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how to measure antenna impedance ?
On Sun, 04 Dec 2005 15:25:51 +0100, Paul Burridge
k wrote: Say I have a mica capacitor (for example) that I want to check for its SRF. How should I mount this component so as to minimize stray L&C from anything other than the component itself? IOW, what 'platform' (for want of a better word) do I need to construct to permit accurate measurements of this cap's RF characteristics in isolation? Hi Paul, Accuracy and precision is no good unless you can duplicate the test rig to the eventual environment of use. That said, precision capacitors and inductors are three leaded devices. The third lead goes to the shield around them. Obviously for either, a shield changes what would have been the nominal value for the component. However, that change also swamps all the variables that could disturb the accuracy. In other words, the shield enforces a fixed environment that reduces all other stray influences to a minimum. In so doing, I've been able to measure standard capacitors and inductors out to 9 places. Without those third lead configurations, the same components would easily lose 3, 4, or 5 of those digits. So one way to mount a mica cap would be over and close to a ground plane that extends beyond its foot print by a significant distance. This proximity would swamp the effects of other components nearby causing a shift in the resonance (if and when they were added, or removed). Building a cage around the capacitor would reduce these effects even further. Of course, all such measures would shift the native resonance, but you are never going to achieve that frequency anyway. You can, of course, elect to go the other way with a minimal ground proximity. In that case you would use microstrip techniques to build the test rig, making the strip with equal to the width of the component (presumably being surface mount). However, SRF becomes rather meaningless except as a general indicator. This is because changing the board material from alumina to epoxy; or changing from a series to shunt application can shift this frequency by 20% to 40%. Another issue is with the leads themselves. ESR for caps can easily tally up to a tenth of an Ohm and you have to select your caps on this basis as much as for their inductance. In this regard, you measure the D of the cap (dissipation factor) not Q (although each is the inverse of the other, there are D instruments specifically for this). This tenth Ohm is NOT necessarily in the wire lead (a common misconception) but rather in all the parallel (or worse, series of the wrapped cap) plate connections. For surface mount caps, you may want to mount them 90° (up on edge rather than flat on face) to the board to double the first PRF resonance and reduce the insertion losses there and above. The short answer to your question is how stable, and how accurate do you want to reproduce the measurement to your application? 73's Richard Clark, KB7QHC |
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