KA-116: Comparison and tests

Comments on the KA-116 circuit

The most important feature of this circuit is that the oscillator's tuned circuit is also the antenna's tuned circuit! This peculiar feature has the following advantages: economy, because you save one coil and one variable condenser.
Perfect tracking, because there are no circuits to track.
Single knob tuning, because there is only one circuit to tune. Note that the oscillator is operating contnuosly.

Sensitivity at no additional cost, because the oscillator tube also amplifies, to some degree, the signal coming from the antenna. Since this signal is applied to the oscillator's control grid, the mixing of signals take place right there.

KA-116 short wave converter in its operation position

Conclusion: A single pentode valve, or in my case a tetrode, performs the funtions of Antenna Amplifier, Oscillator, and Mixer with a minimum of parts.

The same feature mentioned above has a few inherent problems:
- Repetition of signals, because every signal is heard at two different positions. Example: An incoming 5 MHz signal will produce an IF of 200 KHz, when the oscillator is at 4.8 MHz and when it is at 5.2 MHz, so every station is found twice in the dial.
- Reduced selectivity, because every signal is de-tuned 200 KHz if your IF is 200 KHz.
- Radiation, because the oscillator's signal is continuosly radiated into space due to the antenna coupling produced by the small antenna coupling condenser.
- Crossmodulation products, because there is no shielding at all of any of the sensitive elements like: the valve, the coil, the control grid lead, etc., etc.

Practical evaluation: By experimenting with the KA-116 I concluded that its advantages outweight by far its disadvantages. It is exciting and practical to hear short wave stations with the KA-116 short wave converter!

I most say that THE LOWER THE IF IS CHOSEN THE BETTER THE KA-116 WILL PERFORM. I use a BC 348-Q tuned to 200 KHz as IF, and the results are just great.
To understand the russian designer when choosing this circuit that requiers a LOW IF, the reader must travel back to 1930 when the incipient superheterodyne BC receiver used 175 KHz IF and in USA the Airways Weather Band was from 150 KHz to 410 KHz. Most important in Europe they had (and still have) a real long wave BC band.

The "SUBMARINER" and the KA-116
The October 1931 issue of "Radio News" published an article on an american short wave converter called the "Submariner". This converter works on the same principle as the KA-116. Both diagrams are posted side by side for the reader to compare.
The main differences are that the "SUBMARINER" uses a triode type 27 and has regeneration control, while the KA-116 uses a pentode with no regeneration control.





The KA-116 with a salvadorean touch
The KA-116 replica was built very close to the original, from parts found in my junk box, a hand made coil, and a hand made power transformer.
The only addition is a voltage divider. This was needed because the DC voltage provided by the power supply was 320 volts DC under very light load.
The voltage divider as well as the complete diagram including parts values and voltage readings are shown in the diagram.