Comment by ltbarcly3 | Hacker News Reader

Comment by ltbarcly3 | Hacker News Reader

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So... Microwave and VHF are not vastly different frequencies, Microwave starts at 300MHz and VHF ends at ... 300MHz.

Yes, they each span a wide range of frequencies, but every frequency range spans a wide range of frequencies. These two are as close as you can be.

https://en.wikipedia.org/wiki/Microwave

https://en.wikipedia.org/wiki/Very_high_frequency

Not coincidently, VHF Radar is defined to be between 50 and 330Mhz, which actually overlaps both Microwave and VHF television.

Now as to the main question, if I set up a device to interact with radar at some frequency and attach it to a stealth airplane, what frequencies would you expect it to effectively interact with it (assuming it's tuned at all, they could just be attaching a big piece of flat aluminum)?

Answer: If my device has peak interaction at frequency F, you will see harmonics, so you will see significant interactions at 1/8 F, 1/4 F, 1/2 F, F, 2 F, 4F, 8F.

Airport radar is around 3Ghz. So if the radar reflector operates at around 3Ghz, it will also work pretty well at 1.5Ghz, 750Mhz, 375Mhz, and even 187.5 Mhz. 187.5Mhz is TV channel 6 or 7, depending on country. Half again is 93.7 is FM (which the article specifically mentions).

A half wavelength at 3 GHz is 50 mm. A half wavelength of 93.7 MHz is 1600 mm. The retroreflector here is not being used as a retroreflector for passive radar as the transmitter and receiver are not in the same spot. Why would a small chunk of stuff stuck to a bigger chunk of stuff somehow disturb the high power VHF signals used more than the big chunk of stuff it is attached to?

I guess the glib answer is that the big thing is specifically designed to avoid disrupting those signals, and the small thing is designed to disrupt them as much as possible.

A tiny radar reflector has the cross section of a huge object, because it intentionally has right angles that send signals back to their source without scattering them in other directions (as much as is possible).

Passive radar still requires the object to reflect signals, it just doesn't require the signals to be reflected back to the source of the signal. Consider a monitor with an anti-reflective coating (so lets take a non glossy monitor). It is effective at reducing the reflection of the window behind you, it uses a thin film to cause destructive interference so that the light coming in the window not reflected, and you see a greenish-purplish image of the window instead of getting a bright reflection. However, if you take a flashhlight and shine it down on the monitor from near the top of the monitor, you will easily see the glare from the flashlight as it illuminates the monitor.

I think this is a good analogy for active vs passive radar. The F-117 was built to avoid right angles, this is like the light from the window behind you. The F-117 won't send radar back in the same direction it came from, so active radars are thwarted. However, if it is being 'lit up' from somewhere else, it is not as effective since by not sending radar back to where it came from it sends it in other directions instead.

However, it also has a coating to just straight up absorb radar. The F-35 is much more reliant on such coatings, and so it should scatter very little EM radiation in those frequencies, regardless of whether the detector is in the same location as the transmitter.

I thought radar absorbing paint didn't work much below 1GHz. So it probably doesn't really help against passive radar.

Harmonics occur at integer multiples of the fundamental. If a body resonates at F, you will see harmonics at F, 2F, 3F...

You will not see harmonics at fractional (1/2 F, etc...) frequencies of the fundamental, though you will likely still see some backscatter.

Retroreflectors[0], to include the Luneburg lenses[1] mentioned in the article, function by directing radar energy parallel to its incidence direction. These reflectors will not significantly increase the plane's RCS for passive/bistatic receivers. Even if these reflectors did increase RCS omni-directionally, it is unlikely that a reflector designed to operate at 3 Ghz (10 cm) would also be designed to operate at 187.5 MHz (1.6 meters).

[0]https://en.wikipedia.org/wiki/Retroreflector [1]https://en.wikipedia.org/wiki/Luneburg_lens#Radar_reflector

More importantly, how does the intensity go with the order of the harmonics?

I don't know enough about the reflector. If it works like a boat's radar reflector, an 20 cm reflector would have a equivalent cross section as a metal sphere with an area of about 30 square meters.

I mean the goal of a radar reflector is to send radar signals back to their source, they are super effective. They don't really work like antennas generally, but the pictures of the F-35's reflectors do not look typical, or like a boat's radar reflector, and I have no idea what principle they use.

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