Elecraft CP1 - Manual (Page 2)

2
light coating of solder. The insulation should be removed to within about 1/8 inch (3.2mm) of the
core.

Mount each inductor in its indicated position. When wound and mounted correctly, the enamel wire
will emerge from the top of each core and connect to the top hole at each inductor location.

Cut two 1.5 inch (3.8cm) lengths of the gray #18 insulated wire.

Remove enough insulation from each end so that 5/8 inch (1.6cm) of insulation remains in the center
of each wire for a 20 dB coupler or 3/4 inch for a 30 dB coupler.

Form the wire into a loop, then connect the wire from the pad in the center of each core, over the top
of the core and to the pad centered between the enamel wire connections. The wire insulation should
rest on the top of the printed circuit board with no bare wire exposed. See the photograph.

Install R1 and R2 in their indicated positions. The resistors should be spaced about 1/16 (1.6mm)
above the printed circuit board.

Install S1 and S2 in their indicated positions. The switches can be mounted in either direction.

Install the 4 BNC connectors in their indicated positions.

Install 4 mounting feet on the back side of the board as shown in the photo.
This completes assembly of the CP1.
Circuit Description
The coupling circuit used in the CP1 is very simple and requires no adjustment. It provides predictable
coupling loss and very high directivity. Power flowing from J1 to J2 is sampled by the transformers so that
it adds in phase at J3 and cancels at J4. Power flowing from J2 to J1 behaves identically, but in this case
adds in phase at J4 and cancels at J3. A more detailed treatment of this circuit can be found in most recent
ARRL Handbooks; for example, see the Tandem Match construction article by John Grebenkemper,
KI6WX.
Application
The CP1 is a very useful tool in the ham shack, especially that of a homebrew enthusiast. The CP1
separates the power flowing in each direction on a transmission line. With the two switches in the down
position, the power from each direction flows to the connector indicated by the silkscreened arrows. By
measuring the power at each connector, an accurate measurement of forward and reflected power can be
made. This is very useful when matching an antenna, the input to an amplifier, or other devices. (SWR
bridges and power meters use a similar coupler, often with inferior performance, driving a diode detector.)
An ordinary oscilloscope may be connected to the forward power connector and used as a monitor scope.
When used with a spectrum analyzer, the CP1 can be used at either port to observe the transmitted
spectrum, including IMD if used with the Elecraft 2T-gen or other 2-tone generator. If a sweeper is
substituted for the transmitter, then swept-frequency measurements can be made.
The two switches allow 3-watt 50-ohm terminations to be connected to either RF sampling port. This is
useful when simultaneous forward and reverse measurements are not required. For accurate readings when
either J3 or J4 is NOT connected to a piece of test equipment, the switch next to the unused BNC connector
should be in the UP position to use the internal 50-ohm resistor.