With all satellite dishes that I’m aware of, big or small, you have to aim your dish at a certain satellite. With the big dishes you can aim at one satellite for some channels, or another for different channels, at least, that’s the way it use to be. With a small satellite, like the Dish Network dish I have, for example, the dish is aimed at one satellite for all of the channel selections.
Then you have the XM and Serius satellite radios. Obviously in order to use them in your car, the antenna has to be directionally independent or else you’d loose the signal very easily.
So, finally, my question. Seeing as how satellite radio doesn’t require a dish pointed at a satellite, is a directionally independent satellite TV system possible?
Since the signal comes from an essential point source, the satellite, it would be of no advantage, and a positive disadvantage, to use a non-directional receiving antenna.
The transmission from the satellite does cover a relatively wide area on the surface of the earth. The directionality is in the receiving antenna which has a narrow reception “beam” so as to exclude unwanted signals from directions other than the satellite and to provide as much capture area and thus gather as much signal power as possible. The capture area of a receiving antenna is the gain*square of the wavelength/4pi, with the gain being defined as the signal received by the antenna in use compared to the signal received by an omnidirectional receiver.
The power intensity of electromagnetic waves is given watts per unit area, such as watts/meter[sup]2[/sup]. In order to find out how much energy a particular antenna will abstract from a wave of say 10[sup]-7[/sup] w/m[sup]2[/sup], you need to have an equivalent area for the antenna.
An omnidirectional antenna that accepts equal energy from an EM wave irrespective of the direction of travel of the wave is taken as the reference with a gain of 1. Suppose we have an antenna that takes 100 times as much energy from the preferred direction as from any other. It has a gain of 100 and suppose it is receiving a wave of 3 cm, 0.03 meters, wavelength.
The capture area is then: 100*.03[sup]2[/sup]/43.14159 = 7.16310[sup]-3[/sup] m[sup]2[/sup].
Multiply this by the field strength: 10[sup]-7[/sup]7.16310[sup]-3[/sup] = 7.163*10[sup]-10[/sup] watts. For a 50 Ohm input this equals about 190 microvolts.
That’s probably not the case. The standard Dish Network install points at birds at 110 and 119 degrees west longitude, and some locals come in on 121 or 105. The dishes started out as 18" diameter, but now are much larger to receive all these signals at once.
Good point. What I was trying to get at is, my dish is in a fixed position, unlike the larger dishes of people I knew where they could aim thier dish at different satellites to get different channels.
That’s not the same thing. Most satellite TV dishes have a single LNB and receive signal from one satellite. Many satellite services also offer special programming, such as foreign language stations or HDTV broadcasts, which require a second LNB (or even a third), and sometimes a slightly larger dish. The dishes themselves are fixed, but each LNB is offset to receive the signal from a different satellite. That’s what Alvis is talking about here.
That’s always the case. An omnidirectional antenna will always (all else being equal) receive less signal than a directional one. This is one reason that a rooftop Yagi antenna is far superior for broadcast TV than a pair of settop “rabbit ear” antennas.
Incidentally, owing to the large amount of RF noise emitted by digital electronic systems, you’re better off mounting your rabbit ears as far from the TV and other sources of local RF noise as possible, and NOT on top of the set, especially with modern sets. DVD players, VCRs and other ancillary electronic devices also contribute to this local RFI.
A small nitpick to start off with. Truley omnidirectional antennas don’t really exist although a biconical antenna comes close. The volume of space from which the ordinary autombile radio antenna will accept signals resembles a donut with a very teeny little donut hole. No signal is generated in the antenna for EM waves that come to it at its end. Maximum signal is generated in the antenna for EM waves that arrive perpendicular to its length.
An ideal omnidirectional antenna is postulated when computing the antenna’s area for convenience. In practice what this means is that all real antennas that you can build have at least some gain, or preferred direction, when compared to the hypothetical omni.
An antenna that accepts signals from all directions is desirable for a radio but not necessary becauses radio stations that you might want to listen to are located all around you. If you only want to listen to one particular station then a directional antenna with its perfered direction aimed toward that station would be the best arrangement.
And that is exactly the case with satellite TV. There is only one station’s signal that you want to receive, that of the satellite. That station’s signal carries all of the TV channels and you want to receive that signal and no others. Or as little other signals as possible. Therefore the antenna is mounted in a reflector (dish) which functions exactly like the reflector in a telescope or the reflector in your automobile headlights. It focuses on the antenna nearly all of the received energy that arrives from a direction perpendicular to the plane of the reflector and virtually none of the energy from other directions.
Having gone back and more carefully read your exact question I think I can give a more direct answer. Yes, a directionally independent TV satellite system is possible. All that would be needed is to use a non directional antenna which will have a relatively low gain with respect to a dish and increase the transmitted power by an amount equal to the loss in antenna gain.
But why do that? A satellite radio receiver in an automobile has to use a low-gain antenna that has little directivity for a couple of reasons. The first is aesthetic. Who would use it in their slick, streamlined, new auto or pickup if it required a big antenna dish mounted on the roof? The second is practical. Not many could, or would, afford the tracking system required to keep a directional antenna pointed at the satellite through all of an auto’s turnings. The satellite TV, on the other hand, is intended for fixed, or at least stationary, sites and the highly directive antenna can be used thus saving the expense of a higher powered transmitter sent into orbit.
The explanation I’ve heard is that XM and Sirius can use nondirectional antennas because they broadcast from newfangled satellites that use a lot more power. Not sure how much I should believe it.
I’ve also noticed that the home antennas are directional. Mine’s on a windowsill, and nudging it up and down or rotating it just a little bit can have a huge effect on the reception.
This is why I was asking about TV dishes. A couple of years ago in the winter, my wife and I were having problems getting a lot of channels. At first we thought it was the weather interfering. After it went on far longer than it should, a then friend of ours checked out the dish. Turns out it got turned, only slightly, but that was enough to screw up our reception. So he turned it one way and the next until we yelled at him that we were getting a good signal.
Seeing how easy the connection between the dish and satellite could be broken if the dish isn’t aimed in just the right direction, and yet, apparently, satellite radios don’t suffer like this, I was wondering if TV dishes could be made so that you wouldn’t have to worry about the dish somehow getting moved and pointing the wrong way.
Well, satellite radio does suffer from the same problem - at home. Only the mobile antenna is omnidirectional.
Another factor might be that the car antenna is mounted on a metal car roof. The manual says it’s important to mount it a certain distance away from the edge, because the car roof acts as a larger antenna, but IIRC it implies that the roof is only used to pick up signals from the terrestrial repeaters. There are none in my area, but Sirius still works equally well no matter which direction I’m driving.
If you used a dish with less directivity, i.e. a smaller dish, it would have a wider angle through which it would receive nearly maximum signal. The problem is I think maybe those satellite dishes have a low noise amplifier mounted on them so as to make sure you get a good signal. This means the dish would no doubt have to be supplied by your TV provider. The satellite company’s aim is to put as low powered transmitter as possible into orbit, allowing for the fact that the satellite wanders slightly around some point in the sky. This means they have to use as high a gain (highly directional) antenna as they can get away with.
Just for kicks, I computed how much extra fuel it takes to put an extra kilogram of mass into geostationary orbit. And if you use a smaller dish with say 10 times less gain, the transmitter power has to be increased by a factor of 10 which adds weight to the satellite.
In order to put 1 kg of payload into such an orbit you have to use additional fuel of almost 2.7 kg. If my computation is correct, that is. So if you add a kg of payload you are really adding 3.7 kg to the liftoff weight. And of course since you have to carry added fuel you need a bigger fuel tank which weighs more and requires even more added fuel weight. That’s why they scimp on transmitter power and use high gain antennas.
English please 
