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# Req: Antenna Gain measurement

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#### Rayengine

##### Full Member level 5
antenna gain measurement

Hi All,

How can we measure the antenna gain in-house ? Assume we only have reference dipole, signal generator and spectrum available only measurement.

Rgs

Rayengine

real problem

You have to avoid several problems. The first is reflections. One way is to use the tops of two adjacent buildings. Also use a gain antenna at the other end of the path from the antenna you are to measure. (This cuts out reflections). First you measure the signal strengh with a half wave dipole then with the antenna you want to know the gain of. Do a sanity check against theory. Even commercial firms have trouble measuring gain accurately.

As Flatulent has said, you need to use good directional antenna in other end. Also you need enough distance between antennas - it will be good to have 15 or more wavelength between antennas. Another factor is a height over ground - better to rise antennas more than 3 wavelength over ground or roof - locate dipole and testing antenna at the same height anyway.

purcell method

there is a also an antenna gain measurement method called Purcell method which is quite simple. Briefly it consist on measuring the antenna input match of the antenna when looking at a ground plane placed in front of it at several distances. From the set of points you can trace a line and some parameter of the line (the slope or the ordinate at the origin) is proportional to the gain. I dont remember exactly the method but i tried it once, and you can make a search on the IEEE or the electronics letters with "purcell" keyword. However, for low gain antennas such as your dipole the uncertainty may be large

Antenna gain measurement

During a seminar on RF measurement, a relator told about a method involving three antennas; first you measure the signal coming from couple antennas A+B, then B+C and at the end A+C, with the corrections and what all the other people already said ( height of the antennas, distance between the antennas, ecc. )
Then you make three equations with three unknow data ( the gain of the three antennas, because you do not have any reference isothropic radiator ).
You solve the three equations and you will have the absolute gain of the three antennas.
P.S. I have never tested it because we have calibrated antennas and field, but anyway I think is worth the test.

Mandi

measure antenna

Make sure the polarisation is not in the direction of the ground....It may induce a lot of errors...

I search with Google and found that there is a article in ARRL web site with the title that exactly what I want? But it restrict to member only! Any have this article on hand?

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