We Tell You How to Specify the Best Directional Air Quick Coupler

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We Tell You How to Specify the Best Directional Air Quick Coupler

Posted By snip snip     December 2, 2020    

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Over the years many different techniques have been utilized to bring about the physical realization of directional Air Quick Coupler . The scope of this article is limited to discussion of quarter-wavelength, coaxial, directional couplers, as this type of coupler represents a balanced compromise between functional range, performance and cost for most broadband applications covering up to an octave bandwidth.

The basic directional coupler is a four port junction that is used in a wide variety of microwave systems to satisfy almost any requirement for sampling incident and reflected microwave power conveniently and accurately with minimal disturbance to the transmission line.

The basic configuration of a single directional coupler is shown in figure 1 which illustrates two parallel transmission lines over a length of one-quarter wavelength, corresponding with the center frequency of operation. The main and secondary lines are separated by a calculated physical distance which determines the coupling factor of the device. The physically closer the lines are to each other, the more power will be introduced on the secondary line The term coupling denotes how much of the input power is sampled to the coupled port and is defined as 10 times ratio of Incident Power and Forward Power C = 10 log10(Pf/Pi). Typical coupling values found in practice are 3, 6, 10, 20, 30, 40 & 50 dB; however, practically any coupling value may be obtained through proper design.

A dual directional coupler is essentially two single directional couplers connected back-to-back sharing a common mainline and providing two output ports with high isolation between those ports. This high isolation is critical for the accuracy of reflectometer set-ups that simultaneously sample input power to a device or load providing a ratio of signals for the purpose of determining return loss in decibels.

Since we are on the subject of measurement errors, we should also deal with the importance of Voltage Standing Wave Ratio (VSWR) because reflections will add and subtract to the incident signal causing uncertainty in the coupling factor. VSWR is defined as the ratio of incident to reflected signals and is ideally 1.00:1, meaning these signals are in phase and will not cancel. The better the VSWR, the less return loss is encountered. Unsatisfactory coupler VSWR will degrade measurement accuracy and is usually attributable to lesser quality connectors or inadequate design techniques.

The frequency sensitivity or “flatness” of a coupler is a measure of how coupling varies over a given frequency range. Optimum coupling frequency response is achieved by “centering” the design within the specified band of interest. Typical coupling flatness for a quarter-wavelength coupler operating over an octave band is within ± 0.75 dB of nominal. All things being equal, stronger coupling factors (3, 6 & 10 dB) exhibit greater flatness than weaker coupling factors (20 through 50 dB). When operating over frequency bands greater than an octave, the flatness tolerance may need to be relaxed due to the inherent characteristics of coupling roll-off.

Another important consideration when specifying a coupler is to ensure the device has minimal mainline insertion loss. Through virtue of their design, coaxial air-line couplers offer the lowest possible loss when inserted in a transmission path. Generally, the insertion loss of a coupler (or any microwave device for that matter) becomes more significant at higher frequency, namely because loss increases with frequency and higher frequency power sources are considerably more expensive. Accordingly, the criteria of low insertion loss will prevent precious power from being wasted on measurement components.

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