Q about balanced feed line
 

Amos Keag wrote:
An RF Ammeter will do just fine

What if the first current is one amp at zero degrees and the
other current is one amp at 45 degrees. Is that balanced?
--
73, Cecil http://www.qsl.net/w5dxp

Q about balanced feed line
 

Dot wrote:
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.

Would you believe that an SGC-230 will match that mismatch
condition and the only clue that you will have is that you
are not making as many contacts as before?
--
73, Cecil http://www.qsl.net/w5dxp

Q about balanced feed line
 

Sorry, when I read "RF ammeter", I thought only of the old type which
resembles a panel meter and has two terminals to connect in line with a
single conductor. A clamp-on type would indeed do the job, as we
discussed here not long ago.

Roy Lewallen, W7EL

chuck wrote:
Wouldn't the clamp-on type ammeter as discussed previously provide that
information (provided a section of twin lead small enough to fit the
clamp were used?

Chuck

Roy Lewallen wrote:
Amos Keag wrote:


YEP!!

An RF Ammeter will do just fine


Balance requires that the currents in the two wires be equal in
magnitude and opposite in phase (or direction). How do you connect an
RF ammeter to determine this?

Roy Lewallen, W7EL

Q about balanced feed line
 

In article ,
Cecil Moore wrote:

Big Endian wrote:
How does one check the balance between two parallel feed wires into a
doublet antenna. Neon bulbs or some sort of meter gizmo?

The easiest way is to use a large sampling toroid such
that the balanced line can be fed through the toroid.
A ten-turn sampling coil will indicate any unbalance.
Note that the balanced line needs to be centered with
respect to the toroid and needs to be moved along the
feedline to obtain maximum accuracy. Ideally, the toroid
needs to be located at a standing wave current maximum
point. Since my choke-balun is always located at a current
maximum point, I can easily monitor my feedline balance.

You can also use smaller toroids on each line and then
superpose the two readings. But the two toroids need to
be identical which is no small requirement.

An unbalanced condition would have a meter indication, like current
flow? Balanced the meter needle would not move? I want to monitor this
condition on the feedline in the shack from the Johnson KW matchbox.

Q about balanced feed line
 

Big Endian wrote:
An unbalanced condition would have a meter indication, like current
flow? Balanced the meter needle would not move?

Yes, for a balanced condition, the meter needle should
not move.
--
73, Cecil http://www.qsl.net/w5dxp

Q about balanced feed line
 

Hi Roy,

I have a couple of RF current meters with freq. response from 10KHz to
100MHZ that are the types used for FCC part 15, DO-160E testing etc. and
have a transfer impedance of 0dB-ohm. They have an opening of about
1.6" dia. and I sqeeze the feedline together (spacing about 1.5")
temporarily and clamp over both conductors for measurement #1 and clamp
over just 1 of the conductors for measurement #2. I read the output of
the current probe with an RF power meter. In my case the current in a
single conductor was approximately 15dB above the differential current
which seemed to be pretty well balanced. Is there anything wrong with
this approach?

Larry Benko, W0QE


Roy Lewallen wrote:
Amos Keag wrote:


YEP!!

An RF Ammeter will do just fine


Balance requires that the currents in the two wires be equal in
magnitude and opposite in phase (or direction). How do you connect an RF
ammeter to determine this?

Roy Lewallen, W7EL

Q about balanced feed line
 

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

Roy Lewallen wrote:
Larry Benko wrote:

Hi Roy,

I have a couple of RF current meters with freq. response from 10KHz to
100MHZ that are the types used for FCC part 15, DO-160E testing etc.
and have a transfer impedance of 0dB-ohm. They have an opening of
about 1.6" dia. and I sqeeze the feedline together (spacing about
1.5") temporarily and clamp over both conductors for measurement #1
and clamp over just 1 of the conductors for measurement #2. I read
the output of the current probe with an RF power meter. In my case
the current in a single conductor was approximately 15dB above the
differential current which seemed to be pretty well balanced. Is
there anything wrong with this approach?

Larry Benko, W0QE


Not that I can see. In my opinion it's the best way to make the
measurement. A homebrew version of this is entirely adequate, though, as
described in the earlier thread. It's very important to terminate the
secondary with a fairly low impedance so the transformer doesn't present
a significant impedance to the line. I assume your meter does this
internally -- maybe that information is in the transfer impedance you
mentioned and which I don't really understand.

Roy Lewallen, W7EL

Q about balanced feed line
 

Dot wrote:

SNIPPED

Which, of course, explains my continuing reluctance to use transmatches.

Yes it limits my range of antenna usage (and experience) but curse it all,
I'd really like to know when something goes wrong.


You will know that also with a transmatch!!!!

I've used a transmatch for almost 45 years out of 50+ years in Ham
Radio. If my antenna has a problem the settings for a 'match' [in
quotes] change!! That indicates something has happened.

The system SWR increase can be seen in the changes transmatch settings.

Q about balanced feed line
 

Amos Keag wrote:
I've used a transmatch for almost 45 years out of 50+ years in Ham
Radio. If my antenna has a problem the settings for a 'match' [in
quotes] change!!

How do you know when the settings for an SGC-230
autotuner change?
--
73, Cecil http://www.qsl.net/w5dxp

Q about balanced feed line
 

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