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Nakajima J1N-S, Japan fighter aircraft

Picture source.

As per my understanding, a Yagi antenna is a directional antenna with limited gain, which it broadcasts or receives the microwave signal to and from the front side. It can only receive signal if the transmitting antenna is not exactly in front of it, or can receive with limited/very limited quality, depending on the direction. Parabolic, however, has better reception than Yagi.

My question is, why did this old Japan Nakajima J1N airplane use a Yagi rather than a parabolic antenna?

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  • $\begingroup$ Wouldn't parabolic antenna be impractically huge? They worked with relatively low frequencies back then. $\endgroup$
    – Jan Hudec
    Sep 21, 2019 at 20:49
  • $\begingroup$ But normally used by airplane, especially civil. I am not sure for fighter jet. The parabolic antenna is located in the nose. $\endgroup$
    – AirCraft Lover
    Sep 22, 2019 at 2:16
  • $\begingroup$ Note that there is a relationship between radio-wavelength and antenna-dimensions. I'am not an RF engineer, but I guess the wavelength used in the J1N's radar doesn't allow for parabolic antennas. Checkout the Wikipedia article on the german Berlin radar set of WW2. While the predecessor, operated with wavelengths in the tens of centimeters, the Berlin set operated with 9 cm waves. The article notes, that this allowed to shrink the antenna by a factor of 7 in size, thus allowing to place a parabolic antenna behind an aerodynamic fairing. $\endgroup$
    – Dohn Joe
    Sep 22, 2019 at 9:40

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Note that the assertion "It can only receive signal if the transmitting antenna is not exactly in front of it" is wrong. The most sensitive direction for receiving is in fact exactly in front of the antenna, so if the incoming beam is directly in line with the yagi shafts, the maximum signal strength will be received.

A parabolic antenna would be bigger across than the width of the plane's fuselage, causing a huge amount of (unnecessary) drag.

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First, both parabolic & Yagi antennas are "directional" in that their main lobe is designed to have high gain in a limited direction. this is because all antennas have to trade the width of their main beam(s) with the amount of gain in that beam. you can either put a little power everywhere or a lot of power in one direction.

that being said the picture you show is also from wikipedia. From that Yagi antenna article

The Yagi was first widely used during World War II for airborne radar sets, because of its simplicity and directionality

the subtext on the photo reads:

A Nakajima J1N1-S night fighter with quadruple Yagi radar transceiver antennas

Reading through the J1N article it appears these were used for night surveillance and intercept. Much like modern fighters with radars antennas in the front, the likely use was for tracking or targeting targets - probably B29's. With the Yagi (or any highly directive design) the radar can 'see farther' since it has high gain in the forward direction.

With 4 antennas with their beams pointed in slightly different directions their coverage would have been a bit wider (both in azimuth and elevation) for searching.

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  • $\begingroup$ It appears these were used for night surveillance and intercept. Interesting. $\endgroup$
    – AirCraft Lover
    Sep 22, 2019 at 2:15

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