SWR METER
crossed pointer display (2013)



Here is another example of an SWR meter of handbuilt construction.
It uses a crossed pointer type meter display; ($15 with freepost!)
cheaply and readily available ex China (see graphic)
At the point of "crossing" of the two pointers gives a readout on the meter face; of the SWR (standing wave ratio) of your antenna!
Both meter movements are 100uA FSD with a meter resistance of about 1.5Kohm
However, Fwd Power full scale is marked as 10 Watt and reflected power is marked as full scale of 2 Watt.
Because we are esentially displaying a voltage but the meter calibration is in watts;
the value of the multiplier resistance's must be calculated with this in mind.
The "Power" i.e. Wattage dissipated in a circuit is proportional to the square of the voltage applied to the circuit.
This we know from standard formulae.
Therefore the two multiplier resistances would have to be in the ratio of the "square-root" of the ratio of the two Power scales on meter face.
i.e. 10W/2W = 5
sqr root of 5 = 2.2
I have used 33k and 15k multiplier resistors which divide in the ratio of 2.2
An additional 10K common series resistor is added to allows calibration adjustment.
I used 5 Watt as a calibration point as RF output from transmitter - terminated with 50 ohm resistive Dummy Load.
For the directional coupler I have utilized the Electronics Australia Magazine idea,
of a piece of coax with insulated pick-up wires threaded neatly under the copper shielding braid.
As it is only a short 90mm length; sensitivity is restricted to the VHF/UHF spectrum.
I get adequate deflection from a 5 Watt handheld transceiver
(also cheap Chinese) BaoFeng UV-5R ($43 as at 3Q/2013).