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Date: 22 Nov 2008 22:08 GMT
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Subject: Re: LF: More on Admittance
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Dear Peter,

I am using a home brew antenna bridge in which variable R and C are in
parallel. This bridge is about 20 years old, and one reason for this
parallel arrangement has been that at the time of construction all
potentiometers available on the market had a metal case, and the
capacity of the case is in parallel to the resistance. In the parallel
arrangement this does not matter and can even be compensated, but in the
series arrangement the case capacity is especially troublesome at high
frequencies and when the resistance is set at high values. This may have
also been a reason for W6SAI to recommend a parallel arrangement for
homebrewers.

Today there are potentiometers with plastic cases on the market (which
in the past may have been special components for bridges only). In high
impedance amplifiers they may be worse because of lack of screening. But
for building bridges I would like to try them if I felt the need to
design a new one.

I am usually looking for reactance measurements, not admittance. In this
case  measurement results in general (not me, I like to think in
parallel values) want to be obtained in the form of R and X in series
(Rs and Xs). When using a bridge in the parallel arrangement results are
originally in Rp and Xp (or Rp and +pF or -pF as in my calibration), and
they have to be converted to Rs and Xs. This is not necessary when using
a bridge in the series arrangement.

This has been my position so far concerning measuring with bridges.    

HW?

73 Ha-Jo, DJ1ZB


"Peter Dodd" <g3ldo@ukonline.co.uk> schrieb:
> Hi Alan,
> Thanks for the information. Most noise bridges use serial variable 
> resistance and reactance bridges but there is a circuit of a bridge 
> using a parallel arrangement in the W6SAI radio Handbook although it 
> doesn't say what the advantage of such an arrangement is.
> 
> Regards
> 
> Peter, G3LDO
> > Hi Peter I think it more due to the fact that commercial bridges and meters really want to cover the widest range. There is obviously a problem with serial connected reactances when one is very much larger than the other. Then the nulls on bridges become shallow and ill defined. If you use a parallel arrangement this is not a problem. For most Amateur aerial cases you can get away with serial connection because there is not the need for the wide range needed for laboratory equipment.....10^3 compared with maybe 10^6.
> >
> > Alan G3NYK
> >
> >
> >
> >
> > --- On Wed, 19/11/08, Peter Dodd <g3ldo@ukonline.co.uk> wrote:
> >
> >   
> >> From: Peter Dodd <g3ldo@ukonline.co.uk>
> >> Subject: LF: More on Admittance
> >> To: rsgb_lf_group@blacksheep.org
> >> Date: Wednesday, 19 November, 2008, 6:55 PM
> >> Many thanks to those who responded to my question regarding
> >> an 
> >> Admittance to Impedance converter.
> >> I have a further question. Since most of the measurements
> >> we make on 
> >> antennas and transmission line require the answer in term
> >> of impedance 
> >> why are there so many commercial instruments calibrated in
> >> units of 
> >> Admittance, particularly at VHF and UHF. I suspect that it
> >> is because it 
> >> is easier to make accurate variable bridge standards if
> >> they use a 
> >> parallel arrangement; is this true
> >> Regards
> >>
> >> Peter, G3LDO
> >>
> >>
> >>     
> >>> The best program (as far as I am concerned) is called
> >>>       
> >> Z-Y Converter, 
> >>     
> >>> which was sent to me by Rik Strobbe.
> >>> It came direct because it gave the reflector
> >>>       
> >> indigestion although it 
> >>     
> >>> is only 160KB.
> >>> I was given a General Radio 1602-B Admittance Meter as
> >>>       
> >> a pre Christmas 
> >>     
> >>> present, which is calibrated in millimhos, which I
> >>>       
> >> guess is the same 
> >>     
> >>> as millisiemens.
> >>>       
> >
> >
> >
> >   
> 
> 
>