Steve's 6 Band Explor-Air Unicoil Radio
Well here it is, finally the Unicoil radio! After several months working on this radio and some hard turns in different directions along with diversions we have a working radio. The Uncoil itself has worked right from the beginning, with each version getting a little more refined than the last. The one in this radio is the latest, third version.
The idea for this coil was born from the need to replace the missing and damaged original coils in my Explor Air. My Explor Air started out as a piece of junk when I got it. Of course it still is not the prettiest thing, but thankfully it works much better than it looks! The biggest plus about the radio is that is feature packed. You know I like radios that can do a lot of stuff. Here is the list: 12 volt operation (no lethal voltages), wide frequency range ~400 khz - ~30 mhz across six bands(four was stock), sensitive and selective reception, easy vernier tuning, and very important, plenty of speaker volume on all bands.
There have been several projects since my last big one (The Two Tube Low Draw Convertible Battery Powered Radio). This one is the next big one. Of course this one does not have low draw like the last one, in fact you had better have a 12 VDC power pack that can deliver close to an amp or more current for the heaters. That is the only shortcoming on most space charge tube radios. This one uses four tubes, three of the four tubes can be interchanged with several substitutes. The tubes are 1: 12U7-12AU7-12AT7 (12AX7 not so good), 2: 12CX6-12DZ6-12EA6 (12BA6 maybe, you can try others!), 3: 12AE6-12AV6 (try others too!), 4: 12K5 (no known subs). The ones in my rig now are 12AU7 (L-plates), 12CX6, 12AV6, 12K5. These are the ones that seem to work best and were pretty easy to get. Let's not forget the main thing here is to be able to make something from what we have lying around, which was the origin of this radio!
It is important not to lose sight of the origin and purpose of this coil idea. I would never recommend anyone to tear into a perfectly running SWL rig like a GR-81 Explor Air, or Span master. Two of the four original coils in my radio were already missing. If you run across a pile of junk like I did, why not!
The other circumstance is, especially for newcomers to the homebrew SWL hobby there are now more limited resources. Unless one is fortunate enough the already have a set of nice coils or deep enough pockets to buy them this Unicoil idea is the ideal solution. Pre made high quality coil sets are getting harder to find and more expensive all of the time. I see individual ocean hopper coils go for around 40 dollars on Ebay! And you need six for a complete set which would be $240? It starts to sound like an expensive hobby when broken down in those terms. I paid dearly for mine a while back but not that much. The idea is to simplify and cheapen the process of building an all band SWL radio.
The details of coil construction have not changed drastically from the ones charted previously. The last tap on the tickler winding is the only thing that could use another revision. I will just have to draw up the revised schematic on the radio itself also.
Information on the Unicoil is on another page.