Using Oscilloscope to Detect Common Mode Current
 

First thanks for the many replies to posts I make here. I enjoy the
dialogue and am fortunate to have access to the knowledge and
experience here.

Other than my MFJ-259B and my DMM, the only other piece of test
equipment I have is an oscilloscope. Would it be possible to wrap
several turns of light gauge enameled wire around the jacket of the
coax at my operating position, attach the oscope probes and detect and
possibly measure the amplitude of any common mode currents?

Using Oscilloscope to Detect Common Mode Current
 

dykesc wrote in news:db3e01e5-1866-4e43-a795-
:

....
equipment I have is an oscilloscope. Would it be possible to wrap
several turns of light gauge enameled wire around the jacket of the
coax at my operating position, attach the oscope probes and detect and

Do you understand that the common mode current is a standing wave. What if
you measured it at only one location, and that happened to be a deep
minimum... what would that tell you?

Owen

Using Oscilloscope to Detect Common Mode Current
 

On Jun 7, 10:12*pm, Owen Duffy wrote:

Do you understand that the common mode current is a standing wave. What if
you measured it at only one location, and that happened to be a deep
minimum... what would that tell you?

Owen

Yes, but I am specifically interested in what's going on near my
operating position. Have had some problems with computer peripherals
when transmitting on 80m.

This may be a better way of going about it:

http://techdoc.kvindesland.no/radio/...6153319205.pdf

I know I could just wind chokes to try and eliminate any common mode
rf at my operating position. I just like the idea of measuring first
to see how effective the chokes are.

Using Oscilloscope to Detect Common Mode Current
 

dykesc wrote:
First thanks for the many replies to posts I make here. I enjoy the
dialogue and am fortunate to have access to the knowledge and
experience here.

Other than my MFJ-259B and my DMM, the only other piece of test
equipment I have is an oscilloscope. Would it be possible to wrap
several turns of light gauge enameled wire around the jacket of the
coax at my operating position, attach the oscope probes and detect and
possibly measure the amplitude of any common mode currents?


That will work, but here's a way to make an actual quantitative
measurement, assuming the bandwidth of your 'scope is at least a few
times the highest frequency you want to measu

1. Find a ferrite core large enough to fit over the coax and connector.
Common type 43 is fine, or any of the lower frequency ferrites (e.g.,
Fair-Rite/Amidon 70 series) if you're measuring at HF.
2. Wind 10 turns (10 passes through the core) of enameled wire on the
core. Connect a 100 ohm resistor across the winding with leads as short
as possible.
3. Slide the core over the coax. Measure the voltage across the resistor
with the 'scope. The common mode current in amperes = 10 * the 'scope
voltage(*).

(*) The current through the resistor is 1/10 the common mode current due
to the 1:10 turns ratio. The voltage across the resistor is i * R = 1/10
the common mode current * 100 = 10 * the common mode current. The
transformer will look like a one ohm series resistance (R / N^2, where N
is the turns ratio of 10) to the common mode current.

Roy Lewallen, W7EL

Using Oscilloscope to Detect Common Mode Current
 

dykesc wrote in
:

....
Yes, but I am specifically interested in what's going on near my
operating position. Have had some problems with computer peripherals
when transmitting on 80m.

Well, doing is learning.

My own view is that common mode current meters are seen as a magic
bullet. It is true that if you make a single measurement and you observe
substantial current (meaning you have some absolute calibration), then
you know there is a problem.

The problem is that if you don't observe current in that single
measurement, you cannot infer that there is not a common mode current
problem.

Some things that you might do may just move the standing wave pattern.
Again, measurements at a single point are a very limited perspective.

The best countermeasure is design of antenna *systems* for low common
mode current.

Nevertheless, we need to make some compromises on suburban blocks and
physical symmetry can be one of the sacrifices.

Take a leaf from the books of lightning protection. They usually firstly
try to shunt the undesired current to ground rather than trying to impede
its flow through the equipment room.

If that isn't enough, then impede its flow into the equipment room so
that more of it is shunted via the ground path.

Owen

Using Oscilloscope to Detect Common Mode Current
 

On Jun 7, 11:39*pm, Roy Lewallen wrote:


That will work, but here's a way to make an actual quantitative
measurement, assuming the bandwidth of your 'scope is at least a few
times the highest frequency you want to measu

Roy Lewallen, W7EL

Thanks very much Roy. This is something I can implement quite easily.

73

Dykes AD5VS