----- Original Message ----- From: "Jim Brown" <[email protected]> Cc: <[email protected]> Sent: Friday, July 13, 2012 4:37 PM Subject: Re: Topband: Mother of all ferrite common-mode coaxial chokes
> On 7/13/2012 12:09 PM, ZR wrote: >> ** Since when is using an established procedure called not knowing how >> to measure? > > When one does not understand (or take into account) the limitations of > the measurement method. The most common method of measuring Z at RF is > an S11 measurement in a 50 ohm system. This method has the limitation > that VERY small errors in the data, or VERY small calibration errors, > result in HUGE errors in the result when the unknown impedance differs > from the system impedance by more than about 5:1. The chokes he (and I) > were measuring have impedances on the order of 5,000 ohms, so an S11 > measurement will yield erroneous data. Thats correct at UHF and up but what is the error you have actually measured at 1.8 MHz? > > Quoting from Wes Hayward on the subject of measuring high impedances: > > "It is also possible to measure an L or a C attached as a load on a > bridge attached to the VNA. This is termed a "reflection" measurement. > The results are similar and remain equally difficult. The > severe errors of this direct method are discussed in Agilent > Applications note 1369-6. Much better measurements are obtained when > one uses a scheme called RF I-V where a radio frequency source is > applied to an unknown impedance. Then the current through the > impedance and the voltage across it are both measured. The vector > ratio of the values is calculated to obtain a better complex impedance > value. This method is discussed in Agilent Applications Note 1369-2. > <SNIP> N2PK has built his own version of the Agilent RF I-V scheme and > has obtained much better data. >From a quick glance thru that paper I dont see much benefit of RF I-V below about 100MHz. What am I missing here? > > Also see a brief discussion of RF-IV in > http://sdr-kits.net/DG8SAQ/VNWA/VNWA_HELP.pdf which is the manual for > the software for DG8SAQ's Vector Network Analyzer, and which supports > the N2PK RF-I-V fixture. > > 73, Jim K9YC > > _______________________________________________ > UR RST IS ... ... ..9 QSB QSB - hw? BK > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 10.0.1424 / Virus Database: 2437/5129 - Release Date: 07/13/12 > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
