Roy Lewallen wrote:
Larry Benko wrote:

Roy,

It is amazing how a simple concept such as impedance can be made
obscure but most current probes are spec'd this way by the
manufacturers and test procedures for DO-160E (FAA aircraft testing)
call out probes the same way. By saying that a current probe has a
transfer impedance of 0dB-ohm means 0dB relative to 1 ohm, but R = V/I
so 0dB-ohm means that for 1A of current thru the primary of the probe
produces 1V across a 50 ohm load. Similarly a -20db-ohm transfer
means that 1A produces .1V across a 50 ohm load. For most of us, this
means a 1 turn primary and a 50 turn secondary which yields a
theoretical primary impedance or 0.02 ohms. One of the probes I have
says the primary impedance is less than .1 ohm.

Larry Benko, W0QE


Thanks for the explanation. My concern is with the insertion impedance,
which at 0.02 or even 0.1 ohm, is certainly adequately low for this
device -- as long as it's properly terminated.

Roy Lewallen, W7EL

That's why these probes which are about the size of a small donut cost
over $1000. Since they are used to qualify EMI emissions from other
equipment they are priced like test equipment and certified to be
accurate. Generally the frequency response if terminated properly is
flat to within a fraction of a dB. Occasionally they show up on eBay
and usually go for less than $100. With no parts to wear out they never
go bad unless they have been used to test equipment for EMI
susceptability where you transmit RF into them, sometimes at high power
levels.

Larry, W0QE

Larry Benko wrote:
Roy Lewallen wrote:
Larry Benko wrote:

Roy,

It is amazing how a simple concept such as impedance can be made
obscure but most current probes are spec'd this way by the
manufacturers and test procedures for DO-160E (FAA aircraft testing)
call out probes the same way. By saying that a current probe has a
transfer impedance of 0dB-ohm means 0dB relative to 1 ohm, but R =
V/I so 0dB-ohm means that for 1A of current thru the primary of the
probe produces 1V across a 50 ohm load. Similarly a -20db-ohm
transfer means that 1A produces .1V across a 50 ohm load. For most
of us, this means a 1 turn primary and a 50 turn secondary which
yields a theoretical primary impedance or 0.02 ohms. One of the
probes I have says the primary impedance is less than .1 ohm.

Larry Benko, W0QE
Thanks for the explanation. My concern is with the insertion
impedance, which at 0.02 or even 0.1 ohm, is certainly adequately low
for this device -- as long as it's properly terminated.

Thanks, Larry. "dB-ohm" was a new one on me, too.

That's why these probes which are about the size of a small donut cost
over $1000. Since they are used to qualify EMI emissions from other
equipment they are priced like test equipment and certified to be
accurate. Generally the frequency response if terminated properly is
flat to within a fraction of a dB. Occasionally they show up on eBay
and usually go for less than $100. With no parts to wear out they
never go bad unless they have been used to test equipment for EMI
susceptability where you transmit RF into them, sometimes at high power levels.

They also come in larger sizes than donuts. A friend who works in
radiation protection uses them to measure RF currents in the wrists and
ankles of workers, eg operators of machines for RF welding of plastics.

But all these things have to be tested first... which is how he found
*himself* with one of these big toroids clamped around his ankle,
standing outdoors on the chicken-wire groundplane of an HF monopole, in
bare feet, in November. All in the name of "Occupational Health"... but
definitely not his own.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Ian White wrote
But the original questioner wants to measure what's *really*
happening.
========================================

But the original questioner can tell what's *really* happening just by
inspecting the antenna and feedline. He doesn't need any electrical
measuring instruments.

Of course, it may be in the middle of the night, in which case it
would be best to wait till the sun comes up.

Or he could rely on his memory of how he installed the antenna.

The trouble with you experts is that you overcomplicate matters and
make quite simple, obvious things appear to be mysterious.

But you have to justify your existence in one way or another. smiley
----
Reg.

In article ,
"Reg Edwards" wrote:

"Ian White wrote
But the original questioner wants to measure what's *really*
happening.
========================================

But the original questioner can tell what's *really* happening just by
inspecting the antenna and feedline. He doesn't need any electrical
measuring instruments.

Of course, it may be in the middle of the night, in which case it
would be best to wait till the sun comes up.

Or he could rely on his memory of how he installed the antenna.

The trouble with you experts is that you overcomplicate matters and
make quite simple, obvious things appear to be mysterious.

But you have to justify your existence in one way or another. smiley
----
Reg.

Actually I want to monitor the antenna because I have a splice in the
feedline and the feedpoint is subject to wx, snow ice and rain. Some
times the feedline itself is covered with snow and ice. I would find it
interesting to see what is happening when I see my SWR shoot up and
retuning my matchbox. I also have a strange thing happen when I apply
high power, I get a slight bump up in SWR suddenly. I often wonder if
one of my end insulators are breaking down under 1KW of RF or if there
is some imbalance on one leg due to ground or trees swaying in the wind.

On Sat, 25 Feb 2006 18:18:35 GMT, Big Endian
wrote:
"Ian White wrote
But the original questioner wants to measure what's *really*
happening.
========================================
But the original questioner can tell what's *really* happening just by
inspecting the antenna and feedline. He doesn't need any electrical
measuring instruments.

Actually I want to monitor the antenna because I have a splice in the
feedline and the feedpoint is subject to wx, snow ice and rain. Some
times the feedline itself is covered with snow and ice. I would find it
interesting to see what is happening when I see my SWR shoot up and
retuning my matchbox. I also have a strange thing happen when I apply
high power, I get a slight bump up in SWR suddenly. I often wonder if
one of my end insulators are breaking down under 1KW of RF or if there
is some imbalance on one leg due to ground or trees swaying in the wind.

Hi OM,

Reggie is particularly obtuse to this matter. Simple observation of a
dipole that is symmetrical to earth does not guarantee balance. When
that twin lead arrives at your gear, you can easily wipe that out (the
presumed balance) through an inappropriate connection to ground
(through any number of paths that are commonly overlooked in the
shack).

Snow and ice on the line are not likely to impart a common mode
current, but as revealed by bench test by contributors here, the
characteristic Z of the line can change drastically, which then upsets
the tune, where it follows SWR begins to change - noticeably. If your
feedline is coaxial, then the ice is of no consequence, but that does
not remove the jeopardy of common mode current. There you need to
look at how you isolate the line from the drivepoint.

73's
Richard Clark, KB7QHC

"Dot" wrote in message
...
On Fri, 24 Feb 2006 19:39:31 GMT, Cecil Moore
wrote:

What if one is broken and you don't know it?

I one side of a dipole gets loose, you'll know all about it... the
swr is
likely to go to 6 or higher. Your receiver would be mysteriously
quiet and
you wouldn't be getting normal power from your transmitter. In the
worst
case it could damage your equipment.

Antenna failure is not trivial.
==========================================
I quite agree.

Of course, if there's anything the matter with the antenna then you
will be aware of it long before you discover whether or not there's
any need to get out your measuring instruments to find how much your
feedline is unbalanced.

Just go out into your back yard and see whether your antenna is still
up in the air or not.

Many years back I once had a neighbor who sabataged my inverted-L at
1am in the morning while I was on the air. I did NOT measure
unbalance on the feedline - I was not equipped to do it - I just
called the police!

It turned out that my neighbor had very sensitive ears and could hear
my microphone voice through the walls of our adjoining houses.
----
Reg.

"Big Endian" wrote
Actually I want to monitor the antenna because I have a splice in
the
feedline and the feedpoint is subject to wx, snow ice and rain.
Some
times the feedline itself is covered with snow and ice. I would
find it
interesting to see what is happening when I see my SWR shoot up and
retuning my matchbox. I also have a strange thing happen when I
apply
high power, I get a slight bump up in SWR suddenly. I often wonder
if
one of my end insulators are breaking down under 1KW of RF or if
there
is some imbalance on one leg due to ground or trees swaying in the
wind.

=======================================

Perhaps you should have mentioned all this in your original enquiry.
The people you expect replies from are not mind readers!
----
Reg.

Reg Edwards wrote:

"Big Endian" wrote
Actually I want to monitor the antenna because I have a splice in
the
feedline and the feedpoint is subject to wx, snow ice and rain.
Some
times the feedline itself is covered with snow and ice. I would
find it
interesting to see what is happening when I see my SWR shoot up and
retuning my matchbox. I also have a strange thing happen when I
apply
high power, I get a slight bump up in SWR suddenly. I often wonder
if
one of my end insulators are breaking down under 1KW of RF or if
there
is some imbalance on one leg due to ground or trees swaying in the
wind.

=======================================

Perhaps you should have mentioned all this in your original enquiry.
The people you expect replies from are not mind readers!

It was clear enough to everyone else that he wanted to know how to make
measurements. It was also clear enough for you to declare that
measurements are not necessary.

Maybe this is a good time to remind you of your own favourite quotation:

"When you can measure what you are speaking about and express
it in numbers you know something about it. But when you cannot
measure it, when you cannot express it in numbers, your knowledge
is of a meagre and unsatisfactory kind. It may be the beginning of
knowledge but you have scarcely in your thoughts advanced to
the state of science."

William Thomson, Lord Kelvin, 1824-1907.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Reg Edwards wrote:
. . .
The trouble with you experts is that you overcomplicate matters and
make quite simple, obvious things appear to be mysterious.
. . .

I recently finished reading Richard Hofstadter's _Anti-Intellectualism
in American Life_ (written in 1962, won a Pulitzer prize in 1964), where
the author shows in great detail that a general disdain for education,
sophistication, and complexity is a very deeply seated American
tradition. That general outlook was virtually a basis of the founding of
a number of now-mainstream religions, and has had lasting impacts on our
educational and political systems. In short, it's a long and dearly held
American tradition.

So, Reg, that was a very American thing to say. You'd fit right in here.

Roy Lewallen, W7EL

Ian White GM3SEK wrote:

It was clear enough to everyone else that he wanted to know how to make
measurements. It was also clear enough for you to declare that
measurements are not necessary.

Maybe this is a good time to remind you of your own favourite quotation:

"When you can measure what you are speaking about and express
it in numbers you know something about it. But when you cannot
measure it, when you cannot express it in numbers, your knowledge
is of a meagre and unsatisfactory kind. It may be the beginning of
knowledge but you have scarcely in your thoughts advanced to
the state of science."

William Thomson, Lord Kelvin, 1824-1907.

I recently came across some other notable quotes from Lord Kelvin:

"Radio has no future."
"Heavier than air flying machines are impossible."
"X-rays will prove to be a hoax."

-- quoted in _Unweaving the Rainbow: Science, Delusion, and the Appetite
for Wonder_, by noted British writer Richard Dawkins. Dawkins also notes
that "William Thompson, first Lord Kelvin, was one of the most
distinguished and influential of nineteenth-century British physicists.
He was a thorn in Darwin's side because he 'proved', with massive
authority but, as we now know, even more massive error, that the earth
was too young for evolution to have occurred."

The value of Kelvin's contributions is unquestionable. But even he
didn't get it right all the time.

Roy Lewallen, W7EL