I have measured antenna data in far field and I want to analyze it using matlab. I have theta and phi values of E field for a particular frequency. How can I calculate Gain of an antenna with the help of this data. So far what I know that I can calculate total E by adding these two components and then squaring it and integrating it over a sphere will give me radiated power. But for antenna gain calculation you need input power information which I dont have. Also if I suppose that data is already calibrated internally from the instrument, does that mean that by finding maximum value from Etotal will be the maximum gain of the antenan ?
Dear Muhammed,
The antenna gain is a measure of he directivity D of the antenna and its power efficiency Eta . The directivity D is the ratio of the solid angle of the isotropic radiator to the radiation solid angle of the directive antenna. To calculate the radiation solid angle one would divide the total radiated power by the maximum power density of the radiator Emax^2. This some effective radiation angle of the antenna. Assume this is termed Omega, then the directivity gain factor of the antenna will be 4 Pi/ Omega. One can calculate the antenna efficiency Eta by calculating the input power delivered to the antenna to deliver such radiation power. The ABSOLUTE VALUE OF THE RADIATED POWER CAN BE CALCULATED BY DIVIDING YOUR INTEGRATED VALUE BY THE CHARACTERISTIC IMPEDANCE OF THE AIR Za,
In this way one can calculate the gain of an antenna.
For more information please follow the link:http://www.antenna-theory.com/basics/gain.php
Best wishes
The gain of the antenna is a relative value measured against a standard. In antenna gain measurements, the gain is with respect to the gain of the standard gain horn. Try to get the similar values for the standard gain horn and with the calibrated gain sheet of the horn , calculate the gain of your antenna
If you are talking about transmitted gain, you MUST know the total power input in order to calculate the real gain against the "standard", as correctly identified by Ratan Sanjay Doma above.
However, if the situation were such that you could be conficdent that there were no losses such as near field coupling with an absorbing medium such as the earth or nearby resonant materials, then you could simply add up the measured powers over an approximation of the shape of the output field profile to obtain that total power and hence determine the gain. For absolute calculations of the gain of course, you need to be sure that the power supplied to the antenna input terminals is through properly matched - usually of course, 75 ohm, 300 ohm or 600 ohm feeders to prevent reflective losses at the input which will in general add to power losses not directly connected with the antenna characteristics, but losses just the same, which of course cause a deterioration in the real value of the "gain"..
Dear Muhammed,
The antenna gain is a measure of he directivity D of the antenna and its power efficiency Eta . The directivity D is the ratio of the solid angle of the isotropic radiator to the radiation solid angle of the directive antenna. To calculate the radiation solid angle one would divide the total radiated power by the maximum power density of the radiator Emax^2. This some effective radiation angle of the antenna. Assume this is termed Omega, then the directivity gain factor of the antenna will be 4 Pi/ Omega. One can calculate the antenna efficiency Eta by calculating the input power delivered to the antenna to deliver such radiation power. The ABSOLUTE VALUE OF THE RADIATED POWER CAN BE CALCULATED BY DIVIDING YOUR INTEGRATED VALUE BY THE CHARACTERISTIC IMPEDANCE OF THE AIR Za,
In this way one can calculate the gain of an antenna.
For more information please follow the link:http://www.antenna-theory.com/basics/gain.php
Best wishes
Thank you for your reply's. I have now some further question. If I get Etheta, Ephi and total gain values as an output for each angle in elevation and azimuth from measuring instrument will that mean combining Etheta and Ephi will not give me power pattern but its instead gain pattern. Is this true ? In simple words if I try to find maximum value from Etotal this is will give me relative gain value or not.
In reality you will have measured both. The term gain however, relates to the relationship, (the ratio) between the maximum power in a given direction, hopefully the direction in which you want the signal to go, and the standard, which strictly speaking is taken as a sphere. i.e. the standard referred to in earlier replies to your question, is defined as the power in any direction the power at the input to the antenna were radiated uniformly in all directions.
Just trying my math skills if Antenna Gain is defined as the independent variable G then; G=4 Pi f2 Ae / c2 where Ae is the Effective Area, f= carrier frequency and C is the speed of light.
Hello Muhammad,
I would like to add a little to what has already been said by our previous experts.
The antenna gain is always referred with respect to that of an isotropic radiator. This antenna is assumed to radiate equal power in all directions i.e. the sphere.
For practical purposes, it is always better to have such reference antenna and then compare the response of your ANTENNA UNDER TEST.
As you are determining the above values (data) for your practical antenna, why don't you also try to construct one such reference antenna and then compare the results.
You can find some useful construction tips on ARRL website or for that matter the ARRL Antenna Book.
Secondly, to know the exact RF power input at the antenna you can use a RF power meter so you can also validate your calculations.
Thank you everyone for your kind response. To add more clarification to my problem. Actually I didn't do the measurements myself. I obtained the measured data from someone which I don't have any control and I was told that this data is already calibrated which means it was already compared to some standard. So I thought if I just search for some maximum value from (total electric field) it may give me relative maximum gain value. But of course to find absolute value I need to take account other loss factors also.
A useful definition of the "gain" of an antenna is of course, as you would all know, the ratio of the maximum intensity of radiation from that antenna (hopefully in the required direction!!!) to the intensity of an isotropic radiator which is an hypothetical one that radiates uniformly in all directions. From the point of view of a receiver, the gain can be obtained from your calculations of the intensity in all directions, from which you can simply add to obtain the total power represented, after multiplying each intensity by the element of area over which it is estimated to cover, the accuracy depending on the size of the individual areas used. The power obtained is that for the equivalent omnidirectional (hypothetical) antenna, the power per solid angle being that obtained by dividing by 4*Pi.
The relative gain is then simply obtained by taking the maximum intensity you have calculated from the radiated relectric field (I=1/2 c*Epsilom*E^2) and determining from this the power per unit solid angle for comparison with that from the isotropic radiator.
For a transmitting antenna you need to know the power input at the antenna terminals which if the transmitter is perfectly matched to the antenna in terms of impedance, then the measured intensity in the direction of the required gain will provide the answer. In many cases, there may also be other loses besides those at the input, which will reduce the measured intensity below that of its theoretical value. Typical of this are low slung - or not so low slung - HF antennas from which the ground below will absorb considerable energy (or power!) unless the ground plane is a perfect conductor or nearly so. Ideally, this may require placing a copper mesh over the ground below the antenna.
John Nicol.
how to calculate Directivity and gain of an antenna using S-parameters.
- Badges
- Science topic