This may be a simple question and I feel somewhat ridiculous asking, but I haven't seen it clarified in the CWNA book.
When discussing antenna gain, the book discusses the gain of receiving antenna as a variable factor. Meaning that the gain of a receiving antenna can be greater or lower.
It was my understanding that a receiving antenna was merely a passive device that converts radio waves to electrical signals, as they are received when they reach the point of said antenna. If the receiving antenna is not within range of the transmitted waves, then the signal cannot be received.
The way the book sometimes discusses the gain of receiving antennas is somewhat confusing. It makes it seems like both the transmitting and receiving antennas transmit a signal, but the receiving antenna's signal "receives" the other?
This may not make sense, as I have a tendency to look too deeply into things. But in my attempt to wrap my mind around the material, I'm struggling with this concept.
If gain on transmitting antennas equates to a stronger signal, hence the ability for said signal to travel further...
Could someone clarify "gain" as it pertains to receiving antennas?
When talking about antenna gain for a transmitter, I like to think of a magnifying glass burning wood. The magnifying glass doesn't change the power of the sun, it just focsuses it.
When speaking of an antenna receiving a signal, think of a big cheerleading horn, the cone kind. If someone is trying to talk to you from a long distance you may not be able to hear them because there isn't enough sound (signal) getting to the receiver (you). However if you put the horn to your ear you can hear better because it has a larger receiving surface and can bring in more sound. That is how an antenna has gain when receiving.
Does that help?
I don't mean to be rude, but not really.
What exactly does gain do for a receiving antenna? It obviously cannot change the physical aspect of the antenna, so what is it doing to help that antenna receive a signal which would otherwise be unintelligible?
Gain on a transmitting antenna adds transmission power.
A receiving antenna does not "transmit" (correct?), so what is the gain doing to help?
I suppose I'm looking for more of a technical explanation.
I get your analogy, I'm just curious as to what is really going on.
Hi Usr of Ohio:
I imagine it this way.
An ideal transmitting antennae radiates equally in all directions as if from an impossibly small point. When energy that would have been radiated in one direction is instead sent in one of the other directions because of the physical size and shape of the antennae, that preferred direction experiences passive gain.
An ideal receiving antennae receives equally well in all directions as if by a dipole antennae element optimally sized for a given frequency. When energy that would have passed by is instead received because of the physical size and shape of the antenna, that preferred direction experiences passive gain.
I hope this helps. Thanks. /criss
Great question /usr,
I hope to be able to shed some light on it. We can get as technical as you want and if I do not know the answer, I will ask some of my amateur radio/RF engineer friends for their comments.
We will start with analogies if that is OK. First, I see that you mentioned that gain on a transmitting antenna adds transmission power. That statement has to be refined. It does not increase the transmission power in the same way an amplifier would, it is what is called passive gain as Criss has mentioned.
My favorite analogy is if you had a balloon with a fixed volume (the fixed volume concept is important) that was located around what Criss called as an isotropic radiator (ie perfect antenna). This means it is radiating equally in all 3D space. The amount of RF energy measured anywhere in that space would be equivalent.
Next, let?¡é?€??s move to the vertical dipole antenna which has a positive gain over the isotropic radiator of approximately 2dBi (that is what the i stands for). If you take that same fixed volume balloon and place it around a dipole antenna (which is similar to those found on most wireless devices), the shape of the balloon would change but not the volume of the balloon.
If you look at antenna radiation patterns you will see these differences due to their physical design. First this link gives a good description as to what antenna radiation patterns are and how to use them.
OK, so what does a dipole do to get a 2dBi gain over the isotropic radiator? It takes the perfect sphere and flattens it out along the long axis of the antenna. Remembering that the volume in the balloon can not change, something has to give so it forces the outer perimeter to expand. But the expansion is only in a direction that is perpendicular to the axis of the dipole antenna. Hence the tradeoff, one gives up 3D space coverage for more radiation in a specific area/direction.
Please look at the dipole radiation 3D representation on this next link.
One other concept that is important to understand is that published radiation patterns are not actual physical distance plots. The radiation pattern is formulated by testing and documenting where in 3D space that particular antenna is radiating a prescribed amount of energy. In most cases, it can be extrapolated to distance, but that is not the exact or accurate concept.
This analogy can be then applied to even more directional antennas (ie panel, sector or parabolic dish antennas). If you continue the thought process, the balloon not only gets squished along one axis, but it also reduces the coverage angle in the other 2 axis.
That information was important to obtaining an understanding of the reciprocal or receiving antenna science. The balloon analogy is no longer applicable and we have to get a bit more technical. I have linked two WiKi web pages that explain quite well as to why an antenna that has transmitting gain will also have the same benefit in the receiving mode.
Now, if that bored the heck out of you I have a simple analogy similar to what Criss was alluding to. Gain on a receiving antenna is likened to the ability to concentrate.
I first want to mention that you are correct that every antenna that is located in the coverage area of a transmitting antenna will resonant if designed for that frequency. But the difference seen in antennas with more gain is the ability to have increased resonation or capture more RF energy.
Here is where I get a bit different and use an analogy of RF being likened to particles, which if you are interested in quantum mechanics is not theoretically a totally false comparison. I digress, sorry. Let us say you have a gram of dust and it is equally dispensed in a sphere that had a diameter of 1m. That would give you a certain dispersion. Now let?¡é?€??s take that same gram of dust and disperse it in a sphere that had a diameter of 1Km. The dispersion would be dramatically less. This is the same basically what happens to RF energy per the logarithmic function.
Let us now say that you wanted to capture those particles. Obviously one choice would be to move the antenna as close as possible to the transmitting antenna, to take advantage of the increased density. If that is not possible then a larger/more efficient collection scheme would be a second choice, because it has the ability to concentrate/capture (ie a larger sail has more potential energy) more particles. This same process applies to antennas with higher gain. They have the ability to concentrate/capture more RF energy from what is available at that given point in space. That is my simplistic attempt to explain the concept of reciprocity in relation to RF theory.
To advertise an even simpler example is to compare a parabolic dish antenna to a Cassegrain reflector telescope. They use the same process of concentrating electromotive energy albeit light or RF.
A little different view of antennas, with the statements below in the "more true than not", since there are almost always exceptions and special cases.
1. A passing radio wave will generate a voltage in an electrical conductor.
2. You can change the size and shape and number of conductors and position to increase the voltage available from some particular distant transmitting antenna.
3. The changes are designed to get you a higher signal voltage at the receiver input, but also, in many cases, a higher signal to noise ratio.
4. Antennas can take many forms, including fractal shapes, loops, straight lines, cones, and flat squares, all designed to improve the signal for some specific set of conditions. Multiple separate conductors can be designed to work together as if they were a single larger antenna producing more signal voltage.
5. The reason why people don't just use a tiny and convenient receiving antenna and amplify the voltage to the level needed is that amplifiers introduce distortion, limitations of noise from various causes, and the size and expense of really good amplifiers.
6. It is often more cost effective to improve an antenna for a particular purpose than to improve other parts of the whole system.
I will take a stab at it with my favorite analogy. Its unfortunate that we have to resort to analogies, but the actual physics behind antenna reception and transmission require all kinds of math that is beyond most people's (including me!) background.
Imagine a still lake. The water in the lake represents the "medium" through which RF waves pass. (In reality, no such medium has been found to exist, but it's important for our analogy.)
Imagine an antenn as a paddle poking up through the surface of the lake. When the paddle vibrates back and forth, it creates waves in the surface of the lake, which propagate out from the antenna.
Imagine a second antenna somewhere else in the lake. When the first antenna starts vibrating, making waves, those waves eventually hit the second antenna, which causes it to start vibrating too. This is an analogy for transmission and reception of RF signals. The antenna transmits electromagnetic energy, which propagates through space until it impinges upon another antenna, where it induces current ("vibrations") in the antenna.
Antenna gain can be roughly correlated to the size of the two paddles. A bigger paddle will set up bigger waves on transmission. Additionally, a bigger paddle will be vibrated more upon reception due to its larger surface area.
In reality, antenna gain is not just a question of the "size" of the "paddle." A higher-gain antenna doesn't just put out or receive more energy. It shapes or focuses the signal. There is a principle called the law of antenna reciprocity that states, more or less, that the exact same things that cause an antenna to focus its signal on transmission also cause it to focus the signal on reception.
If you really want details, I recommend the book, "RF Engineering for Wireless Networks," by Daniel M. Dobkin.
The paddles and water provide good visualization for teaching.
Especially considering that some scientists are once again proposing that there may be an ether medium for electromagnetic waves. No kidding.
So, gain on a receiving antenna basically means the antenna can "hear" more in a certain direction, as opposed to being able to receive a signal equally at all points of a sphere around the antenna?
Gain on a receiving antenna shapes it's coverage pattern to cover more a certain direction?