The OIB series is made to work around broadcast arrays, up to about
+/- 400j reactance and I forget the real range. I used to work for
Delta Electronics Inc, and we hand calibrated every one of them. The
advantage of the OIB was to work under power; Delta made the CPB-1
for common point installation into a directional array at 50 kW, and
the OIB for portable under 10 kW sort of power. This allowed
measurments in the presence of adjacent channel RF power coming in
while the TX was running.
They also made the RG-1 through 4, synchronously modulated (chopped)
receiver/generators to use with the Delta bridges as well as the
General Radio systems. It gave one more degree of isolation to the
measurment, but only detecting the null in the bridge from the
chopped 3 Watt signal, not the incoming CW rf from another carrier.
The OIB-2 bridge is the HF only version of the Delta equipment. It
was made mostly for SW broadcasters and gov't systems, and allowed 1
kW max thru power. In the 1970's there was a QST article on building
your own.
With the advent of the MFJ/Bird/Autek/etc and microprocessors, these
inexpensive jobbies seem to fill the bill for most ham antenna uses.
Hp did have a great article on the accuracy of couplers and uses for
impedance measurement. They used a direct sampling V/I system in some
of the impedance meters that go up to 100 MHz or so, to get around
this limitation and allow measurment up to 100K and down to a few
Ohms.
73
John
K5PRO
At 11:46 PM -0400 10/11/00, owner-amps-digest@contesting.com (RF
Amplifier Discussion Dige wrote:
>Even the $5k+ Delta OIB-1 and OIB-3 operating impedance bridges tend to pull
>a bit when the reactances start to get really high. I also use a GR-1606A
>for critical impedance measurements. The other nice thing about the MFJ is
>that I can use it as a signal source to excite the GR bridge when making the
>critical measurements. But for getting in the ballpark, the MFJ works fine.
>
>73........de Goose W8AV
>
>------------------------------
>All systems based on a directional coupler lose accuracy as the load departs
>from the design impedance. This is because at 'high swr', quite large changes
>in load impedance can produce quite small changes in the reflected
>power. There
>was a very good article on this some 18 or so years ago in the Hewlett Packard
>Journal, in an article describing a component measuring meter.
>
>It was shown that with a 50 ohm directional coupler system, loads of around
>10kohm can give answers between , if I remember, 3k and 30k or something like
>that, all depending on the directivity of the coupler.
>
>The same applies to network analysers, of course.
>
>The old GR or equivalent bridges or even the just pre-WW2 Boonton impedance
>meter don't have this particular problem - just different ones - like size and
>weight and power consumption and no direct read out, and they're not made any
>more, and the few that are around tend to be expensive.
>
>73
>
>Peter G3RZP
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