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Aaronia released this ultra-wide-range antenna, that can measure up to 35GHz. This was really impressive for only 3000 Euros, so I was considering purchasing it, when I figured I would look at the spec sheet to compare it with their previous model. And that's when I realized that, above ~23GHz, the antenna's gain is all negative!
PrintScreen of spec sheet

Does this mean that when I point the antenna toward the source of the signal, then the reading would actually decrease?

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    $\begingroup$ yes, I'd guess so – at higher frequency, the main lobe is probably just pointing somewhere else. $\endgroup$ Mar 16, 2019 at 10:22

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The vertical axis is in dB, which indicates a ratio. The literature probably mentions that it's the ratio of the antenna's gain to that of an isotropic antenna, or a dipole for the frequency in question (i.e. which is 2.15dB more). So a negative dB means the gain is less than that of a dipole or isotropic antenna. As far as directionality goes, I'd say this is the gain at the boresight. Again, the literature should make this clear.

EDIT: corrected gain of dipole to 2.15dB

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    $\begingroup$ And, more specifically, the vertical axis is the logarithm of a ratio. If the ratio is less than unity, the logarithm is less than zero, producing the negative gain in dB. $\endgroup$
    – Brian K1LI
    Mar 17, 2019 at 23:49

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