How to interpret antenna gain in CST?

How can we interpret gain in CST. For a microstrip antenna it shows 10dB gain. For a fractal antenna it shows -2.6 dB gain. How can it be 10 dB in one antenna and -2,6 dB in the other. I an interpret in two ways (and both are wrong).

1) 10dB indicates radiation & -2.6 dB absorption.
2) 10 dB indicates radiated power is more than input power (violation of conservation of energy) and -2.6 dB shows radiated power is less than input power.

Can anybody help in correct interpretation of the gain? Thank you in advance.

Re: Antenna gain in CST.

Hmm. I do not have CST, but .. 2)

10 dB indicates radiated power is more than input power (violation of conservation of energy)

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"I think 10dB" does not mean that at all!

What it really means is the ratio of maximum value of the radiation intensity (it will be in a particular direction), as compared to a reference radiator such as the theoretical isotropic source which radiates the same intensity of radiation in all directions equally. In this case, the ratio is denoted "dBi"

If the reference is taken to be a standard dipole, then the gain is cited as "dBd" This gain has a value 2.15dB less than when using dBi.

There is no violation of any conservation law. The energy that would have been radiated equally everywhere is, by the action of the the antenna, concentrated in a particular direction at the expense of all other directions.

-2.6dB is also possible, if an antenna has such dissipative losses that a plot of the gain lobe has a maximum value that is less than the value a isotropic source would have managed.

Let us not inadvertently mis-apply the term "power" either.
Power (Watts) is the rate of energy transfer (Joules/Sec).
dB are highly convenient for expressing values of power that span many decades, and we commonly use dBm (relative to 1 milliWatt) or dBW (relative to 1 Watt). Easy to see that this rate, raised by the antenna gain, gives a value much bigger. This does not mean that many Watts was fed into the antenna! Its the Watts that one would have had to offer a theoretical isotropic radiator to get that value.

This is why the input power, raised by the antenna gain, is known as the Effective Isotropically Radiated Power (EIRP)

Re: Antenna gain in CST.

Most of us are aware of the differences between dB, dBi, dBd, dBm etc. The problem is, CST gives the result in dB, which means the power ratio is not wirh respect to a standard antenna like dipole or isotropic radiator. In CST, the gain must be defined in a certain way, which I am unable to understand. Though, apparently it seems to violate Energy Conservation Law, it can't be incorrect. I want to know, how it defines the gain. Same is the case with efficiency, which is in some cases less than one and in some cases more than 1 (again violation of Energy Conservation).

Re: Antenna gain in CST.

faculty3 said:

Most of us are aware of the differences between dB, dBi, dBd, dBm etc. The problem is, CST gives the result in dB, which means the power ratio is not wirh respect to a standard antenna like dipole or isotropic radiator.

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You misunderstand things. CST and most other EM simulator give a gain in dB relative to isotropic -> dBi

Re: Antenna gain in CST.

faculty3 said:

Most of us are aware of the differences between dB, dBi, dBd, dBm etc. The problem is, CST gives the result in dB, which means the power ratio is not wirh respect to a standard antenna like dipole or isotropic radiator. In CST, the gain must be defined in a certain way, which I am unable to understand. Though, apparently it seems to violate Energy Conservation Law, it can't be incorrect. I want to know, how it defines the gain. Same is the case with efficiency, which is in some cases less than one and in some cases more than 1 (again violation of Energy Conservation).

Click to expand...

I don't know CST, and don't have a license, but I suspect if you upload your cst file, someone will be able to look at it and tell you if you are mus-interpreting things, or if there is some problem.

Re: Antenna gain in CST.

Ah-Ha - I speculate now, but..

I have often had to look twice before I notice that some output plot, or gain formula was normalized to some handy, but arbitrary value, to get the plot to start at the top of the graph. eg. we nearly always show a filter transmission loss relative to a perfect lossless line, with 0dB at the top.

It is just possible you have a setting somewhere in CST that will cause results to be normalized inadvertently.

I suspect if you upload your cst file, someone will be able to look at it and tell you if you are mis-interpreting things, or if there is some problem.

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DeboraHarry is quite right. You need more than my speculations.

Re: Antenna gain in CST.

Heya faculty3 - as volker_muehlhaus pointed out, CST returns a gain relative to an isotropic radiator.

Direct from the help file [Farfield overview]:


Personally, I find the 'realized gain' plot to be more useful, since it includes the effect of the input feed. There's no point getting excited about potential directivity if you can't couple power into your antenna!

Re: Antenna gain in CST.

thylacine1975 said:

Heya faculty3 - as volker_muehlhaus pointed out, CST returns a gain relative to an isotropic radiator.

Direct from the help file [Farfield overview]:


Personally, I find the 'realized gain' plot to be more useful, since it includes the effect of the input feed. There's no point getting excited about potential directivity if you can't couple power into your antenna!

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Thylacine1975,
Thanks for this information on Gain in CST. What is your opinion on efficiency. At times it is less than 1 (e.g. 0.86), where as some times it is more than 1 (e.g. 1.6873).

Re: Antenna gain in CST.

faculty3 said:

What is your opinion on efficiency. At times it is less than 1 (e.g. 0.86), where as some times it is more than 1 (e.g. 1.6873).

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I am not a CST user, but in other tools you can cause strange efficiency and directivity results if you don't define the integration area (theta and phi sampling angle) properly. These need to be set carefully, so that the "total radiated power" is sampled over the correct area, without missing some area, and without sampling it twice. One popular mistake is to integrate over Theta = -180° ... +180° and Phi = 0° ... 360°, which effectively samples the sphere twice.