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" <[email protected]> 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.
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