Satelite TV/Radio

This question has bugged me for awhile and I cannot seem to find a good answer online, so im hoping someone here can enlighten me.

Why do I have to point my sat. tv dish to a particualr point in the sky, but my sat. radio reciver just sits on top of my car? If my tv dish is not pointed correctly I get nothing, but my car reciever gets a signal no matter where im driving.

Both get signals from a satelite, so whats the difference here? Are FM and TV waves that different? Or is it something else?

It’s basically a matter of signal strength. Some satellite services are designed to send a signal to a fixed location (i.e. satellite TV sending to your house) and thus use a much weaker signal level requiring a very high gain antenna (and dish antennas have some of the highest gain figures of any antenna type) to receive the signal. Dishes achieve this high gain by being very directional, which means they need to be very accurately aimed.

Other satellite services, those designed with mobile users in mind, need to be received by devices with much smaller, usually nondirectional, antennas. GPS is a perfect example of this, as is satellite radio. The power output of the satellite radio signal is much higher than that of the satellite TV signal, allowing it to be received with a much more modest antenna.

There’s a lot more technical detail here, but that’s basically what’s going on.

Thanks pestie. But of course that leads to another question…why cant the Sat TV people use a stronger signal to allow more people to use dish. Rather than limit to people with SW sky exposure??

That may be true for GPS, but I don’n know about radio. I think that the antenna tracks the satellite(s) to ensure that it is always getting a signal. At least this is how mobile satellite TV works.

Mobile satellite TV is an application that was never meant to be. That’s why those antennas are so hideously expensive - they’re complicated, and there’s a limited market for them. XM and Sirius satellite radio don’t require tracking antennas. I can’t find a cite for it right now, but XM’s antennas are made by Antenna Specialists and Terk, so if you want to check those out you might find more information on exactly what type of antennas they use.

XM, at least, uses two geostationary satellites (named “rock” and “roll”) and repeaters on the ground to relay the signal into areas where satellite coverage is insufficient. Without these terrestrial repeaters you’d tend to run into the same problem satellite TV does with a lack of a clear view of the southwest sky.

To answer your question regarding that, Dob, more power won’t help in the case of satellite TV. You still need a clear view of the satellite without those nifty terrestrial repeaters XM uses. You might get a better signal through some light tree cover or in heavy rain, but you’d still have problems if the signal had to travel through too much in the way of obstructions. Frequency has something to do with it, too - unfortunately, the frequencies allocated for satellite TV tend to be absorbed fairly well by water (trees, rain, etc.)

Hmm… I have a plug-n-play Sirius receiver, with separate power supplies and antennas for my home and car, and a single tuner that travels between them. The car antenna is nondirectional, but the home antenna tilts up and down (which noticably affects the reception). Why wouldn’t the home and car kits use the same kind of antennas?

Listening to XM right now. On the boom box and it doesn’t require a directional antenna. I don’t know why Sirius would do it that way.

Don’t forget that the amount of information being transmitted for radio (even with XM’s source data function) is much MUCH lower than for any form of television. That image requires and enormous amount of bandwidth.

I always assumed that was the reason for specific antenna locations for television and general antenna for radio.

Nah. Higher bandwidth doesn’t necessarily produce a need for higher gain. It affects antenna design in other ways but it’s not like satellite TV needs a dish because the data rate’s so much faster.