Couple of years ago I have done SWR vs frequency sweeps of two antennas through simulation software. The sweep was done from 100 MHz to 2500 MHz.
I used models of following two antennas for this sweep:
(1) A ΒΌ π ground plane antenna (Cantenna)
(2) A Wire Collinear antenna similar to Flightaware antenna
The frequency sweep results of a Cantenna (π/4 ground plane antenna) show that it gives low SWR at and near its designed frequency only, and at out-of-band frequencies its SWR is very high. Hence the ΒΌ π ground plane antenna picks much less out-of-band signals.
The frequency sweep results of the Wire Collinear antenna show that its SWR is good at multiple off-band frequencies. This necessitates a tuner circuit or a filter at front end (i.e. between antenna and receiver).
(1) CANTENNA (1/4 Wavelength Ground plane Antenna)
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(2) WIRE COLLINEAR ANTENNA (π/2 - phasing coil - π/4 - sleeve)
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