Question re radio transmitter power

[coldplugs]

I was looking at the status page for my satellite internet connection and noticed that the radio transmitter power is listed as 1 watt. I presume this is the transmitter attached to the dish that is used to send data up to the satellite.

The satellite sits about 22,300 miles away. 1 watt doesn't seem like much power for this purpose, especially since it has to punch through clouds, weather, and tree branches. I seem to recall that CB radios use 4 watt transmitters and they don't have that kind of range.

My question for folks who understand this stuff is: How can we get such range out of a single watt of power? Is it because it's so directional compared with CB and other "non-directional" transmitters?

 

[aerog]

My question for folks who understand this stuff is: How can we get such range out of a single watt of power? Is it because it's so directional compared with CB and other "non-directional" transmitters?

That's part of it.

I'm not so sure they're only using 1-watt, but that doesn't matter a whole lot. You're using an antenna that focuses every last bit of energy from the satellite onto an antenna that further amplifies the signal before it hits the receiver. The typical net "gain" in a cheap directTV antenna system (including preamp I think) is 50-60db.

There's a lot involved of course, but you're comparing apples and oranges. The typical CB uses a poor antenna in a less than ideal installation, mobile (usually), being received by the same type of equipment. Nothing is ideal, nothing is directional. Put two CB stations on a clear horizon (ie: line of sight) from each other, with 8db-gain antennas pointed at each other and I think you'll find they work just fine.

By the way - when conditions are good you can talk quite a long way on a few watts down near where the CB band is. The CB band is on 27mhz, the ham radio 10-meter band is 28-29Mhz. I used to talk to South Africa regularly from the car, on a converted CB radio using a shortened CB whip antenna. Conditions these days are lousy and band openings are infrequent, but last month I talked to New England and NY with 5w on the 29Mhz band.

There's also at least one amateur-radio satellite that operates on the ham-radio 10 meter band (close to the CB band). It runs about 1w I think, and is in orbit about 900-miles up.

 

[DrEntropy]

And with the network of repeaters scattered everywhere you can get a hand-held transceiver and blab to Australians, too. :thumbsup:

 

[CraigFL]

You're right! It is a directional thing. Even though it is only one watt, antennas are rated in ERP -- Effective Radiated Power. Because the dish antenna focuses on a specific location, and there is some "gain" by the system, ERP can be many times greater than the actual power.

 

[Don_R]

It's the focused directional thing. I was a Sat Tech in my active army days. the sat station I was at had 2-10K transmitters but we rarely used more than 5 watts of power. This was on a 38' parabolic dish using the best 1970s sat technology. Our dish was a 60db passive gain on the dish. I'm sure the technology has gotten better since then.

 

[Billm]

-also frequency, the higher the frequency the longer the range (generally).
Bill

 

[aerog]

Our dish was a 60db passive gain on the dish. I'm sure the technology has gotten better since then.

I don't think you can change the physics that much, except maybe the materials used in the reflector/dish. The 50-60db number I came up with was on an 24" (I think) antenna, but I'm sure that included the preamp.

Coldplugs:
Just FYI, there is at least one amateur radio satellite still in operation that is still on 10-meters (CB is "11 meters"). It's in orbit around 900-miles up and has been in operation since the mid-70s - not 22000 miles but you certainly can have HF on satellites.

 

[coldplugs]

Thanks everyone. It all makes sense.

The power of the parabola, so to speak.

 

[DrEntropy]

Remember th' training: "FOCUS!"

 

[Sarastro]

Everything said about the antennas is true, but there are two more components of this. The first is that the receivers in satellites are unbelievably sensitive these days, so much so that the dominant limitation to their sensitivity is the "thermal noise" of the earth--the very weak level of radio noise generated by the earth, by virtue of the fact that it is warm. For you physics geeks, this is blackbody radiation.

The second is the progress in the theory of communication since about the 1940s. This is very mathematical, dealing largely with statistics, since communication signals are fundamentally statistical--before communication occurs, you don't know what the message will be, only its probabilistic properties. There is even a field called "information theory," which involves the study of information and how to quantify it mathematically. Gets really strange, but makes your cell phone work.

Scratch a communication theorist, find a mathematician underneath. I started out in this field and eventually decided it wasn't what I wanted to do. I like hardware too much, as my presence here probably indicates.

 

[Keoke]

I was looking at the status page for my satellite internet connection and noticed that the radio transmitter power is listed as 1 watt. I presume this is the transmitter attached to the dish that is used to send data up to the satellite.

No that is the power used to access the ground station { Repeater}which in turn via a rather complex and powerful up link talks to the satellite.---Keoke

 

[coldplugs]

I was looking at the status page for my satellite internet connection and noticed that the radio transmitter power is listed as 1 watt. I presume this is the transmitter attached to the dish that is used to send data up to the satellite.

No that is the power used to access the ground station { Repeater}which in turn via a rather complex and powerful up link talks to the satellite.---Keoke

Don't think so. I have to point it directly at Maxsat-5, pretty high up in the southwestern sky from here.

 

[angelfj1]

This may give you some perspective on this topic. A good friend of mine in 1977 worked on both Pioneer and Voyager spacecraft. When these probes were near the inner planets there was plenty of solar energy available to the solar panels. But once past Mars they could not use the solar panels. The solar intensity near Saturn, for example, is about 1 percent of that near earth. They needed very low power consumption transmitters and receivers. The maximum power was approx. 3 watts transmitted by a parabolic dish <span style="font-weight: bold">9 feet in diameter. </span> of course the earth station had huge dish antennae, maybe 30 meters or more in dia.

Todays systems are much different using much higher microwave frequencies. In the old days, for space communication the prime power developed by silicon solar cell arrays, which are about 11 percent efficient in converting solar power to electrical power, limited spacecraft transmitters to about 10 watts of microwave power. The limitation in size of the booster rocket limited antennas to dimensions between one and two meters in diameter. In the 1990s, with the development of large GEO spacecraft using large arrays of <span style="font-weight: bold">gallium arsenide solar cells, which are 22 to 32 percent efficient, and unfurling parabolic antennas up to 10 meters (30 feet) in diameter, transmitter powers have increased to over 4000 watts in direct to the home television systems. </span>

Getting back to your initial question, with the inherent gain of a geosynchronous satellite with a 30 ft dish, you only need milliwatts to communicate in a beamed microwave transmission. And with 4000 watts in orbit, the transmitter has no problem transmitting to your small roof-top dish.

 

[Keoke]

Yes you must optimise the look angle by positioning your dish. This is because the powerful satellite signal's down link which talks to your antenna has a specific foot print on earth and your dish must be properly oriented within that footprint to optimise your service. There are some good Tutorials over on "How Stuff Works"--Keoke