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I'm interested in characterizing the radio signatures of magnetrons that appear to be installed in most large aircraft, some examples appearing on this list:

http://www.mpdcomponents.com/pdfs/FAA-PMA.pdf

My understanding is that these magnetrons were the first form of radar, but other technologies have largely supplanted them. However, they still seem to appear in the parts lists of modern Boeing and Airbus jumbo jets.

My question is what are these magnetrons used for, and when they are used, how they are used? Do pilots still use these regularly?

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    $\begingroup$ Hm, neither of the aircraft models in the list you link to seem to be recent designs. It tops out at 747-200, 737-200, Learjet 36 and so forth. I didn't see one newer than 30 years; most are from the 1970s. $\endgroup$
    – hmakholm left over Monica
    Commented Sep 28, 2015 at 13:16
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    $\begingroup$ First thing that comes to my mind is for heating meals in the galleys. $\endgroup$
    – Rob Vermeulen
    Commented Sep 28, 2015 at 13:22
  • $\begingroup$ @HenningMakholm Thanks! Honestly the parts list I posted was just from some Google sleuthing, and I don't know if more modern craft have these installed. How many 1970s-era craft are still in operation? $\endgroup$
    – Guillochon
    Commented Sep 28, 2015 at 13:30
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    $\begingroup$ @RobVermeulen My understanding is that food is typically heated in convection ovens on aircraft, microwaves are specifically avoided because of the potential for RFI. $\endgroup$
    – Guillochon
    Commented Sep 28, 2015 at 13:33
  • $\begingroup$ Guillochon...the magnetrons I discussed below were installed in OLD radar systems. Even though the radar systems were upgraded numerous times, the magnetron was still a part of the receiver-transmitter installed on Boeing's KC-135 (delivery from Boeing ended in 1966). That particular radar system was removed and replaced with a solid state color weather radar system on the KC-135. The Search and Weather Radar I mentioned below is still installed on some C-130 aircraft. Many U.S. military aircraft manufactured in the 1960s are still in operation. Google C-135, C-130, and B-52 for specifics $\endgroup$
    – Justice Jack
    Commented Nov 17, 2015 at 23:45

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The magnetron is a high-powered vacuum tube, that works as self-excited microwave oscillator. They are used mainly in radar transmitters as either pulsed or cw oscillators at frequencies. In (civil) aircraft, they are mostly used in weather radars.

They have relatively simple construction but have been replaced in almost all aircraft due to their disadvantages:

  • They can usually work only on a constructively fixed frequency.
  • They have poor reliability.
  • They have high power consumption and have noise issues.
  • They have finite life, considerably lesser than solid state devices.

Most of the aircraft operating today use solid state devices in their Radars. It may be still used in some older aircraft and helicopters, but they are becoming increasingly rare.

Weather radars are regularly used by pilots to detect the weather conditions in front of them. According to Airbus Flight Operations Briefing Notes: Optimum Use of the Weather Radar The weather radar only detects:

  • Rainfall
  • Wet hail and wet turbulence
  • Ice crystals, dry hail and dry snow. However, these three elements give small reflections compared to water droplets.

It does not detect clouds, fog or clear air turbulence.

Most of the aircraft in the list were designed 25+ years ago and have (mostly) switched over to solid state devices. Aircrafts are rarely used after 30 years as maintenance becomes expensive.

However magnetrons are still used in some aircraft due to their simple construction and high power output. For example, the WU-880 Radar Receiver Transmitter Antenna in Honeywell Primus 880 radar used in Falcon 2000EX uses magnetron.

Note: Almost all commercial aircraft use convection ovens to heat food (they won't have aluminum foils if they microwave ovens were being used); however, microwave ovens are available for aircraft and are used in some executive aircraft.

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  • $\begingroup$ Thanks very much for the thorough answer. Are these magnetrons continuously powered, or are they only switched on in the presence of inclement weather? Do you have any idea what the magnetron signal would look like to a receiver on the ground? It would be great to contact you directly if you have expertise on these devices. $\endgroup$
    – Guillochon
    Commented Sep 28, 2015 at 20:16
  • $\begingroup$ Didn't transponders originally use magnetrons? $\endgroup$
    – rbp
    Commented Sep 29, 2015 at 2:37
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    $\begingroup$ @Guillochon The magnetron (weather radar) is usually used according to requirement. It is rarely powered on continuously. A magnetron (usually) produces radio waves. To a receiver in ground, the signal would be similar to any other radio source, for. e.g. a radar, though the power would be very less. $\endgroup$
    – aeroalias
    Commented Oct 1, 2015 at 11:21
  • $\begingroup$ @rbp Yes. They did use magnetrons initially, though this has reduced since. $\endgroup$
    – aeroalias
    Commented Oct 1, 2015 at 11:22
  • $\begingroup$ @aeroalias Thanks again for the useful info! $\endgroup$
    – Guillochon
    Commented Oct 1, 2015 at 12:48
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Aeroalias provided a great answer! Additionally, don't think of a magnetron as a component installed on an aircraft, think of it as a component installed within a black box (in this case, a transmitter, that's part of a system installed on an aircraft).

There are multiple magnetrons listed within the link you provided above. The first part number, 257-0261-020, is the original part number installed within a Collins black box. The next two paragraphs are a cut and paste from the MPD Components, Inc.'s Products website (http://www.mpdcomponents.com/products.htm):

MPD Components, Inc. is a diverse manufacturer capable of providing total solutions from ceramic insulators to complete RF assemblies. Our background and expertise is in applied electronics, microwave RF, Vacuum Electron Devices (VED's) and ceramic-to-metal sealing.

Magnetrons (VED's)--MPD offers positive and negative pulse magnetrons. Our magnetrons are for pulse applications with the majority being used for airborne weather radar and beacon magnetrons. Typical peak powers run from a minimum of 1.3 kW to 10 kW for Honeywell, Sperry, RCA, Bendix, King, AlliedSignal and Narco radars. Several magnetrons have FAA-PMA applications.

You later asked if the transmitter is continuously powered and the technical answer is, "yes...as long as the system is turned on." That might seem like a cop-out answer, but it's true. I "think" you meant to ask, "Does the magnetron continuously transmit?" Without knowing the specific system the magnetron is installed, I don't think anyone can definitively answer how long it transmits, but it wouldn't be continuous or it would overheat. For example, let's say this magnetron is installed in an aircraft radar system--the duty cycle (amount of transmission versus the amount of non-transmission) is going to vary based on what the radar system is designed to display (mode and range). When the system is turned on and placed in a transmitting mode (not "standby"), the system will transmit a pulse, long enough to send the pulse. Then the transmitter will shut down and the receiver circuits will open. This prevents the receiver from taking in a high-powered signal (a received echo is very small). For a pulse radar system, how many times the system transmits is called the "PRF" (Pulse Repetition Frequency or Pulse Recurring Frequency)--measured in pulses per second--and could be hundreds or thousands of times per second (depending again on the purpose of the particular radar system.

One last thing, somebody asked if transponders use magnetrons. I think the answer is "No", but there could be a transponder system that used them. I believe most used a klystron. Regardless of what generates the power of the transmission, what I wanted to say is that a transponder does not transmit the way I described above. For example, an IFF transponder could be set to "normal" mode, operate correctly, and not transmit one time during flight. How? Because a transponder doesn't initiate the process like a radar system. With transponders, an interrogator (airborne or ground system) initiates the process with a "Who are you?" type of signal. "If" the airborne system receives a valid interrogation, the system will transmit a "This is who I am!" type of reply...otherwise, the system will stay in receive mode until it receives a valid interrogation. If an air traffic control radar system or an airborne system believes they're properly interrogating, but there's no reply, they can use a communication system to verbally request the pilot to "identify"...and the pilot flip a switch, forcing the system to electronically transmit. Again, what I described is "normal" IFF operation. In an emergency (fire, hijacking, etc.), a pilot can select an "emergency" mode and the system will automatically start transmitting (squawking) the emergency code.

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  • $\begingroup$ Guillochon, I'm new here, but I can already see this web site is going to be a true test of my ADHD! I went to the help link because somebody provided an answer and I was wondering if it's possible to send a private message or if a general chat is allowed within a question. In any case, magnetrons (I don't know specific part numbers) were components in an RT-289 (numerous versions) as part of the Search and Weather Radar System (AN/APN-59) installed in C-135 and C-130 (possibly other aircraft as well). The KC-135 has since upgraded their system, but some C-130s still use this system. $\endgroup$
    – Justice Jack
    Commented Nov 17, 2015 at 22:14
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Magnetrons are still used in most All general aviation radars. Echoes are returned from targets that are wet. Ice returns only one fifth of water droplets of a similar size. air carrier aircraft use solid state radars because they are capable of coherence. These coherent radars are capable of predictive wind shear capability, which is why airlines no longer use magnetrons.

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