Hi anum_pirkani
All quarter-wave resonant combiners have some kind of limited bandwidth.
The very nature of
any splitter that uses trick quarter-wave resonant line lengths in its structure is going to be limited in bandwidth in some way. Where you see attempts to mix or add a several of such matching sections, sized to favour other frequencies to cover a wider band, it usually also involves more insertion loss, multiple peaks in the return loss and pass characteristic.
There are some great designs, and also, some Wilkinson-like arrangements have bands wide enough to be very useful, but the bandwidths in quarter-wave resonant networks will always be limited by the fact they are, inherently, quarter-wave.
Do invoke the search on this forum for the previous thread on Wilkinsons.
It was deeply discussed, and for me, it taught me several key points.
1. The addition of a resistor between the outputs brings together ports that are 180 degrees out of phase, loaded in a pseudo-terminated way, akin to a termination across a balanced line, and so they will cancel. This means that for signals at the split ports that are in phase, there is
isolation. If re-combining signals that were previously split, and amplified, this is perfect. Also good for combining signals from multiple antennas, the coherent signals coming from a single source can be combined without some being re-transmitted, and yet be matched.
This property decides at the outset whether the Wilkinson is OK for the application. If it has to combine separate unrelated signals, it will not have the isolation property, and you may have to use something else.
2. The multi-stage, multi-terminated Wilkinsons can offer stretched bandwidths, with some trade-off, but it brings about a layout issue. The need to "bring together" the branches to meet the resistor, and the fact you cannot have "zero length" resistors, forces characteristic multiple semi-circles or squares. This is where the closely related
"Gysel" variant that splits the resistor into two unbalanced terminations (of a sort), gets around the problem.
3. The Wilkinsons, via Gysel, morph into the
"Rat-Race" class of combiner-splitters. Just like anything that depends on quarter-wave features, they have limited bandwidth, which is often good enough if the frequency is high enough. Just like Wilkinsons, coherent in-phase signals are necessary if used as combiner. If as a splitter only, there is no problem.
4.
Regular quarter-wave splitters. These are very well known. Essentially, the input has to be matched to some impedance at the junction which is the result of two branches coming together in parallel. Then, for each branch, they need their own quarter-wave transformer sections to make their ports also match to the impedance seen at the junction. The junction impedance cannot be the same as that of the ports. The impedance of a quarter-wave transformer line between them is usually the geometric mean of the impedances at the ends.
The splitter in your picture looks like one of these.
5.
Wider band things
Various forms of coupled line. The
Lange Coupler comes to mind here. Usually, there is some attempt by various tricks to equalise the odd and even mode impedances of the coupled section. Coaxial sections, with magnetics added to inhibit currents on the outside of the shield, with balun tricks can yield a splitter-combiner with more than an octave of bandwidth, and impressive performance. See the pictures of the topside and underside of a older technology ANZAC H-8-4. It works from about 900MHz to 2GHz. Not easy to implement in stripline or microstrip, but great for when a Wilkinson just will not do.
P.S. If your layout version fails, then maybe the substrate definition is different to the circuit input version.