I was wondering: If anyone else bothers to look at telecommunications towers, maybe they might have wondered the following: (My question assumes the specific dishes I see mounted on these towers are for “bouncing a signal off of.”)
A) Does the outgoing signal from a concave dish become inverted in comparison to the incoming signal? You know, like a parabolic mirror (or your teaspoon) will invert the image IF the original source is outside the focal length?
B) Does a convex dish create an outgoing signal with a smaller amplitude than the original signal? Again, I am thinking about how images appear when reflected off a convex mirror. Also, I don’t quite understand how a convex surface can be used to bounce a signal off of…being that convex surfaces create virtual images. Or, doesn’t that matter? (I was taught you cannot project a virtual image, but this may not be correct.)
Are you sure you are looking at a dish and not the weather cover over the end of a feed horn? Do these dishes have a feed point mounted out in front? Any photographic examples?
The signal doesn’t "bounce off’ the same way it would from a mirror. The radio signal is received by one dish (think of your home satellite receiver dish), then amplifed and re-broadcast by a separate dish.
Gary, I’ll see if I can find a link to some photos. In the meantime, what is the “feed horn”? Would this be a post or arm (for lack of the proper term) positioning a receiver (or transmitter) at the focus of the dish? Regardless, the dishes I see mounted on these tower structures have nothing at the focus. Thus, I assume it is for the sole purpose of transmitting a signal over a great distance by bouncing off these dishes (like a mirror)… otherwise, the curve of the earth would greatly limit the distance of a transmission.
The curve of the earth is the precise reason that these towers are needed. They are known as “repeaters.” Say you want to send a signal from city A to city B, but you can’t send it directly because you don’t have line-of-sight between the two. (Remember that most of these installations were put up in pre-satellite days). You need to set up one or more repeaters between A and B to relay the signal. The transmitter in city A sends the signal to tower 1, where it is amplified and beamed to tower 2. Tower 2 receives the signal from tower 1 and beams it to city B. Each hop is determined by line of sight and the curvature of the earth.
What you’re most likely seeing are (what I call) terrestrial links. They’re microwave communication links using line-of-site antennas.
Most microwave antennas consist of a reflector, feed horn, and amplifier. When an electromagnetic wave reaches the antenna, it first “sees” the reflector and reflects off of it much the same way as light reflects off a mirror. (at least it’s a good approximation of what actually happens.) The reflector is parabolic, and thus focuses the reflected wave toward a focal point located in front of the reflector. This is where the feed horn is located. The feed horn collects the wave energy and funnels it into a waveguide. But not for long… as soon as possible the wave is amplified using a low noise amplifier (LNA) or low noise block (“LNB,” which is an LNA with built in block down converter.)
The protective “weather cover” GaryM referred to is called a radome.
You asked about “signal inversion.” This is rarely if ever an issue, as it’s never an issue when a TV or radio signal gets bounced around.
There are a ton of things I’ve left out. I used to work at COMSAT, so email me if you want to learn more about this stuff.
There isn’t any image involved. Microwave signals carry information based on their frequency, phase, or amplitude changing over time (really, really fast), not on one part of the beam being different from another. I suppose there could be polarization modulation, but that’s still not an image like you’d receive with a telescope.
HEY! A topic that I deal with every single day - man, now I feel like maybe I can actually contribute something real to this board.
Here’s a picture of a high performance microwave antenna with a flat fabric radome.
This shows what the parabolic antennas look like without the radomes.
and this one shows the typical plastic dome radome (ignore the wierd shielding doohickies around the perimeter of the antenna)
All microwave antennas are polarized based on the orientation and type of the feedhorn. In my trade, we make use of antennas in the 2, 2.5, 6.5, 7, 13, 18 and 23 GHz bands (among others.) Most of the time, we use linear polarization - horizontal or vertical. For some applications, we use circular polarization - right hand or left hand circular (a.k.a. CW or CCW)
The signal polarization doesn’t change from transmit antenna to recieve antenna unless the beam impinges upon a reflector of some kind. In fact, in some fixed point-to-point microwave paths, we actually make use of passive reflectors to “bend” the beam instead of installing expensive electronics - in these cases, the signal really is being “bounced” - but most of the time, the dishes receive or transmit (or both) directly - no bouncing involved. A vertically polarized beam will become horizontally polarized if it hits the reflector at precisely a right angle. It almost never does, however, and this is why the feed horn assemblies on microwave antennas can be rotated at installation.
The antennas are simple parabolas. The feed horn is located at the focus of the reflector. Transmitted signals are emitted from the feedhorn, hit the reflector and are collimnated into a fairly tight beam - 0.5 to 1 degree in width for most of the antennas I use. Receive signals are focused on to the feedhorn assembly by the reflector and are then fed down the waveguide or feedline to Low Noise Amplifiers (which can be located at the antenna or in the receive electronics down at the bottom of the tower - just depends on the design)
