Antenna Reciprocity: Gain & RX Sensitivity Explained

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In summary, antenna reciprocity means that the gain pattern of an antenna will be the same for both transmitting and receiving. This allows for the mapping of the receive gain pattern based on the transmit gain pattern. Having an antenna with transmit gain also means having the same gain for receiving, allowing for a larger RX signal for a given signal power and increasing the distance at which a signal can be received. The physical properties of an antenna, such as directors and reflectors, that conserve energy and create gain in the transmit direction, also contribute to increasing gain in the receive direction. This is possible because gain in one direction is offset by a loss in another direction.
  • #1
Dave9600
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Could someone give me a quick explanation of antenna reciprocity and if/how antenna gain can increase RX sensitivity? Thanks.
 
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  • #2
Dave9600 said:
Could someone give me a quick explanation of antenna reciprocity and if/how antenna gain can increase RX sensitivity? Thanks.

Welcome to the PF Dave. Reciprocity just means that the gain pattern of an antenna will be the same whether it is receiving or transmitting. So if you transmit a test signal and map out the gain pattern in the far field of the antenna, then this tells you the receive gain pattern as well. Here's an intro to antennas:

http://en.wikipedia.org/wiki/Antenna_(radio)

So yes, if you have an antenna with TX gain, it will have the same gain in the RX direction. A multi-element Yagi antenna has gain for both TX and RX, for example.
 
  • #3
berkeman said:
Welcome to the PF Dave. Reciprocity just means that the gain pattern of an antenna will be the same whether it is receiving or transmitting. So if you transmit a test signal and map out the gain pattern in the far field of the antenna, then this tells you the receive gain pattern as well. Here's an intro to antennas:

So yes, if you have an antenna with TX gain, it will have the same gain in the RX direction. A multi-element Yagi antenna has gain for both TX and RX, for example.
Thanks.

I guess what I'm really asking is how does gain increase the distance at which you can receive, loss being what it is in free space? As an example: -80dB is -80dB at the antenna whether you're receiving on an omni-directional antenna or a high-gain Yagi. If there is enough signal to reach your antenna what difference does RX gain really make? Maybe I'm misunderstanding gain?
 
  • #4
Dave9600 said:
Thanks.

I guess what I'm really asking is how does gain increase the distance at which you can receive, loss being what it is in free space? As an example: -80dB is -80dB at the antenna whether you're receiving on an omni-directional antenna or a high-gain Yagi. If there is enough signal to reach your antenna what difference does RX gain really make? Maybe I'm misunderstanding gain?

Gain gives you a bigger RX signal for a given signal power. So if you have two antennas, where the 2nd antenna has a 3dBi gain over the first, then the RX signal will be 3dB higher for the 2nd antenna. This let's you receive signal sources that are farther away.
 
  • #5
berkeman said:
Gain gives you a bigger RX signal for a given signal power. So if you have two antennas, where the 2nd antenna has a 3dBi gain over the first, then the RX signal will be 3dB higher for the 2nd antenna. This let's you receive signal sources that are farther away.
Okay, so what you're saying is that I would now be able to receive a signal which is -83dB where before I could only receive a signal of -80db because the antenna is "creating" 3dB of signal?

I don't mean to sound ignorant. Thanks for your patience.
 
  • #6
berkeman said:
Reciprocity just means that the gain pattern of an antenna will be the same whether it is receiving or transmitting. So if you transmit a test signal and map out the gain pattern in the far field of the antenna, then this tells you the receive gain pattern as well.

that is true, berk, except for the word "just", i think. i think that if you have a pair of antennae, one for receiving and one for transmitting, that the power "gain" (it's really a big loss) using antenna A as the transmitter and B as the receiver, from terminals to terminals, is the same as if you switched them around and used antenna B as the transmitter and A as the receiver.
 
  • #7
rbj said:
that is true, berk, except for the word "just", i think. i think that if you have a pair of antennae, one for receiving and one for transmitting, that the power "gain" (it's really a big loss) using antenna A as the transmitter and B as the receiver, from terminals to terminals, is the same as if you switched them around and used antenna B as the transmitter and A as the receiver.

Good point, your are correct. Glad that you pointed that out, rbj.

Dave9600 said:
Okay, so what you're saying is that I would now be able to receive a signal which is -83dB where before I could only receive a signal of -80db because the antenna is "creating" 3dB of signal?

Correct. Remember, an antenna that has gain over 0dBi in some direction, will have gain less than 0dBi in other directions. There is no free energy here -- maybe that is the part that has you a little uneasy. If you look down on a horizontal dipole antenna, for example, you have gain in the direction perpendicular to the dipole elements, and you have practically no gain (negative many dBi) in the direction parallel to the elements. What you gain has to come from somewhere. Was that part of your confusion?
 
  • #8
Correct. Remember, an antenna that has gain over 0dBi in some direction, will have gain less than 0dBi in other directions. There is no free energy here -- maybe that is the part that has you a little uneasy. If you look down on a horizontal dipole antenna, for example, you have gain in the direction perpendicular to the dipole elements, and you have practically no gain (negative many dBi) in the direction parallel to the elements. What you gain has to come from somewhere. Was that part of your confusion?
No, it's always made sense to me that an antenna "creates" TX gain by conserving energy. It's never made sense to me how an antenna "creates" RX gain.

Okay, so the same physical properties of the antenna that conserve energy (directors, reflectors, etc.), thereby creating TX gain, also increase RX gain by conserving energy in the opposite direction? Is this correct?
 
  • #9
Dave9600 said:
Okay, so the same physical properties of the antenna that conserve energy (directors, reflectors, etc.), thereby creating TX gain, also increase RX gain by conserving energy in the opposite direction? Is this correct?

Yes. The reflector/director elements are a good way to think about this. Just picture a parabolic dish, for example, where you can make a dish that is several wavelengths across or more. There it's a lot more obvious how the act of reflection is providing gain.
 
  • #10
berkeman said:
Yes. The reflector/director elements are a good way to think about this. Just picture a parabolic dish, for example, where you can make a dish that is several wavelengths across or more. There it's a lot more obvious how the act of reflection is providing gain.
Thanks. It makes perfect sense now. I have a problem thinking backwards sometimes.
 
  • #11
Okay, I'm going to add to this...

Let's say I have an antenna that the manufacturer states has a vertical beamwidth of 30 degrees. Isn't the vertical beamwidth going to change depending on the amount of power applied to the antenna? Or am I misunderstanding antenna propagation patterns? Maybe propagation is limited by the physical characteristics of the antenna and the half-power points on the main lobe will always be the same regardless of applied power?

I guess I'm confused as to what a propagation pattern actually shows me.
 
  • #12
Dave9600 said:
No, it's always made sense to me that an antenna "creates" TX gain by conserving energy. It's never made sense to me how an antenna "creates" RX gain.

Okay, so the same physical properties of the antenna that conserve energy (directors, reflectors, etc.), thereby creating TX gain, also increase RX gain by conserving energy in the opposite direction? Is this correct?

i think what is correct is that the RX antenna has increased gain (in the direction it is pointing) because with the other parasitic elements, it has an increased "aperture". it becomes a bigger net to scoop up radiant energy.
 
  • #13
Dave9600 said:
Maybe propagation is limited by the physical characteristics of the antenna and the half-power points on the main lobe will always be the same regardless of applied power?

That is correct. The antenna pattern is normalized, and is independent of TX power.
 

1. What is antenna reciprocity?

Antenna reciprocity is a fundamental principle in electromagnetics that states that the performance of an antenna in transmitting a signal is equal to its performance in receiving a signal.

2. How does antenna gain affect reciprocity?

Antenna gain refers to the ability of an antenna to focus and amplify the signal in a certain direction. In terms of reciprocity, the higher the gain of an antenna, the stronger its transmission and reception capabilities.

3. What is the relationship between antenna gain and receiver sensitivity?

The gain of an antenna and the sensitivity of a receiver are inversely related. This means that a higher gain antenna can compensate for a less sensitive receiver, and vice versa.

4. How do factors like antenna size and shape affect reciprocity?

The size and shape of an antenna can affect its gain and, therefore, its reciprocity. For example, a larger antenna will generally have a higher gain and better reciprocity than a smaller antenna.

5. Does antenna reciprocity apply to all types of antennas?

Yes, the principle of antenna reciprocity applies to all types of antennas, including dipole, loop, and patch antennas. However, it is more accurate for antennas that are designed and used within their specified frequency range.

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