h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Hello all

I currently have an entry on Satellite & Digital Radio in North America sat in PR ( F48874?thread=3964551 ), and a question has been raised regarding the satellite radio antennas.

The home antenna is a fairly chunky bit of kit, and (according to the blurb on the satellite radio operator's websites) needs to be positioned accurately to get line-of-sight to the appropriate satellite to ensure a good signal.

The car antenna, on the other hand, is obviously a lot smaller and usually mounted in a pod on the vehicle roof - e.g. the "shark's fin" on the roof of a BMW or Volvo.

I can find some detail via Google regarding the technology used in the car antenna, but very little about the home reciever - and I don't want the entry to descend into huge reams of highly technical detail. I know antenna design is a highly specialised area, and I don't have that depth of knowledge.

My question is, why do you need to position your home antenna correctly, but your car antenna can sit happily on your car roof while you're driving around? What technology enables this?

All help welcome!

I'd have to consult some old books for a definitive answer, but first thoughts are:

Satellite broadcasting frequencies are usually quite high, probably microwave, which are very directional. The most efficient antenna is comparable in size to the wavelength of the signal, and microwaves are small enough for a parabolic dish to be used. As the satellite is effectively fixed, the dish can be pointed straight at the source. This maximises the received power giving a good signal.

Shipboard satellite communications antennas are often stabilised so that they always point at the satellite as the ship moves, turns, pitches and rolls. This needs very precise gyro compasses and gimbals that would be far too bulky and expensive for a car.

My guess is the antenna on the car is omnidirectional, like a GPS antenna, and is therefore just a lot less efficient (has a much smaller gain) than the fixed dish. It would be similar to the TV aerials you get for caravans - the signal might be good enough in most circumstances, but not nearly as good as the standard unidirectional fixed ones.

As the signal is digital, you wouldn't notice any difference in quality but the mobile one would be far more vulnerable to drop-outs caused by weather, interference etc.

Traveller in Time on his head
"What I do not get is if and how they compensate for the delay between the direct received from satellite and received via ground based stations.

I know the mobile antenna can expect the satellite to be at a certain inclination. Pointing the receiving 'cone' up will reduce some noise. Still the signal would be very weak compare to fixed antennas.

Perhaps both questions are combined, if they send a minute worth of information in a second and repeat that every five seconds, chances are you can receive it flawless in a car. "

Satellite radio uses frequencies around 2.3GHz, which is a very narrow wave, allowing for very small, conical antennas to maximize your reception. But a quick look at home antennas suggests that they're not quite as focused as they would be for military satellite communications. Because the antenna has a wider angle of opening, you're going to be able to receive a signal with more tolerance for movement. But since your home is a fixed location, you have the luxury of fixing the antenna in a specific direction in order to make your signal a little cleaner.

That really pays off for the XM service, since they're using two satellites in geosynchronous orbit, so the strongest signal is always coming from a fixed point in space, in relation to the receiver. But that also means that any obstruction in a direct line-of-sight to that fixed location is going to ruin your reception.

The Sirius service solves that problem by using three satellites in a figure-8 orbit over the Western Hemisphere. That greatly increases the signal availability around obstructions, which is quite important when you're driving. During many periods of the day you'd actually be able to receive a signal from two of the three sats, with your tuner adjusting to the stronger of the two. The signal availability is further enhanced by terrestrial repeaters in densely-populated areas.

The biggest reason the cone doesn't have to be tightly-focused, though, is that the transmission is digital data. That means the output quality doesn't significantly change when you turn left. As long as you're picking up a signal of sufficient quality that your tuner can recognize the ones and zeroes, the sound quality will remain unchanged. You could, as long as you're not dropping packets (assuming it's packet based data... I'm not inclined to dig quite that deep just now), receive the same clarity at 3 bars as you would at 9.

all, that makes sense.

The vehicle antennas in particular have a dual-arm construction; one arm optimised for satellite transmissions, the other for the terrestrial repeaters. My guess is that the tuner would see which antenna was receiving the best signal at any given moment and use that for the signal.

I'll see how much of this I can put in the entry without making it -y.