative radio with said homemade triode?

 The circuit was patterned after some of the earliest regenerative radio circuits - in hopes that the marginally marginal performance would still be enough to coax out some real RF amplification and/or a solid RF oscillation. I can say with great confidence that the device worked extremely well as a detector, and that it would at times oscillate, but I could not get stable heterodyning, which suggests the oscillations may have been the result of nonlinear negative resistance from such a  gassy tube rather than gain as one would expect from a proper vacuum triode.

The filaments were taken from a 12V tail light bulb, the grid and plate were made from bits of scrap, the structure was assembled with a homemade spot welder, the feedthroughs were sealed with epoxy, and the envelope itself with non corrosive RTV. The vacuum was provided by a brand new dual stage roughing pump supposedly capable of 10 microns, though a check with small hand held tesla coil/violet wand indicated pressures more like 100-200 microns.

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Maybe this post should be about the crystal radio for the FM band that I built, along with the three element yagi antenna I made for it. Yes - this is a passive radio that receives FM, and demodulates it with a phase discriminator rather than just slope detection. This allows remarkable clarity and frugality of signal use. I can't claim that this is an original idea - I mostly just followed Robert Weaver's work: http://electronbunker.ca/FMCrystalSet.html

It is tuned by fringing capacitance - a small square plate of copper is moved by a long leadscrew, which tunes higher if brought away from the resonator, and lower if brought closer. 

To get a sufficiently high Q for this thing to work, a helical resonator was chosen over an LC tank, as the latter simple can't give sufficient performance in the 100MHz range, where the helical resonator promises a Q of 2000 or more, in theory. 

The inside of the case is lined with copper, forming the outer surface of the resonator. With standard 3-element yagi with a folded dipole active element, without audio amplification I can listen to 1 or 2 stations comfortable. Add in some audio amplification, and the number jumps to about 6 or 7 stations, some of which come in with hi-fi quality.