Two Radios for You to Build

On the AMRAD mailing list, Andre, N4ICK, posted a link to the YouTube video, “12AU7 regenerative radio on a tin bake plate” (see photo at right) It’s a very cool regenerative radio made with a single 12AU7 vacuum tube. The only problem is that there are no links to the schematic for the radio.

So, I Googled a bit, and came up with this circuit. The cool thing about this circuit is that it uses a 12V power supply, not the high voltage power supply normally required for a tube circuit.

Over the weekend, I went to a big rummage sale sponsored by the local Kiwanis club. They had quite a few aluminum baking pans there for very little money. I should have picked up a couple of them. :)

Simliar Radios
VK3YE has also posted a YouTube video
of his experiments with a similar circuit. Unfortunately, he also doesn’t include a link to a schematic.

Another related YouTube video is for a one-transistor radio. This is actually a very well-done video. It shows you step-by-step how to build the radio. This is something that we may actually be able to do down at the museum with some kids.

At the very least, watching the videos is amusing. At best, maybe they’ll inspire you to do a little experimenting.

Roofing Filters

Here are a couple of good explanations of roofing filters from the Elecraft mailing list:

  • Don, W3PFR, writes:

    The ultimate selectivity of the K3 is determined by the DSP.
    Now think about the input to the DSP – if one restricts the number of signals entering the DSP (actually the ADC front end for the DSP), the DSP will not have to deal with as much ‘garbage’. The more narrow the roofing filter, the less ‘garbage’ will be present at the DSP front end.

    Consider also that the DSP can handle an S-9 +20 signal input from an unwanted signal – above that level it will be overloaded. In situations where signals greater than that level are encountered within the passband of the roofing filter, the Hardware AGC will limit the signal level presented to the DSP. Even though the DSP may be cranked down to hear only the desired signal, the signal gets through to the DSP front end and will reduce the gain of the receiver because the Hardware AGC is being activated by the strongest signal within the passband of the roofing filter – the result is AGC “pumping” as a result of a signal that one may not even hear. Using a more narrow roofing filter can eliminate this unwanted signal and eliminate the AGC pumping.

    Think of the roofing filter as a roof on a house – the wide filter allows the DSP (and hardware AGC) to respond to all the rain falling on the roof. To reduce the amount of rain falling on the roof, a more narrow roofing filter is required.

    If you are interested in contesting and/or heavy DXing, you are liable to encounter nearby unwanted strong signals within the passband of the roofing filter which will cause the hardware AGC ‘pumping’, and you should invest in narrow filters that will eliminate that effect. For those whose operation is more casual and do not wish to dig for weak signals in the presence of stronger nearby signals, a wide roofing filter will do the job nicely.

  • What “Roofing Filter” means to Elecraft. This is an explanation on the Elecraft website.