negative gain antenna

[hagster]

Generally speaking(there are exceptions. E.g colinear). High gain antennas are like a torches. They emmit most of their power forward. Thats great if if know the direction that you want to transmit or receive from.

Low gain antennas are more like lightbulbs. They 'shine' equally in all directions. Great if you dont know where to point. Your cell phone has a low gain antenna because you dont want to have to point it at a tower to work. This is the same for tx and rx.

Remember that there is 'reciprocity' for antennas. This means that an antenna has the same transmit gain as recieve gain.

The choice of high or low gain is completely application specific. You may want a low gain tx antenna and a high gain rx. E.g. for broadcast TV where the transmitter to houses in all directions, but the houses can point a yagi at the transmitter. But for day GPS the satellite needs a high gain antenna to direct power towards thr earth, but the receiver needs a low gain antenna to pick up signals form satellites in lots of directions.

As always in engineering the answer to a very general question is "it depends"

 

[Warpspeed]

Not if you want your transmitter and receiver to work in ALL directions.

Suppose you had to design an antenna for a ship at sea, or a flying aircraft.
You need to be able to both transmit and receive from all directions.

Some huge high gain antenna that only worked in one particular direction would be pretty useless.

 

[kae_jolie]

1. what do you mean by "colinear"?

2. is cell phone antenna omni-directional? I am under the impression it may have negative gain in some directions due to ohmic losses being next to other metallic parts of phone. Also, I believe the cell tower antenna is omni-directional, too, but with higher gain. Is this correct?

3. is there such a thing as 10dB or 20dB or high gain omni-directional antenna? If so, please cite examples. If not, what is a maximum dB for an omni-directional antenna?

 

[hagster]

Kae Dont take this the wrong way, but can i suggest that you visit www.antenna-theory.com and go through some of the introductions. Its not that we dont want to help, but you would get more from that structured information than a general Q&A here.

But here are some quick answers.
1. Colinear is a bunch of stacked antenna designed to have an omni derectional pattern in azimuth and a very directional pattern in elevation.

2.the cell phone antenna is not perfectly omni directional.

3.high gain omni (like a colinear) see answer 1. Positive gain implies directionality. Thus the extra gain in one direction has to come at the expense of gain in another direction. A basic dipole has about 2.1dBi gain because it has nulls on its end axis. dBi is gain with respect to a mythical isotropic antenna that has perfectly spherical radiation(of 0dBi). It is not possible to create this isotropic antenna in reality.

 

[kae_jolie]

So high antenna gain has nothing to do with how the antenna will propagate in certain direction?

As mentioned before, I am under the impression high gain antenna makes the signal "stronger" in one direction, and I interpret "stronger" to mean that the signal can travel further with less attenuation when compared to a low gain antenna.

I would appreciate if someone would agree or disagree with above explanation without any textbook definitions.

I really know all the textbook definitions like the back of my hand, but real-world applications and variations are a different beast...I have already learned a lot from this thread including stuff I can't find in textbooks.

 

[Aussie Susan]

First things first: any EM radiation will attenuate as it leaves an antenna. It will follow the inverse-square law for a start and also the real world is not quite the ideal 'free space' environment that is generally used (but is close enough for practical purposes). This is so regardless of the initial power and direction that the radiation is launched from the antenna.
Where the direction and power come in is that, if you start off with a higher power signal in one direction then the attenuation will still be the same but at some given distance away in the given direction, because you started with a higher power, you will receive a higher power.
When you design an antenna, you need to think about where you want the signal to go. An ideal "isotropic" radiator will radiate equally in all directions and this is usually used as the "gold standard" for measuring the gain of any other antenna design. A Yagi for example will not transmit very much power directly out of the ends of the active element but will send more power at right-angles to the element. If you stand the element vertically, you can imagine that this will send in a doughnut shape out horizontally in all directions with up and down representing the "hole" in the doughnut.
Often the way an antenna radiates is shown in radiation plots that show the vertical and horizontal radiation patterns with the relative strength shown as a circle around the centre. The isotropic radiator will show up as a circle in both the vertical and horizontal plots. The Yagi I mentioned above that is oriented vertically will show as a circle in the horizontal plane and a couple of "wings" in the vertical plane.
These 'wings' are generally referred to as 'lobes' and the point of antenna design is to make the radiation patterns suit your requirements. You mention a satellite antenna - there you want a single lobe (or beam) that points in one direction and it as narrow as needed (if the satellite is geosynchronous then the lobe can be very narrow; if the satellite moves such as the LEO satellites, you need to move the antenna to track the movement and so you might make the beam a little wider to lower the requirements for accurate mechanical tracking mechanisms).
When you talk about the gain of an antenna, you generally mean the relative strength of the main lobe to the isotropic radiator.
Therefore your statement in the 2nd paragraph is both right and wrong. It is right in that the gain means the main lobe is better (or worse) than the isotropic radiator and as it is stronger it can travel "further" before the attenuation reduces the signal level to below where it can be detected. However you are wrong when you say it has less attenuation - as I outlined above, the attenuation is always the same.
This also should sow that your 1st paragraph is also wrong in that the gain is measured along the main lobe which IS (generally) in a "certain direction".
Susan