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Date: Wed, 26 Nov 2008 20:46:05 +0000
From: Peter Dodd <g3ldo@ukonline.co.uk>
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Subject: Re: LF: More on Admittance
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Dear Hajo.
Thank you for your comments.
Your comments on the reason for using parallel resistance and reactance 
in an RF bridge are in line with the those in The Hybrid Junction 
Admittance Bridge, W. N Carron, The ARRL Antenna Compendium, Vol 3 and 
An RF Admittance Bridge for 2 to 30MHz by G0RVN, RadCom July 1965. In 
both cases a method of converting Admittance to Impedance was not 
specifically given. Now, with the help of the guys on the LF reflector 
this has been solved.
I have not seen these comparisons discussed in any other publication.
I does seem to be discussed in Karl Rothammel's Antennenbuch, 1984 
Edition, Page 613, judging by the the diagrams. Unfortunately, I am 
unable to read German so I don't know what is actually said.

many thanks again

Peter, G3LDO
> 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.
>>
>>