Why does satellite TV need a dish pointed at the satellite while satellite radio doesn't?

[SkinnyDude]

I was wondering, how come with Satellite TV, I have to have a dish pointed directly at the satellite while with Satellite Radio, I can just have an antenna pointed anywhere in the sky? Is it because satellite radio is at a lower frequency or do they just push out more power?

 

[phibonacci]

I am not an electrical engineer. This reply may be completely incorrect. Satellite TV uses radio waves referred to as Sky Waves. The waves bounce off the different layers of the Earth's Ionosphere. Ionosphere is a collection of gases 50 miles above the earth. Thus, the satellite dish must point at the location where the radio waves are bouncing off the ionosphere. The waves must hit the dish at the correct angle.

This reply may be incorrect. If it is someone please correct it. As for satelitte radio, I am unsure what type of technology this implements.

 

[FredGarvin]

What satellite radio are you referring to? If you are talking about a car mounted antenna that is different than a parabolic dish that the TV receivers use. Both types are line of sight. So your assumption that the radio antenna can be pointed anywhere is false. My boombox for my XM definitely has to be facing the south or I won't get a signal. In terms of the car mounted antenna for the radio, I know that the entire roof of one's car is turned into a large ground plane for the antenna which aids in reception. Also, satellite radio has repeater stations whereas TV does not to my knowledge.

http://en.wikipedia.org/wiki/Satellite_television
http://en.wikipedia.org/wiki/Satellite_radio

 

[SkinnyDude]

I'm asking about Sirrus and XM satellite radio vs. DirecTV and DishNet. I want to know why TV needs a parabolic dish whereas radio just needs to point in the general south direction. When I drive around in the car, the car can be in any direction and still get a signal even in the country where there probably aren't any repeaters. If I took the LNB off my TV dish and just pointed it in the general south direction, it wouldn't work. On wikipedia where it talks about the radio it says "It is a type of direct broadcast satellite, and is strong enough that it requires no satellite dish to receive." Why would satellite radio be stronger than tv?

 

[berkeman]

That's a very good question. The satellite radio systems have to make up for the gain difference somehow, but I don't know which method they chose. I spent some time just now reading at howstuffworks.com about satellite TV and radio, and a bit of time googling. I'll research more when I have some time, but the two obvious ways to make up the gain difference (and hence get an acceptable SNR) would be higher broadcast power and narrower channel bandwidth. The bandwidth difference would appear to be a couple orders of magnitude, but I'm not sure about that. The number of channels broadcast appears to be about the same for sat TV and radio (couple hundred each), so the difference in total channel BW may be the couple orders of magnitude difference in datarate for TV and audio.

For sure one of the design goals for sat radio would have been for non-dish antennas that could move around in orientation and still receive okay. So the system designers would have had to figure out how to make up for the dBi they were losing without the dish. GPS had a similar design constraint, but the dog-slow datarate of GPS info helped them a lot.

I'll ask around my RF contacts to see if anybody knows the answer. Welcome to PF, BTW.

 

[SkinnyDude]

Thanks for the warm welcome. This forum is very informative. The only way I can figure is that it uses less bandwidth and lower frequency that can penetrate the atmosphere better. But if that's the case why would C band need such a huge dish pointed in a specific direction also?

 

[chroot]

I think the biggest difference is the data rate. You don't need nearly as strong a signal to successfully recover audio (128 kbps?) versus video (several Mbps).

- Warren

 

[Averagesupernova]

Back in the dark ages when I was in school, satellite TV used NTSC that frequency modulated a carrier. This requires MANY TIMES the bandwidtch that todays satellite radio needs. I don't know what the format is now for satellite TV.

 

[NoTime]

The newer forms of satellite TV use variations on mpeg (digital).
The variations are for encryption.

The data rate for radio is certainly less than tv.
Don't know if they increased the power, but probably.

 

[Ouabache]

Great question, I also did some reading and here are some parameters I've found. (some I would like to know are denoted with ??)

Both TV and Radio sats are at comparable altitudes (for the GEOs that's 22,000 mi above earth), (though Sirius system is an exception)
Transmission Frequency both transmit in GHz range (sat TV 12-14GHz) and sat Radio 2.3GHz in U.S.) ref01(TV),
ref02(radio) Sat TV transmits in Ku Band (12-14GHz) and Sat Radio, at least across No America they are on the S-band (2.3GHz).
Number of channels transmitted; TV 200, radio 100
Downlink data rates are also comparable; TV 5-10Mbps*, http://www.elecdesign.com/Articles/Print.cfm?AD=1&ArticleID=5603 7.5Mbps*
Power transmitted by the satellites (radio: http://www.xmradio.com/how_it_works/satellites.html ,TV: ?? )

Satellite antenna? (gain, radiation pattern)

So what does this mean? The magnitude difference in transmitted data rate, was a good thought. However as the references indicate, they are actually comparable. The explanation for the gain difference between Satellite TV vs Radio is still elusive.

*(see notes #11 & 13 below for clarification)

 

[chroot]

You're mixing bandwith units.

Perhaps the entire aggregate bandwidth of all the radio sat's channels is 7.5 Mbps, but an individual channel of audio is certainly no more than 256 kbps.

A single video channel, however, occupies 5-10 Mbps.

- Warren

 

[chroot]

From the link provided by Ouabache, it sounds like the biggest difference in the systems is the very large amount of diversity (multiple receive paths) built into the sat radio designs.

The same signal is available from at least two satellites at any time (satellites which are nearly directly overhead in the US, rather than closer to the horizon as with most video satellites), and the signal is offset by four seconds from one satellite to the next. This diversity provides the receiver with a couple of different ways to pick up the data simultaneously. The same diversity would be very difficult to provide for video signals, because of the incredible bandwidth that would be required.

The entire Sirius feed (all 100+ channels) has only the bandwidth of a single video channel. Multiplying the bandwidth by two or three for diversity is the equivalent of two or three channels of video bandwidth -- not very much -- but provides very robust reception.

- Warren

 

[Ouabache]

You're mixing bandwith units.

Thanks for pointing that out. That makes more sense. In reviewing my references, video data transmission is 5-10 Mbps/channel and (Sirius) radio 24-64 kbps/channel. So the aggregate bandwidth for sat-TV is on order of 1-2 Gbps versus 7.5 Mbps for sat-radio.

Something I didn't see mentioned yet, but implied, is that when you transmit a wider bandwidth signal, the energy is distributed across that bandwidth. So its gain is smaller compared to a narrower band signal transmitted at the same power. That is why you can get away with a smaller gain antenna for Sat radio, such as an omnidirectional on a ground plane in a mobile.

...it sounds like the biggest difference in the systems is the very large amount of diversity (multiple receive paths) built into the sat radio designs.

The same signal is available from at least two satellites at any time (satellites which are nearly directly overhead in the US, rather than closer to the horizon as with most video satellites), and the signal is offset by four seconds from one satellite to the next. ...

This is only true for the Sirius Sats (which use elliptical nongeostationary orbits), XM actually use GEOs just like the Sat-TV systems.

 

[BobG]

XM and DirectTV both use Boeing satellites, so it's easy to compare the specs.

http://www.boeing.com/defense-space/space/bss/factsheets/702/xmsatradio/xmradio.html uses the Boeing 702 and uses a bigger antenna on the satellite, for one thing, which increases the gain. Radio has a smaller bandwidth, meaning satellite power is more concentrated (as already pointed out).

http://www.boeing.com/defense-space/space/bss/factsheets/601/directv1r/directv1r.html satellites using the Boeing 702. They'll do more than just satellite TV, though.

You want a real power challenge, try using a geosynchronous satellite to provide telephone service. The phones have to be comparable in size to cell phones in order to be competitive. http://www.boeing.com/defense-space/space/bss/factsheets/geomobile/thuraya/thuraya.html uses a 40 foot x 52 foot antenna.

 

[happycj]

I think it is due to the antenna used. Dish antenna is directional. But for XM radio, it is phase array used.

 

[berkeman]

I think it is due to the antenna used. Dish antenna is directional. But for XM radio, it is phase array used.

Not on cars. It's a little monopole. In some cases it's two monopoles, but one is for space reception, and the other is for the terrestrial repeater reception.

 

[pete463251]

I have a Ph.D. in engineering and work as an antenna engineer, and while I'm not the brightest guy in the world I can offer my insights.

When traveling in a car, you won't know where the signal is coming from. Hence, antennas for these systems have a more omnidirectional pattern (usually patch antennas or monopoles). When you are mounting an antenna on your roof, you will always be looking for the signal in the same direction. Hence, a high gain (very directional) antenna works well in this case.

Technically, an omnidirectional antenna would work for a rooftop antenna; however the Signal to Interference Plus Noise Ratio (SINR) would be much less than if only the dominant line of sight signal was used. The highly directional pattern reduces multipath and other sources of interference.

In contrast, a satellite dish would not work well for a mobile antenna, because the signal will come from a random angle. Hence, in most cases the highly directional antenna would suppress the desired signal.

A good source of information about antenna types can be found here:
http://www.antenna-theory.com

Cheers

 

[Pumblechook]

I think you have all missed the point except maybe the prevous post. It is nothing really to do with bandwidth or data rates and the ionosphere is not involved. The signal is a direct point to point transmission. It is the carrier frequencies.

Satellite TV in many countries used frequencies from 10.7 - to over 12 GHz.. about 10,700 to 12,500 MHz and uses dishes with a beamwidth of 2 - 3 degrees so needs accurate pointing.

Satellite radio uses much lower frequencies around 1500 MHz and the aerials have a very broad beam so accurate pointing is not required as much.


The simpler the aerial is the less directional it is.

Although at a lower frequency the aerial is simpler and has less gain (less directional - less focusing) it will receive as much or more power than a higher gain attenna working at a higher frequency because it may be physically bigger.

E.G a simple dipole receiving a signal on 100 MHz in a field of X Volts per metre will generate twice the voltage as a simple dipole receiving a signal on 200 MHz in a field of X Volts per metre simply because it is twice as long. And twice the voltage means four times the power. To make up for the difference you make the dipole at 200 MHz into a 3 element yagi say. So you end up with a more directional aerial on the higher frequency for the same signal strength received.

 

[russ_watters]

This thread is two years old...

 

[Pumblechook]

Oh yes.. So it is.