How the SWR meter works.
 

How does the so-called SWR, Forward pwr, Reflected pwr meter work?

(Or how gullible can you get?)

A small fraction of the line voltage is tapped off. This is V1.

A small resistor is inserted in series with the line. The voltage across it
is V2.

The resistors are proportioned such that when the meter is terminated with
50 ohms, V1 = V2.

The meter can be switched to indicate the magnitudes of either V1+V2 or
V1-V2.

V1+V2 is calibrated to indicate Fwd.Pwr.

V1-V2 is calibrated to indicate Ref.Pwr and SWR.

It's a sort of confidence trick which works provided a lot of assumptions
are made.

Whatever is inside the box, a little toroid or tiny fraction of wavelength
of line, they all work in exactly the same way.

By the way, they make excellent little transmitter-loading indicators and,
provided the transmitter is loaded with exactly 50 ohms, they will also
indicate the power dissipated in it. Otherwise - - - ???
----
Reg, G4FGQ

Reg Edwards wrote:
The meter can be switched to indicate the magnitudes of either V1+V2 or
V1-V2.

V1+V2 is calibrated to indicate Fwd.Pwr.

V1-V2 is calibrated to indicate Ref.Pwr and SWR.

The key to the functionality of such is that the addition
and subtraction involve two phasors so the phase angle
between the voltage and current is taken into account.
Assuming the meter is calibrated for Z0 and actually in
a Z0 environment, the necessary conditions for functionality
are met for all Smith Chart impedance points on a particular
SWR circle.
--
73, Cecil, W5DXP

"Cecil Moore" wrote in message
...
Reg Edwards wrote:
The meter can be switched to indicate the magnitudes of either V1+V2 or
V1-V2.

V1+V2 is calibrated to indicate Fwd.Pwr.

V1-V2 is calibrated to indicate Ref.Pwr and SWR.

The key to the functionality of such is that the addition
and subtraction involve two phasors so the phase angle
between the voltage and current is taken into account.
Assuming the meter is calibrated for Z0 and actually in
a Z0 environment, the necessary conditions for functionality
are met for all Smith Chart impedance points on a particular
SWR circle.
--
73, Cecil, W5DXP
=============================

Assumptions, assumptions, assumptions.

Imagination, imagination, imagination.

And what on earth has it to do with a Smith Chart?

I've never used one in anger in the whole of my life! Don't have one!

Cec, don't worry about it. ;o)

Let's not confuse the gullible younger generation.

Is the transmitter loaded with X ohms or is it not?
----
Reg, G4FGQ.

Reg Edwards wrote:
And what on earth has it to do with a Smith Chart?

The phase angle between the total voltage and total
current can be had from the Smith Chart. Here's a
procedure that I use that works well for ladder-line
fed dipoles.

1. Model the antenna feedpoint impedance. Plot it
as a point on the Smith Chart. Draw an SWR circle
through that point.

2. Knowing the length of the ladder-line and the VF,
follow the SWR circle toward the source for the
appropriate wavelength(s). Mark that point as the
calculated impedance seen by the 50 ohm SWR meter.

3. Measure the 50 ohm SWR. The difference in the
calculated lossless SWR and the actual SWR reading
indicates the variations plus losses.

The above exercise gets me into the ballpark (almost)
every time.
--
73, Cecil, W5DXP

Reg, G4FGQ wrote:
"Whatever is inside the box, a little toriod or tiny fraction of
wavelength of line, they all work in exactly the same way."

In-line power meters separate reflected power from incident power for
determinations.

My second edition of the RSGB "Test Equipment For The Radio Amateur" has
an in-line wattmeter diagram on page 4.7 which uses a toriod but
operates much like a Bird wattmeter.

The RSGB meter uses a 4-inch longth of coax which is grounded only on
one end. It carries the power to be measured through the center of the
toroid, and this coax forms a 1-turn primary for the toroidal
transformer. A center-tapped secondary is wound on the toroid. A
capaciitive voltage divider is connected between the coaz center
conductor and ground. This voltage divider and the transformer center
tap share a common load resistor.

The capacitive ratio of the divider is adjusted so that its voltage out
is equal to that of the magnetically induced secondary voltage on either
side of the center tap. The capacitively supplied voltage is
proportional to the coax voltage. The magnetically supplied voltage is
proportional to the coax current.

Reflected current is flowing in the opposite direction in the coax from
forward current in the short piece of coax, so in the direction of
current flow where induced volts in the transformer secondary add to
those capacitively coupled to the common load the total is douple either
contribution. Output of the combo is proportional to the power
represented by the current and voltage samples. Reflected power doesn`t
affect forward power determination in this instance because its voltage
and current samples are out-of-phase and exactly cancel.

Since the start and finish of the toroid`s secondary winding are
180-degrees out-of-phase, when the start of the coil is phased with the
capacitive sample to produce an output proportional to reverse
(reflected) power, the finish end of the coil is phased with the
capacitive sample contribution to produce an output proportional to the
forward (incident) power.

Calibration procedure is given in the book.

Best regards, Richard Harrison, KB5WZI