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Weird & Wireless: Passive antennas and gain
Since most antennas are passive devices, how can they actually have gain?
Before I knew anything about the language of radios, this was always puzzling to me. I always thought I understood it for those fancy, powered antennas. It always seemed to make sense that some powered antenna would have an amplifier in it that would enhance the signal going out or in.
But the whole concept of a passive antenna was a mystery.
So, for those of you that are in the dark, but afraid to admit it, the only thing you really need to do is to think in the language of radios.
Hence, the radiation pattern is a perfect sphere. In the world of math and geometry, we love spheres because the math is nice and easy. You just need to remember where to put the ?. So, to make it easy, we like to describe radios in terms of isotropic transmitters and receivers because we all like easy math.
The problem is that there is really no such thing as an isotropic antenna. Picture this – an isotropic antenna would be the same as a person, who when speaking, would speak equally in all directions and also have ears that would hear equally in all directions. We all know that, while our mothers may have had eyes in the back of their heads, they couldn’t hear and speak that way.
Okay, so back to the world of antennas. To simplify things, we’ll focus on the world of transmitters first, knowing that the same principles apply to receiving as well, just in reverse.
The key to the principle of antenna gain is to describe what the performance of an antenna as compared to an isotropic one.
Let’s pretend for a moment that we have two antennas: antenna 1 is an isotropic antenna that transmits equally in a perfect, spherical pattern and antenna 2 is half of an isotropic antenna such that is transmits in a perfect semi-spherical pattern.
Now, assuming the same power into the antenna, we can see that the semi-spherical antenna will transmit nothing on one side and twice the power on the other side.
This is because the surface area of transmission is exactly half that of the isotropic or spherical antenna. This will be true at any distance and at any power. We might call this antenna a directional antenna because it sends the signal out in a specific direction.
Finally, in order to complete our understanding of antenna gain, we need to use dB or decibel maths. I like to think that the notion of the decibel was created in order to avoid having to deal with really big numbers.
So, for those of you that love logarithms, here is your chance to shine. The best way to think of it is that a value in dB is equal to 10 times the log of the number.
So, looking at the table left and putting the pieces together, we see that our semi-spherical antenna outputs twice the power in the desired direction so it is said to have a 3 dB gain.
Now imagine an antenna that could focus a transmission into a tight beam, kind of like a flashlight does.
We might very well find that this sort of antenna might reduce the area and hence increase the directed power by a factor of say 200 over its isotropic counterpart – often times antennas like this might resemble a plate or dish. In this case, we would say that it has a gain of 23 dB.
Finally, we need to add one additional letter, the letter “i” after dB. In this case the letter “i” means “relative to” or “increase over” so that we know the measure is relative. For our complete notation, we will describe an antenna in terms of dBi.
All in the name of making the math easy and keeping the numbers small.
So remember, the next time you see antenna described as 7 dBi, know that is just means that its transmission surface area is 1/5 the size of an ideal, not possible isotropic antenna.
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Joel Young, VP of Research and Development and CTO at Digi International, has more than 22 years of experience in developing and managing data and voice communications. He joined Digi International in June 2000 and in his current role he is responsible for research and development of all of Digi’s core products.
Prior to joining Digi, Joel was VP of Sales & Marketing at Transcrypt International where he was responsible for sales, marketing, and product development for all information security products. During his tenure at Transcrypt, he also served as VP of Product Development and VP of Engineering where he was responsible for engineering, research and product development for wireless communications products, cellular telephony, wireline telephony and land mobile radio, data security and specialized digital radio products.
He also served as District Manager for AT&T Business Communications Services where he was responsible for the creation and implementation of voice processing and network database strategies, including deploying new voice processing platforms into the AT&T switched network for private network and other outbound calling services.