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Farnsworth-PT: central claims

The project folder contains one document: US Patent 2,172,152, "Radio Frequency Multipactor Amplifier" (Philo T. Farnsworth, granted 5 Sept 1939). Its text layer was digested and then deep-read on the operative pages (2-3), with page-image checking of p.3 (OCR quality otherwise noisy but legible), so there is no source gap for this document. Note: the digest's p.3 line misattributes the one-to-four ratio to secondaries; the deep read shows it refers to electrons arriving at the screen from the helix. No other Farnsworth works (fusor papers, television patents, notes) are in the folder; any review of Farnsworth's wider ideas would rest on [memory] alone.

  1. A vacuum tube with an electron-permeable, secondarily emissive screen cathode between a helical accelerating anode and a collecting anode can multiply an electron current when driven by an RF carrier.
  2. The repeated-trip (multipactor) action, in which electrons emitted at the right phase are returned through the helix to strike the screen again, gives a greatly amplified output current.
  3. The output current is linearly proportional to the electron supply at the start of each cycle, so a signal on the electron-gun control electrode modulates the carrier in the same tube that amplifies it.
  4. Above about 10 MHz a starting source of electrons (light on a photosensitive screen, or the electron gun, which the patent prefers for its modulation advantages; stray ionised particles are also listed as a possible start) is needed for multiplication to build up in time, and photo-initiated output is linear in light intensity up to space-charge saturation.
  5. The design needs no focusing fields; the patent presents it as a simple method of combined amplification and modulation. (The patent gives no measured gain, bandwidth or noise figures, so these are untested assertions in the document itself.)

Detail for claim 2: about one in four of the electrons arriving at the screen pass through its holes (apertured-to-total area ratio); the rest strike it and release secondaries, whose fate depends on the RF phase at launch. The patent gives no gain, voltage or dimension figures.