Can a regenerative shortwave receiver (especially an old vintage unshielded tube or transistor model) connected to a wire antenna generate HF interference? If so, what are some ways to prevent one from generating such interference (but still receive)?

What are some inexpensive ways (if any) to check to make sure that any radiated power from an old receiver (designed well before Part 15) is below legal limits (in the U.S. on amateur HF and broadcast shortwave bands)?

  • 2
    $\begingroup$ As the regeneration feedback is increased, the receiver sensitivity increases, up to a point. When the feedback reaches unity it will break into oscillation and can produce interference. When in oscillation it is not receiving but oscillating, so you would know. A preamp before the regenerative detector would block feedback to the antenna. Another detector method is "super-regenerative receiver", where possible oscillations were quenched (interupted) this allowed for higher receiver sensitivity. $\endgroup$ Feb 22 '14 at 23:09
  • $\begingroup$ This comment would also make a good answer. $\endgroup$
    – hotpaw2
    Feb 22 '14 at 23:48
  • $\begingroup$ Research "foxhole radio", for an interesting historical perspective. $\endgroup$ Feb 23 '14 at 0:48
  • $\begingroup$ @PhilFrost Though very interesting, a foxhole radio is not a regenerative receiver. It lacks an amplifying element as well as a positive feedback circuit. $\endgroup$
    – on4aa
    May 21 '14 at 21:01
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    $\begingroup$ @hotpaw2 Because of its inherent interfering properties, the use of a regenerative receiver is prohibited in Belgium. $\endgroup$
    – on4aa
    May 21 '14 at 21:05

An HF regenerative receiver needs to be oscillating in order to receive morse code and single-sideband transmissions. Therefore it will radiate RF from the receiving antenna. This signal can be reduced if there is an RF amplifier before the detector stage. If you have another radio that can tune the same frequencies, then you should be able to hear the carrier wave emitted from an oscillating regenerative receiver. With a suitable portable radio, you might be able to check if the signal is travelling beyond your property.

Early valve receivers could radiate as much as one watt of power - whilst this may not sound a lot, many radio amateurs regularly converse around the world at that power using morse code and digital modes.

However, if the radio is just used for AM reception, then it should not be oscillating.


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