Home Brew Shortwave Radio Projects
For years I have been interested in building simple receivers, but never had the time. Now that I'm retired, I can take the time to build things and use the extensive junkbox of parts that I've accumulated. The logical first choice was to start out with simple regenerative circuits. Just one or two transistors and maybe a simple audio amplifier using one integrated circuit. I already have a few simple tube radios that I bought at flea markets, so I will defer any tube projects for a while yet.
Regen Receiver Project
Charles Kitchin has written many articles about regen radios for QST and other hobby magazines. I like his approach to design and have had good luck with his circuits. The first attempt at a "Kitchin" used a printed circuit board that I bought from Far Circuits. This was an excellent learning tool and prompted me to continue experimenting and changing the design and the circuit board.
To house this project, I "re-purposed" a metal container that originally contained Christmas cookies. After drilling the mounting holes for all the controls, I used a bit of steel wool to create a matte finish, cleaned with mineral spirits, and then spray painted. The labels for the controls were made with a Brother P-touch labeler. Thanks to Dave Schmarder, N2DS, for getting me interested in the Brother. It makes a nice laminated label that is good-looking, sturdy, and solidly adherent.
By changing the plug-in coil, I can cover 3.5 to over 10 MHz. It's a bit difficult to copy SSB signals as the detector is not super stable, but it works well enough! I made a few mistakes in this version, such as mounting the speaker on the side of the case. It needs to be on the front or the top cover to direct sound toward the listener. Also, using a large value main tuning capacitor proved to be a problem, even though I used an 8:1 reduction dial. The vari-cap "fine tuning" is definitely needed to copy SSB successfully!
Things I learned:
For drilling large holes in thin sheet steel, a stepped drill bit (Uni-bit) is very helpful. The Harbor Freight Tools version is cheap but serviceable.
Spraying a lacquer "clear coat" over a not-fully-cured enamel paint job can cause an un-intended "wrinkle" finish. You can't see the cover in these photos.
Designing and making your own pc board is challenging and educational. I learned about ExpressPCB software, blue Press 'n Peel transfer film, and using peroxide and muriatic acid to etch boards. I also found that the toner used by Brother laser printers doesn't work as well as HP toner for making etching masks using Press 'n Peel film.
PVC plumbing pipe and fittings make good coil forms when used in conjunction with bases salvaged from defective radio tubes. This design uses a 6-pin tube base. At most ham swap meets, I can buy dead 6-pin tubes for 25 cents.
ZN414/MK484 TRF Breadboard
This simple 3-pin integrated circuit has always been interesting to me. Invented by Ferranti in the UK about 4 decades ago, it was aimed at very simple AM radios that were made in places like Hong Kong. It uses only a single 1.5V battery and a minimum of external components.
This is the circuit I began with; I made my own pc board and used an antenna rod from an old transistor radio.
With a minimum number of parts, I was amazed that I could receive KCBS (740 KHz.) from San Francisco (60 miles) without any external antenna. My chief problem was that KSRO, about 3 miles away on 1350 KHz., came blasting through and covered half the available band! The circuit is sensitive, but not very selective. My interesting innovation for this circuit was to replace the variable 47K resistor with an LED used as a voltage regulator. The forward voltage of the LED keeps pin 1 at about 1.7 Volts. I later learned that this is a bit high and 1.2 Volts will work better.