How an antenna is "tuned" to a frequency

Wavelength sets the size

Radio waves travel at the speed of light, 299,792,458 m/s.^[1] The wavelength is that speed divided by the frequency (λ = c / f).

150 and 450 MHz rows: calculated from λ = c / f.^[1] Other rows: TI Application Note AN058, Table 1.^[2]

So a 450 MHz antenna is about a third the size of a 150 MHz one, and it will not match a 150 MHz radio. Lower frequencies reach farther with the same power and antenna gain, but need a bigger antenna.^[2]

Half wave and quarter wave

• Dipole: usually half a wavelength (λ/2) long. It radiates most strongly broadside, at right angles to the element.^[2]
• Ground plane (monopole): usually a quarter wavelength (λ/4). The ground plane acts as the other quarter, so the antenna depends on it.^[2] ComAnt's quarter-wave whips have an "optimized impedance match with reduced size ground plane"; its half-wave whips need "no ground plane". (ComAnt catalog, pp. 37 and 43)^[3]
• Collinear and stacked arrays: several elements combined for more gain in a narrower vertical beam. ComAnt's offset-pattern collinears run from 8 to 11 dBi (CAC2, CAC3, CAC4). (ComAnt catalog product list)^[3]

Bandwidth: how far from its frequency an antenna still works

An antenna's usable bandwidth is usually taken as the range where its VSWR stays at 2.0 or better (return loss 9.5 dB); a VSWR of 1.5 (return loss 14 dB) is a good match.^[2] Most ComAnt datasheets give VSWR under 1.5 across the stated range.^[3]

That is why CompleTech builds each model in sub-bands: the CAB2 broadside, for example, comes in 14 versions from 83–88 MHz to 880–960 MHz. (ComAnt catalog, p. 20)^[3] In CompleTech's words: "Integrated impedance compensation makes the thin radiator behave like a fat and thus a broadband one with excellent VSWR characteristics." (ComAnt catalog, p. 1)^[3] Choose the version whose range contains every frequency your system uses, transmit and receive.

GNSS is different

GNSS satellites transmit right-hand circularly polarized signals (GPS L1 at 1575.42 MHz), so GNSS antennas are built for that polarization and those frequencies.^[4]