Directional coupler

01

Understand coupling, directivity, isolation and insertion loss

Microwaves101 describes a directional coupler as a passive, reciprocal four-port network with input, through, coupled and isolated ports, realised in microstrip, stripline, coax or waveguide, and used to sample signals, including incident and reflected waves in a reflectometer. For input power P1 and port powers P2–P4 it defines insertion loss = 10 log(P1/P2), coupling = 10 log(P1/P3), isolation = 10 log(P1/P4) and directivity = 10 log(P3/P4); in decibels isolation equals coupling plus directivity. [1]

It explains that multi-hole waveguide couplers couple in the forward direction while microstrip and stripline couplers are backward-wave couplers, that any coupler is bi-directional unless a port is terminated, and that a dual-directional coupler places two couplers in opposite directions with internally terminated isolated ports as the basis of a reflectometer. Finite directivity causes reflectometry errors: 40 dB directivity gives very small error, while 20 dB may be unacceptable. [1]

Editorial calculation: a 10 dB coupler samples 10% of the input power at the coupled port, leaving about 90% (about 0.46 dB) for the through path before other losses; with 18 dB directivity its isolation is about 28 dB. [2] [1]

Give the supplier the frequency band, coupling value and tolerance, flatness, minimum directivity, maximum insertion loss, VSWR, impedance, average and peak power, single or dual-directional form, connector or waveguide flange, temperature range and size limits.

02

Read band, loss, VSWR, power and connectors from the datasheet

Keenlion lists its KDC-0.5/8-10S directional coupler for 500–8,000 MHz with coupling of 10 ± 1.5 dB, insertion loss ≤1.2 dB, directivity ≥18 dB, VSWR ≤1.3:1, 50 Ω impedance, 20 W power handling, 0 to +50 °C operation and SMA-female connectors, and offers coupling values from 10 dB to 18 dB in the series. [2]

Editorial calculation: 500–8,000 MHz is a 16:1 (four-octave) bandwidth; VSWR 1.3 corresponds to a return loss of about 17.7 dB; 20 W into a 10 dB coupler puts about 2 W on the coupled port. [2]

Mini-Circuits' ZUDC10-83-S+ shows how a maker separates coupling tolerance, flatness and temperature effects. For 0.3–8 GHz it lists nominal coupling of 10 ± 1.0 dB, coupling flatness of ±0.8 dB typical (±1.2 dB maximum), mainline loss of 0.9 dB typical and 1.3 dB maximum (excluding the theoretical coupling loss, and degrading to 1.5 dB at 85 °C), directivity of 24 dB typical and 20 dB minimum to 6 GHz and 22/18 dB from 6–8 GHz (minimum 16 dB at 85 °C), return loss on all ports of 24/20.8 dB to 6 GHz and 22/17.7 dB above, 20 W input power, a 1 A DC pass from input to output, an internal 50 Ω termination and −55 to +85 °C operation. [3]

Editorial calculation: with 18 dB minimum directivity, a load reflection 18 dB below the forward wave arrives at the coupled port at about the same level as the forward sample's leakage, so return-loss measurements better than about 18 dB cannot be trusted near 8 GHz with this part; at 85 °C the limit falls to about 16 dB. [3]

03

Compare what suppliers offer

The suppliers linked to this page list directional couplers; their cards link to the pages reviewed.

Ask any supplier for measured S-parameters across the band, coupling and flatness, directivity, insertion loss, VSWR on every port, power rating (average and peak), connector type or waveguide flange, temperature range, dimensions and a test report.

04

Size coupling and power rating for high-power monitoring

Werlatone lists its C10117 dual directional coupler as a high-power design with 40 dB coupling across 700–6,000 MHz, rated at 250 W CW, with less than 0.2 dB insertion loss and better than ±1.0 dB coupling flatness. Its specification table gives VSWR of 1.30:1, directivity of 15 dB and a size of 2.0 × 2.0 × 1.06 in. Werlatone rates the part Mismatch Tolerant® and states that it will tolerate a full reflection at rated power. [4]

Editorial calculation: 250 W is about +54 dBm, so 40 dB coupling puts about +14 dBm (25 mW) on the forward coupled port, and a full reflection puts about the same on the reverse port; check that level against your detector's input rating. An insertion loss of 0.2 dB means about 4.5% of 250 W, or about 11 W, is lost in the main line. With the Microwaves101 definition, 40 dB coupling plus 15 dB directivity gives about 55 dB isolation. [4] [1]

Editorial note: the 15 dB directivity listed here is below the 18–24 dB of the 20 W examples above, so ask for measured directivity across the band if the coupler will be used for return-loss measurement. [4] [2] [3]

05

Trace what each part does

This is a functional reference built from the sources cited; it is not one maker's specification.

06

Fill the specification fields with values for your order

A directional coupler quotation needs values for your system. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the band, coupling and power data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Band: [GHz]
Coupling: [dB; tolerance; flatness]
Directivity: [minimum dB]
Loss and match: [insertion loss; VSWR]
Power: [average; peak]
Form: [single; dual-directional; coax; waveguide]
Connectors: [SMA; N; flange]

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