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Testing Antenna Gain or Loss

jan stevens
Prodigy 100 points

Hello,

I'm working with 433MHz and right now I want to test some antenna's and see what their gain or loss is in dB. So I use a function generator with a 0dB reference antenna (10mV signal) and a spectrum analyzer with the antenna I would like to test.

How do I create or get a 0dB reference antenna? Right now I have a bunch of antenna's but I dont know the gain or loss so its quite hard to determine what the 0dB reference antenna should be. Any other suggestions or help is always welcome :).

Kind Regards

  • 0
    Intellectual 480 points

    Assuming you have a sense of humor, this falls in the category of vast projects with half-vast ideas. A reference antenna is typically something standard like a quarter wave whip, or folded or unfolded dipole as opposed to some meandering circuit trace or ceramic antenna, but you pick it. Note that your 'standard' may not match the generator impedance and most certainly is not omnidirectional. These measurements are usually made in an rf anechoic chamber since reflections in your environment will dramatically affect the readings.

    Then there's the problem of the antenna under test matching the impedance of the spectrum analyser and the orientation of the antenna under test. Are you testing the antenna at just one frequency or a range of frequencies? 

    Here's a helpful link that should get you close http://www.ti.com/lit/an/swra161b/swra161b.pdf

  • 0 in reply to Dave Delage
    Prodigy 100 points

    Hello,

    I just want to have a general idea on what the gain is of a typical antenna, shouldn't be that precise. I've check the function generator and spectrum analyzer and I believe they both have input/output on 50 ohm. I also found an antenna that has a gain of 2dB and I'm planning to use this as the reference (since I know from the spec sheet what the gain should be).

    I'm only testing on 433MHz, another question, what should be my output voltage? And how can I calculate the gain or loss on my spectrum analyzer?

    Thanks!

  • 0 in reply to jan stevens
    Intellectual 480 points

    Create the setup being mindful of reflections, orientation and impedance matching on both ends with the same antenna on both ends (ie. the 2 dB reference antenna). Be sure you have a good clean measurement well above the noise, the absolute signal level is essentially irrelevant. Note the reading and subtract 2 dB to make your baseline 0 dB reference point. Swap in the new antenna. Read the result.

    Note that not many antennas have a 50 ohm impedance and that the impedance changes with frequency so matching, especially if you want peak gain at a single frequency, will be essential.

    To create a leaning experience, change the orientation of the antenna, put your hand near it, move some metal around in the signal path, move the ground plane and/or change the frequency slightly.