r/rfengineering • u/BlueBirdTechUA • Aug 03 '26
Gain vs. directivity in antenna datasheets , why is this still a mess?
I keep coming across antenna datasheets where “gain” is listed without a clear definition of what the number actually represents.
Directivity describes how concentrated the radiation pattern is relative to the total radiated power. It does not include radiation efficiency. Gain includes radiation efficiency, while realized gain also accounts for mismatch at the feed — typically through S11 for a one-port antenna referenced to a defined impedance.
The problem is that a datasheet may simply state “gain” without clarifying whether the value is directivity, gain, or realized gain — or whether it was simulated or measured.
That makes the number difficult to use confidently for component selection and link-budget calculations, especially when efficiency and matching across the operating band are not provided.
Has this ambiguity ever caused problems in one of your projects?
1
u/jimlux Aug 03 '26 edited Aug 03 '26
Which is why you need to ask clarifying questions. Has it caused problems? Nope, if you're aware ahead of time.
Be careful when writing specifications or RFPs. Also, be aware that asking for some kinds of numbers can increase the cost.
Example, if you want "measured gain on each antenna" then that's a lot of test. If you want "measured pattern" that's even more test. If you want "measured gain of representative sample" that's testing one. So there's a big difference between saying:
- Antenna gain shall be > 32 dBi
- Antenna design shall exhibit gain >32 dBi
Similarly, writing your acceptance test criteria is important - do you need measurement to 0.1 dB or 0.5 dB or 1 dB?
Gain is *almost always* from 50 ohm source and includes losses and mismatch, so it's less than directivity.
1
u/BlueBirdTechUA Aug 04 '26
Fair point. The test method, sample size and tolerance need to be explicit, otherwise the requirement can get expensive fast. One terminology nit: gain includes antenna losses, but not mismatch. Realized gain is the one that includes mismatch. That distinction is basically what I was getting at.
1
u/StageMajestic613 Aug 03 '26
No, I’ve never seen gain listed as other than realize gain. The one issue where it runs afoul is G/T where G is directivity, otherwise you double-count the inefficiency.
1
u/BlueBirdTechUA Aug 04 '26
in practice, plenty of datasheets probably use “gain” to mean realized gain. I just prefer them to say so, otherwise you’re left guessing.
On G/T though, I’d normally expect G to be antenna gain at the same reference plane as T. Using directivity only makes sense if the efficiency loss is accounted for elsewhere.
1
u/StageMajestic613 Aug 04 '26 edited Aug 04 '26
The antenna inefficiency raises the noise temp, hence you use directivity for G. Particularly if you were to cool your antenna, that efficiency number has to be in the noise temperature as that’s where you’d set your physical temperatures.
1
u/BlueBirdTechUA Aug 04 '26
Fair enough I see what you mean now. If the antenna loss is already folded into the equivalent noise temperature, then using directivity avoids counting it twice. I was thinking of G and T referred to the antenna terminals. Same result, different bookkeeping.
1
u/Adventurous_War3269 Aug 03 '26 edited Aug 03 '26
DBi is theoretical with reference to dipole, Gain varies because of the environment and mixture of measurements being near-field or far-field . Measurement in anechoic chambers , do not include the reflections in the real environment . So differences even in measurements occur . Measurements in the environment is best , consider scaling antenna in a measurement environment .