Roy,

You are right (as per usual). I didn't check the MFJ269 open circuit
readings. At 30 MHz, with the N to UHF adapter installed, open circuit Z is
0 & j656. Since my capacitor readings were about 10% less than that, my
guess is that the resistor reactances were actually about 1/10th of that (~1
PF). If I remove the N/UHF adapter, x rises to 1389. So, most of the C is in
the adapter, which was in place. I think the 12 Ohm data is OK. The
capacitance washed out, and it showed a series inductance of 21 nH.

As I said, this is not a precision instrument. Unfortunately, it is less
precision than I thought. I hope that somebody with access to an HP or
similar instrument will feel inspired to measure some resistors at RF.
Meanwhile, I am going to look at some resistor manufacturers web sites.

Tam/WB2TT


"Roy Lewallen" wrote in message
...
It's much more likely that the shunt capacitance is in the MFJ.

Calculate the parallel impedance of 11 + j0 (the presumed resistor) and
0 - j600 (the shunt C) and you'll see that you wouldn't be able to see the
shunt C when making the 11 ohm "sanity check".

Even at HF, measurements aren't nearly as simple as they sometimes seem
they should be.

Roy Lewallen, W7EL

Tam/WB2TT wrote:

Hi Gary,

I just measured some random 1/2W carbon resistors with an MFJ at 30 MHz.
This is not a precision instrument, but shows a trend.

Nominal Measured
5.6 K 0 - j586
220K 0 -j 600
1.8K 99 - j539 (convert this to parallel form)

As a sanity check

11 Ohms 12 + j4 (some lead inductance here)

What this is tending to show is that the resistors are showing a shunt
capacitance whose reactance is about 600 Ohms at 30 MHz. That is about 9
PF, which seems high. I was expecting more like 1 PF. I want to redo this
at a higher frequency, might be out of range for the MFJ.

I notice my Kenwood power meter uses a capacitive divider for the voltage
sample. A friend of mine built a meter along the lines of what you want
to do. I will ask him what he did.

Tam