Farnsworth-PT · Source catalogue

A 4-page scan of the patent with an OCR text layer (noisy: the drawing-sheet header misreads the filing year as 1958 and the inventor name is garbled, but the specification and claims are readable). The patent describes a vacuum tube that amplifies a radio-frequency (RF) carrier by electron multiplication using secondary emission. An electron gun (heater cathode with concave emitting face, ring-shaped control electrode, gun-type anode) fires a beam into a multiplying chamber. The chamber has a cylindrical collecting anode, a concentric electron-permeable cathode screen of fine silver wire gauze (open area about one quarter of the total), sensitised for photoemission and high secondary emission (oxidised silver, or oxidised caesium coating), and a helical accelerating anode inside the screen. The carrier is applied between the helical anode and the screen cathode through a tuned input circuit, so the potential between them alternates about a steady d.c. bias. Electrons passing through the helix strike the screen and release secondaries. Of the electrons arriving at the screen from the helix, about one in four (the apertured-to-total area ratio) pass straight through the holes to the collecting anode and the rest strike the gauze and release secondaries; those secondaries, depending on the RF phase at the moment of emission (collector wins when the helix is least positive; some are pulled back through the helix when it is most positive), are either collected or pulled back through the helix and strike the screen again, repeating the multiplication. This back-and-forth trip is the 'multipactor' action (resonant electron multiplication in an alternating field). The output is taken from a tuned coil in the collecting-anode circuit. The output current is stated to be linearly proportional to the electron supply at the start of each cycle, so a signal applied to the gun's control electrode modulates the carrier, and the tube both modulates and amplifies. Above about ten megacycles the patent says a starting electron supply is needed for multiplication to build up in the time available; light on the photosensitive screen (photoemission, linear in illumination up to space-charge saturation) or the electron gun can provide it. No performance numbers (gain, bandwidth, noise, power) are given and no experimental data are presented. The patent refers to two earlier Farnsworth patents on electron multipliers, No. 2,071,515 (Electron multiplying device) and No. 2,071,516 (Oscillation generator). Six claims; claims 1-4 cover the tube structure (gun, control electrode, cylindrical collecting anode, permeable secondarily-emissive cathode, permeable accelerating anode, optionally photosensitive and helical); claims 5-6 add the external circuit (input impedance with blocking condenser, d.c. sources, output impedance). Only a.c. multipliers are addressed; d.c. multipliers are said to be covered elsewhere. Starting mechanisms listed among the objects: ionised particles already in the tube, photoemission, or an electron gun (p.2). The patent states it preferably uses the electron gun 'since it possesses advantages in regard to modulation' (p.3); the light source and optics are not shown, and all six claims require the gun and control electrode (photosensitivity appears only in claims 2 and 4). The only numbers in the specification are the 1:4 aperture ratio and 'above ten megacycles'; no voltages, dimensions, gain or test data are given. The patent also mentions other applications in prosecution, and was assigned by mesne assignments to Farnsworth Television & Radio Corporation, Dover, Del. 'Space change saturation' in the text is presumably 'space charge' (the image reads 'change').

Key claims

• An electron-permeable secondarily emissive cathode screen placed between a helical accelerating anode and a cylindrical collecting anode, driven by an RF potential, multiplies an electron current by repeated secondary-emission impacts.
• The tube output current is linearly proportional to the number of electrons supplied at the start of each RF cycle, so a signal on the electron-gun control electrode modulates the amplified carrier.
• Above about 10 MHz an extra electron source (light on a photosensitive cathode, or preferably the electron gun, which the patent says has advantages for modulation) is required for multiplication to reach useful current in the time available.
• Output current from photo-initiated multiplication is a linear function of illumination intensity up to space-charge saturation.
• With suitable potentials, electrons make repeated trips into and out of the space bounded by the collecting anode, giving greatly amplified output current in a single tube that also modulates the carrier.
• About one fourth of the electrons arriving at the screen from the helix pass through its apertures to the collector, equal to the apertured-to-total area ratio; the rest strike the gauze and release secondaries.

Relevance: This is the only document in the project folder and the primary source for Farnsworth's RF multipactor amplifier: a conventional vacuum-tube device based on secondary-emission electron multiplication. It is not a fusion or free-energy document.

Mainstream conflict: none. Secondary emission and multipactor discharge are established physics, with multipactor resonance in RF fields well documented. The patent makes no extraordinary energy claims; the linear-response and gain statements are plausible but unquantified.