I'm interested in phase shifters and their applications to the antennas. Generally I found a 90 degree branchline coupler to provide necessary shift. But I wonder what determines their bandwidth.

Take an example below, if all dimensions are calculated with theoretically depends on the frequency here, then how to make it broadband (1 octave in VHF band)?

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  • \$\begingroup\$ What do you consider "broadband"? 1/5 of an octave? 8 octaves? \$\endgroup\$
    – The Photon
    Feb 25, 2021 at 19:06
  • \$\begingroup\$ @ThePhoton 1 octave in VHF band \$\endgroup\$
    – murataktas
    Feb 25, 2021 at 19:13

1 Answer 1


Such couplers depend on lines of accurate electrical length ie quarter wavelength. So such designs are inherently narrowband, perhaps 15-20% fractional bandwidth. Visualise these working, by thinking the path the incident wave will take to anyother port via all possible routes. You will see that this sort of coupler essentially relies on interference and hence accurate phase shits. This sort of behaviour is narrowband.

Check out tapered coupled line directional couplers which are very broadband but bulky.

For truly ultra broadband but lossy performance, nothing beats a resistive Wheatstone bridge based directional coupler.


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