RadioBanter - View Single Post

February 7th 06, 08:05 PM posted to rec.radio.amateur.antenna

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