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How could it be 28~30dBi gain? Can anyone explain please!

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coolpixs4

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How could it be 28~30dBi gain "Model: GJX-698-2700-30"? Can anyone explain please!
I search for 915MHz high gain antenna and found this:

Does it means with the max input power +/- 2dBi antenna gain ? (lowband : -2dBi, highband: +2dBi max)
Because the omnidirectional antenna with a very simple structure like such kind of this one without a reflector or built-in amplifier; I wonder how it gets 30dBi antenna gain?

check the attached image for adv-specs & the broken inner of the antenna

I did a rough measurement with lab equipment with our 6.5dB gain horn
(SGA-TX-19dBm, distance: 1m) and observed by SPA -----9dBm RX

by calculation => -2dBi @920MHz (it seems correct for Omni-direction dipole antenna)
 

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30dBi omnidirectional gain!?
Do a substitution test (idealy in an anechoic chamber) using a reference dipole.
 

they are lying.
it might have SOME gain by not receiving straight up and down, and not receiving below the ground plane (where the connector is attached). but more like maybe 5 dbi gain, not 30.

i looked to see if there was an amplifier added inside, but did not see anything implying they got effective gain by that method.
 

They are lying. If you look elsewhere for high-gain omnidirectional antennas you will see them advertised in the range of 5dBi - 7dBi gain, but not 30dBi gain.

If you look at the Wikipedia page for omnidirectional antenna, they cite an IEEE paper (McDonald, Noel) that gives an analysis of the omnidirectional radiation patterns directivity vs the half-power beamwidth based on the assumption of the omnidirectional pattern shape. They got D = 10log_10(101.5/(HPBW - 0.00272*HPBW^2)). If we assume perfect antenna efficiency then we can plug in G = D = 3dBi, 5dBi, 7dBi, and 30dBi to get the HPBW for each radiation pattern and an idea of what it will look like. You'll notice that the 3dBi HPBW is ~60deg, 5dBi HPBW is ~35deg, and 7dBi HPBW is ~21.5deg... which all look like normal omnidirectional radiation patterns, donut-shaped with reasonable sized donuts. However, the 30dBi HPBW is ~0.102deg, meaning it's radiation pattern looks like a CD instead of a donut. In reality you can't realize a 0.102deg HPBW omnidirectional antenna, but it's a fun exercise to try and visualize it.
 

Such "typos" can often be found in cheap antenna advertising. Fake numbers meant to impress customers who have no clue and just look for the "best" specs.
 

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