1
$\begingroup$

B-2 Spirit stealth bomber

Parabolic Dish Antenna Receiver

Here is B-2 Spirit, a stealth bomber by Northrop Grumman. Another one is a depiction of a parabolic dish antenna receiver. As we knew, the B-2 is a stealth aircraft, which is not reflects the received electromagnetic (EM) wave signal (of course, there are several way to keep it stealth, such as reflect to another direction, absorbs the received signal, and so on). With the parabolic antenna, it is receives EM wave signal and reflects it to its dipole/monopole receiver on the focal point. Both the dish and its dipole/monopole receiver are made of, mostly, aluminum, a metal. Aircraft also, in another hand, also made of metal. As we know, the EM wave blocker also uses a-comb like metal bar put parallel to the EM wave polarization. The same thing also with Faraday's cage, it is using metal to block EM wave signal.

My question is, what is the explanation of such that parabolic dish antenna reflects EM wave signal while the dipole/monopole receiver receives, but at the same time required a so very high technology to make an aircraft a stealth (absorb the EM wave signal of the radar)?

$\endgroup$
1
  • $\begingroup$ The surfaces of stealth aircraft are coated with absorptive materials and possibly patterned underneath the coating. Faraday cages work by reflection, they don't absorb the electromagnetic signal, they just keep it out of the interior. That's easy. To make efficiently absorbing materials is hard. I have some practical experience with it because I tried to make my own ferrite materials for the suppression of EMI in very sensitive circuits a couple of times. I wasn't very successful. I got a couple dB better results than with off-the-shelf components but not 20dB as hoped. $\endgroup$ Apr 3, 2023 at 21:46

3 Answers 3

1
$\begingroup$

A metal parabolic dish deflects EM waves the same way as any other "stealthy" metal surface except for axial or paraxial waves that are mapped in to its focus, everything else is splattered to the side and the more off axis the ray bundle is, the wider the splatter; this is related to the coma error of the paraboloid. A B-2 airplane has a very small so-called monostatic radar cross section in the microwaves but that does not mean that its bi-static cross section is also small at an arbitrary angle. Also being stealthy is strongly frequency dependent, certainly the B-2 is quite visible, no pun intended, in the optical frequency range. I assure you that frequency/angle dependence is a highly kept secret.

A thin $\ll \lambda/20$ straight wire is easily excited in phase with a parallel E field and its reflectivity is very small for perpendicular polarized wave. If you have an array of such wires then they behave as a good reflector for parallel polarization but nearly transparent for an orthogonal one for waves coming perpendicular to the plane of the wires, but the transparency changes with the angle of incidence. The reason behind this is a discrete periodic "sampling" effect, very similar to the one allowing discrete samples of high enough rate to reconstruct a continuous function of finite bandwidth. The shallower the incidence angle is, the closer the wires will have to be packed to act as a reflector collectively.

$\endgroup$
6
  • $\begingroup$ As the dish of a parabolic antenna and the dipole/monopole element inside the horn are made of the same material, mostly aluminum, so why does the dish reflects the coming signal but the dipole/monopole receiver inside the horn doesn't not? $\endgroup$ Apr 5, 2023 at 9:03
  • $\begingroup$ The illuminator (horn) of the dish also reflects and in fact it can be a major source for "visibility" of an otherwise stealthy antenna but your question was about the dish so I did not want to complicate the issue. It is very unlikely that a stealthy plane would be using a dish antenna if for nothing else but because it takes up a quite large installation volume especially if it has to scan over an angle larger than a few beam widths. $\endgroup$
    – hyportnex
    Apr 5, 2023 at 9:33
  • $\begingroup$ This is in my post: With the parabolic antenna, it is receives EM wave signal and reflects it to its dipole/monopole receiver on the focal point. Both the dish and its dipole/monopole receiver are made of, mostly, aluminum, a metal. And my question was: what is the explanation of such that parabolic dish antenna reflects EM wave signal while the dipole/monopole receiver receives. Why the receiver element receive while both the dish and the receiver element made of the same material? $\endgroup$ Apr 5, 2023 at 20:35
  • $\begingroup$ The horn/receiver is in the focal point of the parabolic dish, all axial rays hitting the dish are reflected to the focal point. But not all axial rays hit the dish, some are scattered off the boom (rod, spoke) or spars holding the horn, some are scattered off the horn itself. To reduce visibility the holding jig can be made of much plastic and the outside of the horn be coated with absorber but some amount will always be scattered and reflected back. Dish antennas have many advantages but this parasitic scattering is one of the disadvantages. $\endgroup$
    – hyportnex
    Apr 5, 2023 at 21:36
  • $\begingroup$ Let me give another example, Yagi antenna. Yagi antenna consist of driven elements which the feed line connected, the reflectors, and the directors. The driven elements radiate radio wave parallel to the elements. The reflector then reflects the radio wave, and the directors improve the direction to improve gain. All the elements are made of the same material, mostly, aluminum. If the antenna is acts a Tx antenna, why the reflector doesn't just absorb for itself the hitting radio wave? So do the directors. But the fact, their existence improve gain as the direction is more focus. $\endgroup$ Apr 6, 2023 at 5:45
1
$\begingroup$

Here is an easier way to think of this.

Empty space has a characteristic impedance for EM radiation of 377 ohms. If an EM wave strikes an object with that same impedance, no EM radiation will be reflected back from it- all will be soaked up by the object.

So, to reduce the radar cross-section of a plane or ship, you coat it over with something that has an impedance of ~377 ohms, and design its shape so it will not tend to reflect EM radiation back to the source. Thus "buttered up", it becomes much harder to get a radar return from.

Note that in this case, the interior structure of the plane can still be aluminum, titanium or steel, because the skin of the plane has soaked up all the EM energy before it penetrates deep enough to bounce off the metal inside.

But if you omit the 377 ohm skin, the object becomes reflective again and can be seen with radar.

$\endgroup$
3
  • $\begingroup$ Just simple question: the dipole/monopole receiver of a parabolic antenna are made of aluminum, just like the Yagi-Uda antenna: the driven element, the reflector, and the directors are made of the same material, (mostly, commonly) aluminum. The exactly same design is used for both transmitter (Tx) and receiver (Rx). If the driven element can receive (not reflects) the coming signal, why does the reflector reflects and does not absorbs? $\endgroup$ Apr 5, 2023 at 8:56
  • $\begingroup$ When a signal strikes the driven element, part of the signal is absorbed and part of it is reflected off. The other elements in the yagi are there to "capture" as much of the re-radiated signal and urge it back towards the driven element. The principles of yagi operation are well-explained in the ARRL Antenna Book. $\endgroup$ Apr 5, 2023 at 18:25
  • $\begingroup$ After I studied EM more and more, the more I don't understand what I asked here. If we see mirror, the phase of the reflected EM wave is lag 180 degree from the incoming. Our image we see on the mirror is the generated EM by the incoming photon. With the same logic, then the driven element of the Yagi antenna and the driving element will be different 180 degree, do does to the reflector to the driving element is different 180 degree while the reflector to the driven element is different 360 degree or complete one phase. More complicated, not as easy as I guessed previously. $\endgroup$ Oct 2, 2023 at 17:31
0
$\begingroup$

So why does a dish reflect RF signals yet the dipole that the dish directs the signals to actually absorb the RF energy?

My understanding is the dish is not connected to a load per se so when the rf energy hits the dish the induced currents are not used to produce work. For that reason they are reflected or re-radiated.

Now the dipole on the other end is connected to a load. When the RF excites the metal elements of the dipole the energy is transferred into the coax or amplifier section and is used to eventually be demodulated at the radio output.

$\endgroup$
1
  • $\begingroup$ If it is the reason, then it is very easy to make stealth aircraft, right? Just connect all the part of the aircraft connected to ground so it will not reflect any single. As I am an Aircraft Lover, I understand a bit about how expensive is to make the stealth aircraft as required very expensive and extensive research. $\endgroup$ Oct 2, 2023 at 17:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.