ANTENTOP- 01- 2010, # 012

Some Thoughts on Regenerative Receivers

Hallicrafters S-20R

Credit Line: http://www.radionewjerusalem.com/Radio/S-20R%20Hallicrafters.html

In the 1960s, the ARRL handbook showed an interesting receiver along these lines. It used a 1700 kHz IF frequency with the local oscillator between the 40 and 80 meter ham bands. The 1700 kHz IF had good selectivity with a half-lattice crystal filter. The front end was tuned to either 40 or 80 meters, as desired.

As to controlling the regeneration, any scheme that allows you to control the feedback will work. The more common themes:

- Armstrong "RF" throttle - another expensive variable capacitor, plus frequency issues.

- Variable resistor across the tickler coil -- "kill" some of the feedback signal. My original 1T4 regenerative receiver used this scheme.

- Change filament voltage! Used in many of the early directly heated filament tubes.

- Change the screen voltage in a Pentode. Probably one of the cleanest strategies. This is used in the World War II Paraset design.

- Change the coupling between the tickler and tuning coil mechanically.

- Add a variable resistor in the emitter circuit (Kitchin's design, who uses a Hartley oscillator, and he also uses a separate diode detector). This reduces the loop gain of the oscillator.

Change the plate voltage of the detector.

Again, due to the various interactions, and a desire to get smooth transition into regeneration, some sort of good voltage regulation for the detector stage is essential. This doesn't add any great complexity to the design with the availability of nice integrated circuit voltage regulator chips. Kitchin uses a simple string of diodes to get around 1.5 volts for the detector.

Now, for an analysis of a couple of current commercial kit designs. Where I live in rural "Slower Lower" Delaware, it is hard to get electronic parts other than at the local Radio Shack, unless I do a lot of mail ordering. As a consequence, getting back into radio after a 30+ year hiatus, I opted to go the kit route for my initial efforts in regenerative receivers. There are a couple of clear advantages to kits -- all the parts were there, and the design was liable to work the first time round. Any modifications on my part could be done with ease, either during construction, or later, as I might choose.

For my first regenerative receiver kit, I chose the Ten-tec 1253. As I mentioned, the band switching scheme is quite clever -- it uses a series of inductors switched by PIN diodes and a counter. It also has nine bands. This is mostly due to the rather limited tuning range with the varactor diode, but nine bands is pretty cool. It covers from the low end of the 160 meter band up through 13 meters with a few unimportant gaps. The upper frequencies are a bit problematical, but this is typical of both vacuum tube and solid state regenerative receivers. If you want to go to higher frequencies, and use a regenerative detector, then the Regenodyne approach is a good bet.

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