----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
Cc: <topband@contesting.com>
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
>
> _______________________________________________
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