Why do it the hard way. Use a bidirectional coupler made for the frequency
range and power level of the amp such as the Bird 4266(1500 Watts). Then
you can measure power, SWR or hook up your counter, oscope, or spectrum
analyzer etc. without fear of destroying them. It gives 30 dB isolation from
the line and is flat in frequency response ( plus/minus 0.5 dB or less 1.5
to 40 Mhz measured here.)

"Roy Lewallen" wrote in message
...
The problem with a plain resistive divider is that the detector (or, in
the case of a scope probe, the scope input) invariably has an associated
shunt C. Without compensation, the lower arm of the divider gets less
voltage than it should, due to that shunt C. If the capacitive reactance
is high compared to the resistance of the lower divider arm at the highest
frequency of interest, then you can probably get by without shunt
capacitors. But otherwise, you need to make a compensated attenuator by
putting C across at least the upper divider arm.

The rule for making a compensated divider is that the time constants of
the top and bottom should be equal. That is, R1C1 = R2C2 where R1 and C1
are the parallel R and C at the bottom of the divider, and R2 and C2 at
the top.

This kind of divider has a theoretically flat frequency response, that is,
the voltage at the tap is constant assuming that the voltage applied to
the top of the divider is constant. But there's the rub. The impedance of
the divider is *not* constant with frequency -- because of the C, it
decreases as frequency increases. So depending on the circuit it's put
across, above some frequency the divider will begin disturbing the circuit
being measured due to the divider's too-low impedance. Consequently, the
design procedure is usually to first minimize the detector or load C. This
allows you to use the smallest possible C in the top part of the divider,
resulting in minimized loading of the circuit to be measured. In general,
you don't want to add physical C to the lower part of the divider, unless
the load (detector) C is variable, unpredictable, or nonlinear, which
requires it to be swamped by a known, fixed, good-quality C.

Even such a simple circuit isn't trivial, if very accurate division and
wide frequency response (or fast and low-aberration time response) are
required. "Hook", "soak", and other nonlinear effects are well-known to
designers of oscilloscope front ends, as are the tricks and skills
necessary to avoid them. But these problems are much worse for high
impedance dividers than ones designed to operate in a low impedance
environment like 50 ohms.

Roy Lewallen, W7EL

gudmundur wrote:

Resistive dividers don't work worth a crap at rf, unless you use
non-inductive resistors, and then you must consider that your input
to the squaring circuit will have some capacitance to ground, who's
ability to screw up your higher frequency readings will be astronomical.
Why the squaring circuit?

Bottom line is this, all the resistive ladders I have played with
were worthless unless they contained capacitive compensation, which
can be found only by trial and error. I prefer a 'T' connection with
a good quality 'coaxial pad' of 10 db, then I feed it to my Tek 2215A
scope. It's deadly accurate from D.C. to at least 30mhz. More accurate
than any of my inline wattmeters, with the exception of the 'BIRD', and
that is only good across the spectrum of the slug I use.