Please give me some suggestions about a device, which can/should
preferably remain online like a SWR meter, which shows the unbalance of
an antenna connected to the station via a balanced feeder.

On 7 Feb 2006 05:11:27 -0800, wrote:

Please give me some suggestions about a device, which can/should
preferably remain online like a SWR meter, which shows the unbalance of
an antenna connected to the station via a balanced feeder.

A pair of match RF amp meters.

73,
Danny



email: k6mheatarrldotnet
http://www.k6mhe.com/

On 7 Feb 2006 05:11:27 -0800, wrote:

Please give me some suggestions about a device, which can/should
preferably remain online like a SWR meter, which shows the unbalance of
an antenna connected to the station via a balanced feeder.

MFJ recently began marketing a device that goes inline on a balanced
feedline; a couple of RF ammeters that show the current going through
each side of the line. see part mfj-835

bob
k5qwg

wrote:
Please give me some suggestions about a device, which can/should
preferably remain online like a SWR meter, which shows the unbalance of
an antenna connected to the station via a balanced feeder.

To get a relative indication of unbalance on the feeder, run it thru
a toroidal core with a few turns of wire wound around for a secondary
winding. Recitify the secondary voltage, and read with a DC
voltmeter/microamp meter. If you don't have a toroid big enough to fit
around the feeder (likely if you are using 300 ohm line), it should be
ok to transition to smaller line for an inch or so to go through the
toroid.
You are wanting to read the balance of the antenna. The antenna by
itself can be completely balanced, but when you add the transmission
line, the antenna system can become quite unbalanced. The above
detector will give you a relative indication of feeder unbalance, which
I guess you really want to read.
Gary N4AST

MFJ makes a $99 RF current meter (MFJ-854) which does this and provides
30 ma to 3 amp FS readings. They claim 1 ma is detectable. One
milliampere of unbalance is not worth talking about.

Gary's do-it-yourself suggestion is a good one. Easy to build and
interesting to calibrate. Of course, if the interest is only in
determining which of two alternatives creates the greater unbalance, the
DIY approach is hard to beat.

Both methods will probably require reducing line separation. The MFJ
accomodates only 1/2 inch in the clamp. Both methods also leave you with
an instrument useful in tracking down RFI problems.

Chuck NT3G

wrote:
wrote:

Please give me some suggestions about a device, which can/should
preferably remain online like a SWR meter, which shows the unbalance of
an antenna connected to the station via a balanced feeder.


To get a relative indication of unbalance on the feeder, run it thru
a toroidal core with a few turns of wire wound around for a secondary
winding. Recitify the secondary voltage, and read with a DC
voltmeter/microamp meter. If you don't have a toroid big enough to fit
around the feeder (likely if you are using 300 ohm line), it should be
ok to transition to smaller line for an inch or so to go through the
toroid.
You are wanting to read the balance of the antenna. The antenna by
itself can be completely balanced, but when you add the transmission
line, the antenna system can become quite unbalanced. The above
detector will give you a relative indication of feeder unbalance, which
I guess you really want to read.
Gary N4AST

wrote:
wrote:

Please give me some suggestions about a device, which can/should
preferably remain online like a SWR meter, which shows the unbalance of
an antenna connected to the station via a balanced feeder.


To get a relative indication of unbalance on the feeder, run it thru
a toroidal core with a few turns of wire wound around for a secondary
winding. Recitify the secondary voltage, and read with a DC
voltmeter/microamp meter. If you don't have a toroid big enough to fit
around the feeder (likely if you are using 300 ohm line), it should be
ok to transition to smaller line for an inch or so to go through the
toroid.
You are wanting to read the balance of the antenna. The antenna by
itself can be completely balanced, but when you add the transmission
line, the antenna system can become quite unbalanced. The above
detector will give you a relative indication of feeder unbalance, which
I guess you really want to read.
Gary N4AST


This is the best method. Be sure to run both conductors through the core.

The core itself will act as something of a common mode choke (current
balun) unless you terminate the winding in a fairly low resistance.
Terminating it will reduce its effect on the line you're measuring. I
typically use about 10 turns for the secondary with 51 ohms across it.
That gives an insertion impedance of 51 / 100 = 0.51 ohm, which is
negligible. I prefer to do this so my measuring device doesn't disturb
what I'm measuring -- if I want a balun I do it separately. I suggest
using a second core/winding on just one of the wires so you can compare
the total wire current to the imbalance current. It's very important to
terminate the one on the single wire, and if you want to use the two for
comparison, you'll then also have to terminate the other. A
high-permeability "low frequency" ferrite core is best for these, like
Fair-Rite type 70 series (72, 73, 77, etc.). Type 43, a very common type
used largely for EMI reduction, is also ok.

If you terminate the winding in about 50 ohms, you can put your sensor
at a remote location and run 50 ohm coax between the core and
detector/termination without disturbing the reading except for the coax
loss.

I don't recommend two RF ammeters as some others have suggested. They
only tell you amplitude and not phase. You can have some pretty
seriously imbalanced currents and still see equal meter readings.

Roy Lewallen, W7EL

Roy Lewallen wrote:
wrote:
wrote:

Please give me some suggestions about a device, which can/should
preferably remain online like a SWR meter, which shows the unbalance of
an antenna connected to the station via a balanced feeder.
To get a relative indication of unbalance on the feeder, run it
thru
a toroidal core with a few turns of wire wound around for a secondary
winding. Recitify the secondary voltage, and read with a DC
voltmeter/microamp meter. If you don't have a toroid big enough to fit
around the feeder (likely if you are using 300 ohm line), it should be
ok to transition to smaller line for an inch or so to go through the
toroid.
You are wanting to read the balance of the antenna. The antenna by
itself can be completely balanced, but when you add the transmission
line, the antenna system can become quite unbalanced. The above
detector will give you a relative indication of feeder unbalance, which
I guess you really want to read.
Gary N4AST


This is the best method. Be sure to run both conductors through the core.

The core itself will act as something of a common mode choke (current
balun) unless you terminate the winding in a fairly low resistance.
Terminating it will reduce its effect on the line you're measuring. I
typically use about 10 turns for the secondary with 51 ohms across it.
That gives an insertion impedance of 51 / 100 = 0.51 ohm, which is
negligible. I prefer to do this so my measuring device doesn't disturb
what I'm measuring -- if I want a balun I do it separately. I suggest
using a second core/winding on just one of the wires so you can compare
the total wire current to the imbalance current. It's very important to
terminate the one on the single wire, and if you want to use the two
for comparison, you'll then also have to terminate the other. A
high-permeability "low frequency" ferrite core is best for these, like
Fair-Rite type 70 series (72, 73, 77, etc.). Type 43, a very common
type used largely for EMI reduction, is also ok.

If you terminate the winding in about 50 ohms, you can put your sensor
at a remote location and run 50 ohm coax between the core and
detector/termination without disturbing the reading except for the coax
loss.

I don't recommend two RF ammeters as some others have suggested. They
only tell you amplitude and not phase. You can have some pretty
seriously imbalanced currents and still see equal meter readings.

Another option would be to make two identical transformers, one for each
wire, and connect both secondaries to the same terminating resistor.
Reversing one of the secondaries gives true vector addition or
subtraction of the two currents, and then you can detect and measure the
resultant in the normal way.

Out-of-phase connection of the two secondaries indicates the
differential line current, or the in-phase connection gives the
common-mode current (hopefully much smaller).

I think that should work...


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

You can see an exemple he


" A pair of match RF amp meters.

73,
Danny


You can see an example he
http://www.barbaxoops.com/modules/xc...&album=7&pos=1

Regards

Jean-Marc
F4DRH
www.barbaxoops.com

Ian White GM3SEK wrote:

Another option would be to make two identical transformers, one for each
wire, and connect both secondaries to the same terminating resistor.
Reversing one of the secondaries gives true vector addition or
subtraction of the two currents, and then you can detect and measure the
resultant in the normal way.

Out-of-phase connection of the two secondaries indicates the
differential line current, or the in-phase connection gives the
common-mode current (hopefully much smaller).

I think that should work...


Excellent idea. But be sure to use a fairly low value of terminating
resistor for this and any time you run just one of the conductors
through the core. Otherwise you'll be putting a significant impedance in
series with your feedline conductor, causing mismatch and/or loss. The
insertion impedance, that is, the effective series impedance of the
core, will be Rt / N^2, where Rt is the terminating resistor connected
across the secondary winding and N is the number of secondary turns.
Lower values of terminating resistor will reduce the detector's
sensitivity, but this won't be a problem at moderate or even quite low
power levels.

Roy Lewallen, W7EL