This is a brief note to document some experiments done today with a regenerative preselector.
A regenerative preselector is a regenerative amplifier connected to an antenna, where the regeneratively-amplified RF output is then fed into the rest of the receiver chain for detection.
In my experiment, I wanted to see if a short wire antenna, amplified by a varactor-tuned regenerative preselector, would be able to give a strong and clear signal. The result is that it was not able to do so.
The experimental setup was to take my latest varactor-tuned regenerative receiver (documented elsewhere on this blog at http://qrp-gaijin.blogspot.jp/2015/08/a-12-volt-vackar-style-minimalist.html, though any regenerative receiver will do), and to connect a short wire antenna of about 20 cm length to the hot end of the tank. Incoming RF from the short wire antenna would therefore be fed into the varactor-tuned tank and be regeneratively amplified by the regenerative stage.
The amplified RF was fed into a separate, portable commercial shortwave receiver with a high-impedance whip antenna. The whip antenna was completely retracted and folded downwards to ensure minimum signal could enter the receiver's original whip antenna. Then, the short wire antenna from the regenerative amplifier was simply laid on top of the commercial receiver's retracted whip antenna. The high impedance of the commercial receiver's whip antenna, combined with the high-impedance of the regeneratively-amplified short wire antenna, allowed for capacitive signal transfer between the regenerative amplifier and the commercial receiver.
The commercial receiver and the regenerative amplifier were tuned to the same frequency, somewhere within the 41-meter band. Tests were done at night (when 41-m propagation should be good) and inside of a concrete building (which greatly attenuates signal levels).
Regeneration for the regenerative amplifier was adjusted just beneath the oscillation threshold. In this condition, it was possible to weakly discern an AM signal in the 41-meter band. Signal-to-noise ratio was poor.
Then as a comparison, I turned off the regenerative amplifier and disconnected the short wire antenna from the regenerative amplifier. Then, I connected the same short wire antenna directly to the retracted whip antenna of the commercial receiver. The wire antenna was positioned identically as in the previous case. In this condition, reception of the same AM signal in the 41-meter band, while still weak, was noticeably clearer, with a much-improved signal-to-noise ratio.
My conclusion is that my particular regenerative amplifier (used as a regenerative preselector) introduces more noise into the signal chain than does the front-end RF amplifier of the commercial receiver. My particular regenerative amplifier has 3 factors that may contribute to its noisiness: its use of a low-Q varactor 1SV149 for tuning at HF, its use of a garden-variety BJT that is coupled strongly into the LC tank (which exposes the tank to varying parasitic capacitances of the BJT), and its low-voltage biasing technique where base and collector are both tied to Vcc (which may make the parasitic capacitances larger than they otherwise would be with a normal biasing technique where collector voltage higher is held higher than base voltage).
This experiment was done within the context of deciding on the front-end design for my shortwave superhet. I have now decided against using a regenerative preselector. However, it may be possible to build a quieter regenerative preselector with more attention to the above-mentioned factors.