Does a longer wavelength require a larger antenna for emission or reception or both?
The wavelength is tied to the antenna size. A shorter wavelength (higher frequency) allows you to use a smaller antenna and still get the same amount of gain out of it. Using a longer wavelength with the same size antenna will result in less gain, meaning that the antenna won’t be as sensitive to the signals.
For things like airborne radar, you end up with a whole bunch of things that you need to factor in, and antenna size is only one of them. Other important factors are things like the absorption of your desired frequency by water vapor in the air. Longer wavelengths tend to propagate better through the atmosphere. Shorter wavelengths are also scattered more easily, meaning that longer wavelengths can often detect objects that shorter wavelengths will sometimes scatter off of.
In other words, it’s all very complex.
**Radar uses specular reflection. Why not also diffuse reflection? **
You get the strongest return from the specular reflections. Diffuse reflections by their nature involve scattering and radiating the energy level out in various directions.
A good clear specular reflection also allows you to measure doppler shift to determine speed as well as location.
How come stealthy aircraft like the F-35 still have round surfaces underneath? I understand that round shapes are generally avoided because they contain so many angles that at least one of them is bound to be just right for a specular reflection back at the emitting radar.
The F-117 used very angular surfaces, which are great for channeling radar returns along certain paths so that you hopefully steer them away from the targeting antenna. Angular surfaces aren’t so hot for aerodynamics, though. The F-117’s nickname among its pilots is the “wobbly goblin” which should give you some indication of how well it flies.
The F-117, despite its F designation, is more of a light ground attack aircraft than a real fighter. The F-35 on the other hand is a fighter, and must handle and perform like a fighter.
Between the development of the F-117 and the B-2, a lot of the math was worked out so that signals could be reflected and channeled away as desired but you could still have an aircraft with less angular and more aerodynamic surfaces. This has allowed folks to build stealth aircraft that don’t tend to have the word “wobbly” in their nicknames.
Ironically, one of the things that really helped this effort was some Russian publications that were in the public domain.
How effective are differently-placed emitters and receivers at countering stealth aircraft and stealth ships?
I suspect that only people with fairly high security clearances can answer this one. They don’t tend to release data like this, for fairly obvious reasons.
How does the limited range of a radar manifest itself? What happens outside it range that doesn’t happen outside of it and vice versa?
Is it like low light in a camera? When light is insufficient in a camera, there’s things you don’t see or see in low detail. To remedy that, you can crank up the sensitivity but that can get you false positives in the form of noise.
Radar doesn’t work like a camera. It’s more like marco polo. You yell marco and wait for the target to polo back. What happens when the return signals get weak is that they get lost in the background clutter. Simple radars like weather radars will often show all kinds of clutter. Sometimes what you are looking at on the map isn’t weather near the antenna, it’s ground clutter.
Aircraft radar is often more sophisticated and uses all kinds of filtering techniques to differentiate between clutter and real returns.
When using low frequency waves at ranges of 100-200km, you get low-quality information, correct? How does that manifest itself? Do you get a very wide range of possible true values for size, location and speed of objects?
Long distance radars like that tend to be lower resolution due to the wavelength. You can’t really determine size from a radar signal like that. All you can go by is the strength of the return. Generally speaking, larger aircraft return larger signals than smaller ones. The F-117 stealth fighter, on the other hand, returns a signal roughly equivalent to that of a bird.
How can Home-On-Jammer guidance be countered? From what I understand, some missiles will use normal radar illumination but when they get jammed, they switch to HOJ guidance. What can the target do then?
I have no idea what current technologies and strategies are used for electronic countermeasures these days. Googling the phrase “electronic countermeasures” might give you some insights.