Why calibration is required for VSWR meter...

wrote in message
oups.com...
Why calibration is required for VSWR meter...

The meter simply measures the ratio of the power travelling along a feeder
line to an antenna against the reflected power back from the antenna and
feeder due to a mismatch of impedence.

The VSWR meter does not strictly need to be calibrated. Simple meters tend
to be frequency sensitive and are provided with a control to adjust the
meter to read full scale for forward power. Operating a switch or reversing
the connections allows reverse power to be assessed.

A VSWR meter will also only read accurately if the impedence of the antenna
and feeder system is the same as the design impedence of the metrer, usually
around 50 ohms.

More complex measurement circuitry and sensing devices can be used to
compensate for frequency sensitivity and the meter calibrated against known
loads in a factory. The important thing is to know how much reflected power
is in the system because this power largely ends up being dissipated as heat
in the transmitter final stages.

wrote:
Why calibration is required for VSWR meter...

Assume 100 watts in a 50 ohm transmission line
driving a 50 ohm load. The current in the line
is 1.414 amps and the voltage across the line
is 70.7 volts. That's also the current through
the load and the voltage across the load.

A typical SWR meter samples the current using
a toroidal transformer function resulting in a
sample voltage proportional to the *line current*
and equal in phase. Call this phasor voltage V(i).

The line voltage is sampled through a voltage
divider. The result is a sample voltage proportional
to the *line voltage* and equal in phase. Call this
phasor voltage V(v).

To obtain a voltage proportional to the reflected
power, the two sample voltages are subtracted, i.e.
added out of phase, so the total voltage,
V(t) = V(v) - V(i) = 0 for zero reflected power.

The only characteristic impedance and load for which
V(v) = V(i) is Z0=50 ohms. If one increases Z0,
V(v) will increase and V(i) will decrease and not
be equal in magnitude. If one decreases Z0, V(v)
will decrease and V(i) will increase and not be equal
in magnitude. Thus the SWR meter is calibrated to
50 ohms and only 50 ohms. If we switch to 75 ohm
coax and a 75 ohm load, the 50 ohm SWR meter will
indicate reflected power where none exists on the
Z0=75 ohm line.

(One can argue that the reflected power within the
50 ohm meter actually does exist inside the Z0=50
ohm meter environment but that is another discussion.)

An SWR meter is essentially a bridge with one leg
of the bridge typically set to 50 ohms. I have
designed SWR meters to read forward and reflected
power calibrated for Z0=300 ohms and 450 ohms. Those
meters are not useful when used with coax.

Continuing: To obtain a voltage proportional to the
forward power, the two sample voltages are added in
phase so the total voltage
V(t) = V(v) + V(i) = 2*V(v) = 2*V(i)
for the forward power indication in the example above.
A 100 watt calibration mark is written on the meter face
for that condition above.

If one goes through the exercise of the current being
out of phase with the voltage, one will observe that the
forward power and reflected power readings are accurate
as long as Vfor/Ifor = Vref/Iref = Z0 = 50 ohms.

V*I*cos(theta) = forward power - reflected power
--
73, Cecil http://www.w5dxp.com

--
73, Cecil http://www.w5dxp.com

Who's this Theta broad you keep bringing up?

Denny wrote:
Who's this Theta broad you keep bringing up?

In power engineering, the Greek letter, Theta,
is usually used to denote the phase angle between
the voltage and the current. V*I*cos(theta) are
the watts, i.e. real power. V*I*sin(theta) are
the vars, i.e. reactive volt-amps.
--
73, Cecil http://www.w5dxp.com