On Sat, 28 Aug 2010 11:59:21 -0700 (PDT), Wes Stewart wrote: >Equally silly (and applicable here) is measuring a device in a 50-ohm >environment and trying to determine attenuation, when in actual operation, >neither the source or load impedance is known.
Exactly. It is the 50 ohm environment that is the source of much of the measurement error. Measuring S21, by one means or another, is an effective solution, and the one I've used. Measuring shield current, as David has suggested, is an indication of the effectiveness of a choke IN A PARTICULAR CIRCUIT -- for example, a feedline onto which it is inserted, the antenna to which the feedline is connected, and the termination of the feedline at the receiver. The effectiveness of a choke in any given application depends upon the common mode impedances within that particular circuit. These are two VERY different problems, and two very different measurements. Both are useful. Measuring the choke on the bench tells us about the properties of the choke. Knowing that, we can optimize the choke for a given frequency range and use, AND we can insert the properties of a choke into a model and, if the model knows enough about the circuit of interest, predict the effectiveness of the choke in reducing common mode current. There is, of course, another set of unknowns. They relate to the source(s) of the noise, their location, polarization, directivity, proximity to the feedline and to the antenna, the directivity of the antenna, etc. The choke will have little effect on the component of noise picked up on the antenna -- it can suppress noise picked up only on the feedline. In general, the choke will be most effective in suppressing noise that is much closer to the feedline than to the antenna, and/or is rejected by antenna directivity. 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
