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

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

All radio people suffer from delusions of measuring accuracy.

RF power measurements are the most inaccurate of all.

The accuracy of measurements are a function of the instrument user.

They who attempt to grasp support by stating the manufacturer's type number
of the instruments used are most in need of the self-confidence it falsly
generates.

Either that or the statements are gratuitous adverts.

How cynical can one get at this hour of the day?

All radio people suffer from delusions of measuring accuracy.

RF power measurements are the most inaccurate of all.

The accuracy of measurements are a function of the instrument user.


They who attempt to grasp support by stating the manufacturer's type number
of the instruments used are most in need of the self-confidence it falsly
generates.

Hi Reg. What exactly are you talking about? I had a few minutes in between
Hurricane Ivans wrath to get the Emergency generator cranked up and had a
chance to read this. Lucky you don't have these things in the UK.
73 Gary N4AST

Reg wrote
They who attempt to grasp support by stating the manufacturer's type
number
of the instruments used are most in need of the self-confidence it falsly
generates.

Hi Reg. What exactly are you talking about? I had a few minutes in
between
Hurricane Ivans wrath to get the Emergency generator cranked up and had a
chance to read this. Lucky you don't have these things in the UK.
73 Gary N4AST
============================

Gary, we have heard the news over here about the devastating Hurricane Ivan.
We get them here at about 1/2 strength of yours only once every very few
years. And even then it's only over a relatively small area.

If you can't understand what I am wittering about then its due either to the
storm stress you are under or because you are one of those suffering from
delusions of accuracy. In your case I prefer the former excuse.

I hope your generator started up OK and that you and your family suffer the
bare minimum of danger and damage. At least communications between us are
still intact. My best wishes.
---
Reg, G4FGQ

"JGBOYLES" wrote in message
...

All radio people suffer from delusions of measuring accuracy.

RF power measurements are the most inaccurate of all.

The accuracy of measurements are a function of the instrument user.


They who attempt to grasp support by stating the manufacturer's type
number
of the instruments used are most in need of the self-confidence it falsly
generates.

Hi Reg. What exactly are you talking about? I had a few minutes in
between
Hurricane Ivans wrath to get the Emergency generator cranked up and had a
chance to read this. Lucky you don't have these things in the UK.
73 Gary N4AST

Gary,

I saw an interesting curve at a resistor manufacturer's web site. It plotted
resistor error as a function of F(MHz) x R(Meg) for 1/4 W carbon resistors.
To make a long story short, the resistor error will be about 20% where the
Megahertz x MegaOhms = 1. That means the resistor value will be 1/
Frequency. So, at 30 MHz, the resistor will be in error by 20% if it is
bigger than 1/30 =.033 Meg, or 33K. That, I believe ignores capacitive
effects. Personally, I have never tried to put RF through a resistor bigger
than a few hundred Ohms.

It occurs to me that you can ignore capacitive effects if you make all
resistors identical. For instance, if you want a 3:1 divider make the series
resistor 10K, and the shunt resistor two 10K resistors in parallel. Of
course, you will need a high impedance load on it. Let's see if anybody
shoots this down.

Tam

On Thu, 16 Sep 2004 21:26:48 -0400, "Tam/WB2TT"
wrote:

To make a long story short, the resistor error will be about 20% where the
Megahertz x MegaOhms = 1.

And not so curiously Trc = 1 MOhm · 1 pF = 10^-6
F = 1 / T
F = 1 MHz
perhaps the product rule should be:
Megahertz x MegaOhms x picoFarads = 1

The 20% error is, of course, simply the rolloff response at the RC
inflection point described by 1/Trc.

Let's see if anybody
shoots this down.

Hi All,

I think chipping at the clay feet of saints is more appropriate
metaphor.

What is the saint? The RF response of the resistor. It should be
suspect right out the gate. Being suspect, you employ the
conventional techniques already evidenced even by the cheapest Power
Meter builder (MJF) by swamping the stray capacitance with series
capacitors (paralleling the resistors). One capacitor is either
variable, or further paralleled with a trimmer. The saint is also the
unspecified requirement: is this divider BEFORE OR AFTER the detector?

If before, and thus subject to RF, the simple RC compensated divider
has served for eons. If after, and thus subject to only DC - who
cares? The one clay foot of the discussion.

The other clay foot of the discussion is that for placement before OR
after the detector, ALL ratios are post-hoc determinations (in other
words, design with variable components fully expecting you WILL be
wrong). Further, ALL descriptions to this point have been of
normalized levels. With the RC compensated divider, you are throwing
the knee if rolloff into lower frequencies so that ALL frequencies of
interest reside on the same slope. Hence the common "calibration"
procedure has you adjust the resistors for the low frequency readout,
and the capacitors at the high frequency readout. This "calibration"
is simply distributing the error so that it doesn't accumulate
outrageously.

The greater challenge is how do you know how much power you are
setting your meter to read? Compounding errors are common in RF.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Thu, 16 Sep 2004 21:26:48 -0400, "Tam/WB2TT"
wrote:

To make a long story short, the resistor error will be about 20% where the
Megahertz x MegaOhms = 1.

And not so curiously Trc = 1 MOhm · 1 pF = 10^-6

I think the curve ignores C, and is based on skin effect only. There is no
explanation for the data.


F = 1 / T
F = 1 MHz
perhaps the product rule should be:
Megahertz x MegaOhms x picoFarads = 1

Go to http://www.xicon-passive.com/resistor.html and click on CC. There is
also info on resistor performance vs frequency in the W6SAI book. He shows
curves for 5 different carbon resistors vs frequency without identifying the
resistor values. As a gross average, they show about 50% error at 15 MHz.

The 20% error is, of course, simply the rolloff response at the RC
inflection point described by 1/Trc.


The curve goes from 0 - 100. I arbitrarily picked 20 % as being a point
where there is apreciable error.

Tam/WB2TT

Let's see if anybody
shoots this down.

Hi All,

I think chipping at the clay feet of saints is more appropriate
metaphor.

What is the saint? The RF response of the resistor. It should be
suspect right out the gate. Being suspect, you employ the
conventional techniques already evidenced even by the cheapest Power
Meter builder (MJF) by swamping the stray capacitance with series
capacitors (paralleling the resistors). One capacitor is either
variable, or further paralleled with a trimmer. The saint is also the
unspecified requirement: is this divider BEFORE OR AFTER the detector?

If before, and thus subject to RF, the simple RC compensated divider
has served for eons. If after, and thus subject to only DC - who
cares? The one clay foot of the discussion.

The other clay foot of the discussion is that for placement before OR
after the detector, ALL ratios are post-hoc determinations (in other
words, design with variable components fully expecting you WILL be
wrong).

So true. I notice the series C in the Kenwood meter is variable.

Further, ALL descriptions to this point have been of
normalized levels. With the RC compensated divider, you are throwing
the knee if rolloff into lower frequencies so that ALL frequencies of
interest reside on the same slope. Hence the common "calibration"
procedure has you adjust the resistors for the low frequency readout,
and the capacitors at the high frequency readout. This "calibration"
is simply distributing the error so that it doesn't accumulate
outrageously.

The greater challenge is how do you know how much power you are
setting your meter to read? Compounding errors are common in RF.

73's
Richard Clark, KB7QHC

The other clay foot of the discussion is that for placement before OR
after the detector, ALL ratios are post-hoc determinations (in other
words, design with variable components fully expecting you WILL be
wrong). Further, ALL descriptions to this point have been of normalized
levels.

Hi Richard, I haven't been able to keep up with this like I wished because of
that pesky Hurricane. If you put the detector circuit before the voltage
divider, then the resistors see DC which they are a lot happier with. The
detector diode will have to be 700VDC PRV rating, and the filter cap. will have
to be sized properly.
I guess the diode will have some frequency dependent properties, but as long
as it still acts like a diode, and the forward bias drop is around .6V it ought
to work. This looks like good alternative to frequency dependent resistors.
What say you?


73 Gary N4AST