A base station diplexer should be judged by trade-off logic, not by one isolated target
In real base station projects, diplexers are often introduced because the system needs feeder sharing, co-siting support, or a more controlled installation footprint. That immediately turns the discussion into a balance problem: the project wants compact hardware, but it also wants clean electrical behavior under real deployment conditions.
The engineering question is therefore not only whether two paths can be combined. It is whether the chosen cavity envelope still leaves enough room to achieve acceptable insertion loss and isolation for the site architecture that the project actually needs.
Compact cavity targets usually create electrical pressure elsewhere
Base station teams often want a smaller diplexer because the site has limited space, the feeder path is already crowded, or the installation logic becomes easier when the hardware envelope is reduced. Those are valid project drivers, but they do not remove the underlying RF design burden.
When the cavity becomes more compact, the available design margin typically becomes tighter as well. The engineer has less room to manage coupling behavior, rejection shaping, energy flow, and physical layout at the same time. That does not mean compact designs are impossible. It means the project should expect a harder trade-off discussion rather than assuming all targets will remain equally easy to hold.
Insertion loss and isolation are often the clearest signs of the size trade-off
In practice, insertion loss and isolation are usually where the consequences of cavity pressure become easiest to see. If the envelope is pushed tighter, the design often becomes less forgiving. That makes it harder to keep transmission loss low while also maintaining strong separation between the combined paths.
For engineering teams, the important point is not to chase an abstract "best" number in isolation. It is to decide which electrical behavior matters most for the actual system. In some projects, every fraction of insertion loss matters because the link budget is already tight. In other projects, isolation becomes the harder boundary because the co-siting environment is aggressive and the unwanted interaction risk is high.
This is why a diplexer should not be reviewed only as a compact mechanical part. It should be reviewed as an RF structure whose size directly affects how comfortably the loss and isolation targets can be met.
Good engineering input defines where the compromise should sit
Before an RFQ or design review gets serious, the team should decide which constraint is truly leading the project. Without that, the diplexer target can become self-conflicting: very compact, very low loss, very high isolation, and easy to install, all at once, without a clear ranking of priorities.
A better engineering process starts by stating the project limits clearly enough for the trade-off to be judged openly instead of discovered too late.
For base station diplexers, compact size only creates value when the electrical compromise still makes sense
A smaller cavity can help installation and integration, but in a comparable design target it usually makes it harder to hold low insertion loss and high isolation together. The right engineering decision is therefore not "make it as small as possible." It is "set the right balance for the actual project constraint."