little thing I need to understand about antennas

[sram]

Let's see if I can explain this well. I have seen some tactical radios which are SATCOM( Satellite communications), and can use different types of antennas. The fixed radio station will have some sort of a sat antenna (It doesn't have to be a dish antenna)that will have to be directed to the satellite being used to get a good signal. Now you can use the same radio but in the move(like installing it on a combat vehicle or an aircraft). The antenna will be different of course and it doesn't need to be directed in order for it work. The aircraft/vehicle will be able to use the radio while in the move.

From what I understand, the signal needs to be concentrated to form something like a beam so that it can reach far distances --just like what the dish antenna do in TV sat stations--So, what happens with the other case? The case that uses a different type of an antenna while using the radio in vehicles and aircrafts. How could an antenna that is not directional send a signal powerful enough to reach the satellite? And you know the vehicles/aircrafts are moving, so how come they don't lose the satellite?

And if the solution adopted in moving vehicles is superior, why isn't it being used in fixed stations? Instead of going through the trouble of directing the antenna every time we need to setup a station?

I hope I made it clear.


Thanks.

 

[KJ6EAD]

For receive only applications, patch, slot and low profile parabolic antennas work. For mobile transmission, either a parabolic or circularly polarized Yagi mounted on a mechanically steerable gimbal is typical while a phased array is another option.

https://en.wikipedia.org/wiki/Yagi-Uda_antenna

https://en.wikipedia.org/wiki/Circular_polarization

https://en.wikipedia.org/wiki/Phased_array

https://en.wikipedia.org/wiki/Fractal_antenna

**broken link removed**

**broken link removed**

 

[RCinFLA]

The application determines what type of antennas will work.

Concentrating available power in the intended direction requires less power to make the communication link.

Sat phones, evolved from the Iridium system, use omnidirectional antennas for convinence of user. They sacrifice power consumption efficiency by doing so but intent was a small phone resembling a cellphone. The modern cell phones with internal antenna is also sacrificing battery power and battery life compared to old external antenna design. Internal antennas require more cell sites but evolution of needed capacity of mobilephone population made this an acceptable tradeoff. The first cellphone, the Motorola Dynatac (the Brick) used a very efficient half-wave external antenna, but would be unacceptable to most mobilephone users today.

The system overall design has to satisfy the overall link power budget to achieve the desired results.

 

[sram]

The application determines what type of antennas will work.

Concentrating available power in the intended direction requires less power to make the communication link.

Sat phones, evolved from the Iridium system, use omnidirectional antennas for convinence of user. They sacrifice power consumption efficiency by doing so but intent was a small phone resembling a cellphone. The modern cell phones with internal antenna is also sacrificing battery power and battery life compared to old external antenna design. Internal antennas require more cell sites but evolution of needed capacity of mobilephone population made this an acceptable tradeoff. The first cellphone, the Motorola Dynatac (the Brick) used a very efficient half-wave external antenna, but would be unacceptable to most mobilephone users today.

The system overall design has to satisfy the overall link power budget to achieve the desired results.

Thanks RcinFLA. That's the kind of post I was looking for. So, If I understand you correctly, those antennas installed on the vehicles in my example are supposedly omin-directional, which means they can receive from any direction. So if they are in the spot beam of the satellite, it should just work. And just like you said, this is not an ideal situation for signal strength but we are willing to scarify some factors for convenience......who is going to always adjust the antenna to see the satellite???

That's very much it....right?

 

[RCinFLA]

Yes.

Sometimes things evolve based on technology advancement. GPS has been pushed a lot farther then its original intent with improved multi-correlators that can achived usuable signals as lower received powers. Some airlines have installed electronically steerable phased array antennas to track geostationary satellites for onboard sat TV receptions. As the plane maneuvers around the phased array antenna automatically moves main antenna lobe to keep it on the geostationary satellite.

You still have to have prior knowledge of exactly where you need to point the antenna. Omni directional antennas are used where communications can come from any direction without prior knowledge of what direction, but you pay the price of requiring higher transmit power then would be needed for a highly directional antenna.

Cellphone tower are using smart antennas, where they employ multi-directional antenna arrays to improve reception by dedicating a focused beam at a given mobilephone. They also have ability to steer nulls in the pattern to reduce interferance.

 

[sram]

Excellent explanation. Again, thanks RcinFLA.

 

[Nepaliman]

A satellite in the space is moving around the earth (like 14 round per day) fastly, but why we should have to focused our receiver dish in same direction/ same position? How does it receive the signals?

 

[ericgibbs]

A satellite in the space is moving around the earth (like 14 round per day) fastly, but why we should have to focused our receiver dish in same direction/ same position? How does it receive the signals?

hi,
Google: Geo synchronous earth satellite

https://en.wikipedia.org/wiki/Geosynchronous_satellite

 

[JimB]

Also consider that there are different satellites.

Some are in geosynchronous orbits, that is to say that they are always in the same position in the sky.
These satellites are about 25000 miles away, hence there is a high path loss and a high gain directiontal antenna is very useful for both transmit and receive.

Other satellites are in a fairly low orbit and are only 100 to 150 miles away. This is not very far and path loss is relatively low.
A low power transmitter and an omni directional antenna is quite practical for communicating through such satellites.
Each individual satellite is only within range of someone on the earth surface for a few minutes at a time.
As a result, sat-phone systems such as Irridium have many satellites in orbit so that there is usually at least one satellite within range at ant time.

JimB