ANSWR meter is a fixed-ratio resistance bridge. The external "unknown arm" is the input impedance, R+jX, of the antenna system. The bridge unbalance voltage is displayed on a meter calibrated to indicate a choice of system parameters.
This program assists with design and models the behaviour of the most
common form of HF SWR meter. It is located in a coaxial line between the
transmitter and the antenna where, subject to certain conditions, it indicates
SWR on the section of line between transmitter and meter. It is often placed
immediately at the transmitter output where an indication of the reflection
coefficient of the system input impedance is a more appropriate parameter
to display.
Some or all of the following meter scales may be used:
- SWR. Very non-linear scale from 1 to Infinity. SWR = 3 at 1/2-scale
- Magnitude of Refln.Coeff. Scale graduated linearly 0 to 1 at full scale
- Reflected Power, watts. Square-law scale, cramped at lower end
- Forward Power, watts. Same square-law scale. Full scale > Tx rated power
Actual Tx Power = Indicated Forward Power - Indicated Reflected Power.
Principle
of Operation
A generator with internal impedance Zs feeds a coaxial line of impedance
Zo. The end of the line passes through the meter. The rest of the system
beyond the meter, for the purposes of these notes, is considered to be
the Tx load ZL. Inside the meter two relatively small voltages are obtained.
One is proportional to and in phase with the load voltage. The other is
proportional to and in phase with the load current.
From this pair two other voltages are produced: their difference Vd
and their sum Vs. The bridge is standardised by terminating the bridge
with a dummy load equal to Zo, then adjusting the fraction of load voltage
tapped off such that the rectified Vd is precisely zero as indicated on
a high-Z DC voltmeter.
Any load other than Zo will now unbalance the bridge,
the unbalance voltage Vd being proportional to the reflection coefficient
RC = (ZL-Zo)/(ZL+Zo).
The RC scale is graduated linearly from 0 to 1. SWR = (1+RC)/(1-RC)
may be indicated on the same meter face, the scale being graduated
non-linearly, 1 to Infinity.
The square of the meter deflection Vd is proportional to power reflected
back from a mismatch at ZL. The scale, calibrated in watts, can be shared
with the forward power indication which is proportional to Vs squared.
But forward and reflected watts cannot be displayed simultaneously on a
single meter movement.
Basic Meter Circuit (Fwd/Refl switch not shown)
Current Transformer
Primary
,-------------------W-------------o-------------,
¦ Turns ratio N --------- ¦ ¦
¦ ,MMMMMMM, Sec. RRR ¦
Source RRR ¦ ¦ RRR R1 RRR Load
Internal RRR ¦ R3 ¦ RRR RRR RL
Resistance RRR o-SHUNT-o ¦ RRR
¦ ¦ ¦ RRR ¦
¦ ¦ '----K-->RRR Cal potr. ¦ ZL=RL+jXL
¦ ¦ RRR ¦
Source ¦ ¦ XXX Load
Internal Diode rectifier RRR XXX jXL
Voltage + DC voltmeter RRR R2 XXX
¦ ¦ RRR ¦
¦ ¦ ¦ ¦
Ground --->o---------------o-----------------o-------------'
R1 & R2 is a voltage divider across the load. Fraction
K = R2/(R1+R2). Voltage across R3 is load current times R3/N which
can be switched either to add to or subtract from the fraction K. When
subtracting the meter responds to reflected waves and when adding it responds
to forward waves. The calpot is adjusted for a voltmeter null for reflected
waves when ZL equals reference resistance Zo.
To minimise power dissipated in the divider resistors
R1 should not be less than 100 times Zo for a 100 watt transmitter.
The preset potentiometer is included in the divider chain to permit precise
adjustment of the ratio K.
If a capacitance divider is used connect an RF choke across
C2 to provide a DC path for the meter current. Alternatively a high-value
resistor can be shunted across C2 which then must be included in the voltmeter
calibration resistance. Fine ratio adjustment is possible when C1 or part
of C2 is a preset capacitor. The ratio K = C1/(C1+C2). The reactance
of C1 should not be less than 12*Zo.
The voltmeter may be a 100 uA fsd moving coil meter with
sensitivity adjusted by a series preset calibrate resistor. The program
computes resistor setting.
Voltmeter Circuit
Ge diode Preset cal resistor
---> from SWR bridge --->o------>¦------o-------RRRRRRR------,
¦ ¦
¦ ¦
Disk ceramic === 0 to 100
0.01 uF ¦ micro-ammeter
¦ ¦
¦ ¦
Ground --->o--------------o--------------------'
Example of Toroidal Current Transformer
The ferrite ring core is of a size which when wound with 24 AWG wire
can be slipped over the coaxial polythene insulant. Typical dimensions:
OD=1/2"=13mm. ID=1/4"=7mm. Thickness 1/6"=4mm. With a permeability
of 200, 1uH needs 3.4 turns. 15 turns gives 19.5 uH which
has a reactance of 220 ohms at 1.8 MHz. A 33-ohm shunt resistor
gives a satisfactory reactance to resistance ratio of 6.7 This transformer
would be suitable for a 30 to 100 watt, 50-ohm SWR meter.
How to
Estimate Transmitter Internal Resistance (program asks for value)
For accuracy do the measurements on the lowest frequency band, say
at 1.9 MHz. Two dummy loads are needed, at least one of which has
the standard Zo for the transmitter. The other should be of the same or
preferably a higher value. Set the PA drive to give roughly 1/2 full rated
power into Zo and measure voltage V1 across Zo. Use T-junctions as necessary.
Do not readjust Tx drive again.
Now use a second T-junction to connect the second dummy
load in parallel with Zo and measure the load voltage again which will
fall to V2. Keep all coaxial leads short. Let the resistance of the two
loads in parallel be R2.
Source resistance, Zs = (V1-V2)/(V2/R2-V1/R1) ohms. Depending
on usage of ALC and type of output circuit Zs may lie between several 100-ohms
and Zo or less.
How to
Estimate Transmitter Internal Resistance (program asks for value)
For accuracy do the measurements on the lowest frequency band, say
at 1.9 MHz. Two dummy loads are needed, at least one of which has
the standard Zo for the transmitter. The other should be of the same or
preferably a higher value. Set the PA drive to give roughly 1/2 full rated
power into Zo and measure voltage V1 across Zo. Use T-junctions as necessary.
Do not readjust Tx drive again.
Now use a second T-junction to connect the second dummy
load in parallel with Zo and measure the load voltage again which will
fall to V2. Keep all coaxial leads short. Let the resistance of the two
loads in parallel be R2.
Source resistance, Zs = (V1-V2)/(V2/R2-V1/R1) ohms. Depending
on usage of ALC and type of output circuit Zs may lie between several 100-ohms
and Zo or less.
More Notes
Ideally, the ferrite core of the current transformer should have
a permeability large enough to need only one turn on the primary winding.
The single turn is then the inner coaxial conductor plus polyethylene
with a short gap in the braid. The gap in the braid should be no longer
than is necessary to obtain a connection from the inner conductor to the
voltage divider. Stray capacitance between the exposed inner and the secondary
winding can be included in the divider chain with careful layout. A frequency
range from 1.8 to 30 MHz should be possible.
Accuracy at the higher frequencies falls off due to stray
capacitance in the divider chain, to self-resonance in the transformer
secondary winding and to increase in ferrite core loss. However, core loss
is equivalent to a shunt resistance across the winding and adverse effects
can be reduced simply by experimentally increasing the value of the actual
shunt resistance above the computed value.
Core permeability should exceed 100. 200 is preferred.
500 may be too high due to associated high core loss. As a guide secondary
turns should be between 10 and 30. Few turns results in greater power dissipation
in the shunt which has to be physically small, preferably 1/2-watt or less,
for a good HF response.
Use metal film resistors. The shunt resistor should be
between Zo/4 and 3*Zo, the smaller values for high power transmitters,
higher values for QRP.
Calibration
Procedure
The program performs this procedure automatically after design parameters
have been entered and the circuit completed. The procedure modeled is as
follows:
Connect the meter between a transmitter and a dummy load =
Zo ohms. Switch the transformer secondary winding to the 'reflected'
position. At medium trasmitter output power, at a low frequency, vary the
potentiometer K for a voltmeter null. The ratio K is now equal to R3/Zo/N
and the RF voltage available to drive the meter full scale for forward
power is 2K times the load voltage at full rated transmitter power. Do
not adjust K again - that part of the calibration procedure is complete.
Now reverse transformer connections to the 'forward' position.
Alternatively reverse direction of transmission through the meter which
will now indicate a large value on the forward power scale. Drive transmitter
power output to a known value, say 100 watts into Zo ohms and
adjust the voltmeter calibration resistor for full scale = 100 watts,
or another preferred meter deflection bearing in mind that some loads may
result in an indicated forward power greater than 100 watts. Both
reflected and forward power, in watts, can now use this common scale and
transmitter output power = indicated forward power minus indicated
reflected power.
When calibrating the power scale, e.g., full scale =
100 watts, the 'known' power level should be checked by measuring
the RF voltage across load Zo.
Measuring Errors
after the Previous Calibration Procedure has been Performed
SWR meters function correctly only when used in transmission systems
having a Zo for which they have been designed. Meters and Zo's are not
interchangeable.
It is unnecessary for the whole system between transmitter
and antenna to have a common Zo, only that the meter, the line between
transmitter and meter, and transmitter internal resistance should all have
that value for minimum error.
Under these conditions the only internal meter errors
are due to tapping off a small fraction of line current and a small fraction
of line voltage which are compared with each other and then used to drive
a moving coil voltmeter. This upsets the line impedance by one or two percent
and something less than one watt of Tx power may be dissipated inside the
meter in the process.
When the source/generator resistance is not Zo - it seldom
is - reflected and forward power and also Tx output power will still be
correctly measured.
BUT the indicated reflection coefficient and SWR will
be in error, especially when the load has a large impedance angle. However,
if needed, the reflection coefficient can be calculated exactly from RC =
Sqrt(Refl.Watts/Fwd Watts). If the SWR is needed then SWR = (1+RC)/(1-RC).
How
to Obtain Direct Indications of Reflection Coefficient (RC) and SWR
As already described, when the source impedance as seen looking back
towards the transmitter from the meter is not Zo, the indicated values
of RC and SWR are incorrect but the true values can be calculated from
forward and reflected watts.
Note that RC = Reflected/Forward meter deflections regardless
of power level. So to obtain correct direct indication of RC and SWR proceed
as follows:
- Switch to forward power. At any suitable Tx output power adjust meter sensitivity for full scale deflection to read 1.0 for RC and infinity for SWR.
- Switch to reflected power and read RC and SWR directly on the meter.
- (1) and (2) must be repeated whenever the load impedance changes. This is because the Fwd power reading changes whenever the load changes unless the source resistance is equal to Zo. This meter characteristic provides a simple means of checking how close the source resistance is to Zo.
Note that adjustment (1) upsets forward and reflected
power measurements. The meter must now be re-calibrated at a standard power
output level into a dummy load having a resistance of Zo.
How to Obtain Simultaneous
Display of Forward and Reflected Power
Two separate 100 uA moving coil meters are needed. Each could could
have its own current transformer but, in practice, both meters can be driven
from one common transformer with a centre-tapped secondary.
Use the same core as used for one transformer and wind
on twice the number of secondary turns with a centre-tap. Connect the centre-tap
to the line voltage divider. Shunt each half of the winding with a resistor
twice the value computed for single-meter operation. Connect the rectifier
diodes to each end of the winding. Each meter circuit will require its
own decoupling capacitor and sensitivity control resistor. Input resistance
of the primary winding, sensitivity control resistors, and the amount of
RF power extracted from the line to drive the meter circuits will be the
same as for one-meter operation.
Location
of Current Transformer
The circuit diagram shows the current transformer in series with
the source. To make internal source resistance exactly equal to Zo, deduct
the transformer's input ohms from Zo before entering source resistance
into the program. Insofar as the source is concerned the voltage divider
impedance is in shunt with the load.
The program assumes RF power dissipated in the divider
chain is negligible.
Operating Reminders
- After calibration the meter will accurately indicate all parameters only when the source (transmitter) internal resistance is equal to Zo. This condition is checked by varying the load and looking for constant forward power indication.
- When the source resistance is not equal to Zo only forward and reflected power will be accurately indicated. Reflection coefficient and SWR will be indicated accurately only when RC = 0 and SWR = 1. (The primary purpose of the meter).
- Other values of RC and SWR can always be calculated from power readings.
- The meter can be made to indicate RC and SWR directly by adjusting sensitivity on every occasion a reading is required. But this necessitates re-calibration of the wattmeter scales.
- It is possible to prevent this adverse interaction between meter functions by selecting from the front panel a different sensitivity control for each of the meter scales. The program leaves the experimenter to design the circuitry.
- The data screen displays both indicated and true values for comparison.
