RF Utilities
Port levels, coupler selection, the coupling and directivity and isolation relationship, and reflectometer measurement uncertainty in one utility built for systems integrators.
A directional coupler is one of the most useful passive parts in an RF system, and almost every job an integrator gives it is a level problem rather than a design problem. You tap a transmitter to feed a monitor, a power meter or a spectrum analyser. You inject a pilot or a second carrier into a running line. You put a dual directional coupler in front of an antenna to read forward and reflected power during commissioning. In each case the question is the same: given the coupler figures on the datasheet, what power turns up where, and is it the level you need.
The noIM3 Directional Coupler Calculator answers that question directly. It is scoped for systems integrators and does not touch coupled line design. There is nothing here about even and odd mode impedances or section lengths. Instead it takes the figures a datasheet actually gives you, coupling, insertion loss, directivity and isolation, and turns them into the port levels, the part selection and the measurement accuracy you work with on site.
Four modes cover the everyday work. Port Levels reports the power at the through, coupled, isolated and dissipated ports for a given input, with a live diagram. Select Coupling turns a target tap level into the exact coupling required and the nearest standard value. The solver reconciles the coupling, directivity and isolation relationship from any two figures. Reflectometer Accuracy shows how much a coupler directivity limits a return loss measurement, which is the difference between a VSWR reading you can trust and one you cannot.
Enter the input power and coupling and read the power at the through, coupled, isolated and dissipated ports at once, in both dBm and watts. Add the datasheet insertion loss for a real part and the directivity to place the isolated port. The power budget closes, so you can see exactly where the input power goes.
Because a coupler taps power off the main line, the through port loses that fraction even with no dissipation. The tool computes the theoretical minimum through loss from the coupling, and when you enter a datasheet insertion loss it separates out the dissipative portion, so a tight coupler and a loose one are compared on the same honest basis.
Give the level you want at the coupled port, for example a safe input for a spectrum analyser or a power meter, and the tool returns the exact coupling required, the nearest standard value from 3 to 40 dB, the actual level that value produces, the margin against your target, and the through loss penalty of each option in a single table.
Isolation always equals coupling plus directivity. Datasheets often quote only two of the three. Enter any two and the tool solves the third, so you always have the full picture of a part before you specify it.
When a directional coupler measures antenna or feeder return loss, its finite directivity sets the accuracy. Enter the directivity and the reading and the tool returns the true return loss and VSWR band, and warns when the directivity margin is too thin for the reading to mean anything. This is the check that stops a good looking VSWR number from hiding a bad match.
A reference tab carries the core relationships, the standard coupling ladder with the through loss and coupled fraction each value implies, and the directivity margin rules of thumb, so a quoted figure can be placed in context without leaving the tool.
Runs entirely in your browser. No power levels, coupler figures or measurements are submitted to a server. Useful for commercially confidential work or any environment where information security policy prohibits sending engineering data to third party services.
Reach for the Directional Coupler Calculator in any of the following situations.
It turns the figures on a coupler datasheet into the levels you use in a system. Given the input power and coupling it reports the power at the coupled port, the through port, the isolated port and the power dissipated in the coupler. It also selects the coupling for a target tap level, solves the coupling, directivity and isolation relationship, and works out the return loss measurement uncertainty when a coupler is used to measure an antenna.
The coupled port power is simply the input power minus the coupling factor. A 10 dB coupler taps 10 dB below the input, so a plus 43 dBm input gives a plus 33 dBm coupled level. The tool reports this in both dBm and watts, along with the coupled fraction as a percentage of the input power.
Because the power that reaches the coupled port has to come from somewhere, and it comes off the main line. Even a lossless coupler therefore has a through loss equal to minus ten log of one minus the coupled fraction. It is small for loose coupling, about 0.04 dB for a 20 dB coupler, and significant for tight coupling, about 3 dB for a 3 dB coupler. A real coupler adds its dissipative loss on top, and the tool separates the two.
Coupling is how much power the coupler taps to the coupled port. Directivity is how well it distinguishes forward power from reflected power, which sets the quality of any measurement it makes. Isolation is the total loss from the input to the isolated port and always equals coupling plus directivity. A datasheet that lists any two defines the third, and the solver in this tool fills in whichever is missing.
The directivity leakage adds to the reflected sample, so the measured return loss could correspond to a range of true values. The band is set by the margin between the directivity and the reading. A 20 dB reading taken with a 25 dB directivity coupler could be anywhere from about 16 to 27 dB of true return loss. As a rule of thumb you want the directivity to exceed the return loss you are measuring by at least 10 dB, and the tool warns you when the margin is too thin.
Systems integrators and RF technicians who use off the shelf couplers in a signal path, rather than circuit designers who design the coupler itself. Everything here is about applying a coupler in a system: port levels, part selection, and measurement accuracy. It does not cover coupled line design such as even and odd mode impedances or section lengths.
No. The calculator runs entirely in your browser. No power levels, coupler figures or measurements are submitted to a server, which suits commercially confidential work and any environment where information security policy prohibits sending engineering data to third party services.
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