antenna gain

[insename2]

can some1 explain to me how antenna gain is measured? it is just how well the radiation is focused in one direction? if so how can satellite dishes have a gain of around 17 dbi when its not very directional?

 

[xxXXDeathXXxx]

gain is always 10*log (powerout/powerin) in units of dB. Performance is a function of geometric shape the environment and the applied field. Most satellite dishes I've seen are parabolic reflectors and are highly directional. The gains should be very high with this type of antenna.

PS the little i is just an indication that this is as compared to the theoretical isotropic radiator. Obviously a directional antenna will be focusing it's power into a more columnated beam rather than in the isotropic spherical radiation pattern.

 

[Born2bwire]

They measure the gain of an antenna by placing it in an anechoic chamber. A calibrated antenna emits a signal into the chamber and the received power is measured from the device under test. They rotate the antenna to get the measurements along different planes. The chamber is designed to minimize any reflected power within a certain bandwidth.

 

[xxXXDeathXXxx]

It seems like the device under test would need to be rotated about two axes.?.?.?

 

[Born2bwire]

Yep. Depending on the sophistication of the chamber you can do that. One of the chambers I worked with could have an entire computer chassis on it and rotate along two axes. The chambers will vary in size but the ones that I am familiar with probably have the square footage of a garage. But there are ones that are much larger. I know Northrup-Grumman has a very large chamber for them to do aircraft model measurements. One interesting thing about EM anechoic chambers is that they are also very effective at absorbing acoustic waves in addition to electromagnetic radiation. The dead silence in the chamber is really freaky, makes you realize how how we continually hear some amount of feedback or noise in all our environments. The chambers can be used for other purposes. We had a smaller semi-anechoic chamber that was used to measure the EM radiation emitted by devices to see if they were within specifications. And there are chambers designed specifically for acoustic waves.

 

[insename2]

thx guys

hope no one minds me adding an additional question...

why is it that some wireless devices can get overpowered when i a really strong em field? wouldnt that increase signal strength??

 

[Born2bwire]

Originally posted by: insename2
thx guys

hope no one minds me adding an additional question...

why is it that some wireless devices can get overpowered when i a really strong em field? wouldnt that increase signal strength??

I can't quite figure out what you are asking, but I am going to guess that you are asking why some devices can be overpowered by a strong EM field EDIT: Ahh, forgot an "n." For most purposes, it should be difficult to induce noise on a signal such that it becomes useless to a commercial device (prototypes are usually fairly easy to ah heck up unfortunately). For a wireless device, the received signal is demodulated and brought from, say 1.2 GHz for GPS, down to a few MHz and filtered. The signal is effectively bandlimited so for a device to be overpowered it would first need to be receiving noise that lies in the same frequency range of its operation. Secondly, if the field is of constant strength, then the device will have no difficulty in still picking out the underlying field, it would just be at an offset. So to my mind, the noise that is introduced to the signal must be just that, noise. Wireless devices, especially communication devices, generally have adaptive filtering that is very effective at removing random noise. Usually what occurs is some sort of predefined handshake. For example, when a modem connects, it first sends a long stream of data that has been preset. The modem then receives a preset stream of data. The data that is received will have noise from its journey, but since the modem knows what should have been sent, it will use the received data to train its filters to remove the noise that that particular line is experiencing. Still, there is a limit to the ability for filters to remove noise. In addition, there is usually some noise being induced by the device's circuits itself. If one would subject the device to an unusually large amount of noise, the filters of course can fail at some point.

I guess another way to defeat even the most robust of devices would be to subject it to a noise of such large magnitude that it saturates the DAC's of the device. The received signal for any device is analog, and so the device must sample and convert the signal to process it digitally. This means that there is a limit to the dynamic range of the device and if we subject it to a very large field in the correct frequency range, then the underlying data signal will be too small in comparison to be accurately picked out when the signal is converted.

I am sure there are better (and more correct) explanations. But this isn't exactly my expertise. My job is to figure out how to model and create the fields, not interpret them.

 

[insename2]

thx born2wire

Originally posted by: xxXXDeathXXxx
gain is always 10*log (powerout/powerin) in units of dB. Performance is a function of geometric shape the environment and the applied field. Most satellite dishes I've seen are parabolic reflectors and are highly directional. The gains should be very high with this type of antenna.

PS the little i is just an indication that this is as compared to the theoretical isotropic radiator. Obviously a directional antenna will be focusing it's power into a more columnated beam rather than in the isotropic spherical radiation pattern.

then how can a omnidirectional high gain antenna exisit?

such as this
http://www.costcentral.com/proddetail/C...unt_Antenna/AIRANT1728/410780/froogle/

 

[Born2bwire]

Originally posted by: insename2
then how can a omnidirectional high gain antenna exisit?

such as this
http://www.costcentral.com/proddetail/C...unt_Antenna/AIRANT1728/410780/froogle/

I wouldn't call a 5.2 dBi gain antenna high gain. A simple halfwave dipole has 2.15 dBi. That omni directional 5.2 dBi antenna is probably similar to a dipole array. Most wireless LAN application antennas are low gain. This is probably due to FCC requirements, compact antenna size, and getting omnidirectionality. You've probably heard about those "cantennas." Those are antennas made out of Pringle chip cans designed for 2.4 GHz Wifi. You could design and build one of those after taking an undergraduate waveguide class. Very simple stuff but you can get 12 dBi easily out of these things.