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<body>I am wondering why the documentation on Pure Signal implies the use of a directional coupler as necessary to obtain the required sample of the transmitted RF. I can understand why that could be desirable when the feedback was not adaptive, or where the sampler is remote from the final amplifier.. However when the predistortion is being generated adaptively I do not understand why a simple resistive non directional coupler would not be perfectly satisfactory and much easier to implement. The antenna system should be quite linear and the voltage sensed adjacent to the final amplifier output should surely always be a valid measurement of the transmitter transfer function. Of course a directional coupler is always useful to measure VSWR and is usually already present in modern amplifiers, although often not readily accessible for use with Pure Signal.<div><br /></div><div>I have implemented Pure Signal on an ANAN 10, Juma 1000 PA combination and used both the published directional coupler design and a simple resistive pick off at the PA output with equally excellent results.</div><div><br /></div><div>What am I missing?</div><div><br /></div><div>Dick, W0QM, G3FNL </div></body></html>