Elecraft CP1 User Manual
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The Elecraft CP1 is a dual directional coupler that provides high directivity, can be internally or externally
terminated and can be constructed with a coupling loss of either 20 dB or 30 dB. It can be used to sample a
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measurements in conjunction with the Elecraft 2T-gen (2-tone generator).
Specifications
Frequency Range
1 MHz to 30 MHz
Coupling Loss
20 dB or 30 dB (29.8) +/- 0.25 dB
Directivity
30 dB typical
Power Rating
30 dB coupler, 250 watts; 20 dB coupler, 25 watts
Parts List
(1)
CP1 Printed Circuit Board
E100191
R1, R2
(2)
Resistor, 51-ohm, 3W Resistor
E500178
S1, S2
(2)
Miniature slide switch
E640009
J1-J4
(4)
BNC Connector
E620020
See text
(2)
Toroid Cores, FT50-43
E680008
(2)
Toroid Cores, FT50A-61
E680025
(4)
Self-adhesive mounting feet
E700024
(10 ft.)
#26 red magnet wire
E760002
(3 in.)
#18 gray insulated wire
E760021
Assembly
Before winding the toroid inductors, decide whether you want to construct the CP1 as a 20 dB
coupler or a 30 dB coupler. For power levels of 25 watts or less, a 20 dB coupler will result in greater
useful measurement range with commonly-used detectors. With power levels higher than 25 watts, a
30 dB coupler will be more useful.
Use a permanent marker to write the coupling loss in the space provided on the printed circuit board.
See the photographs on page 3.
If you are constructing a 20 dB coupler, 10 turns will be wound on each toroid core. If the coupling is
to be 30 dB, then 31 turns will be used. For 10-turn windings, cut two 12 inch (36 cm) lengths of
magnet wire. For 31-turn windings, cut two 40 inch (102 cm) lengths of magnet wire. Note that two
types of cores are supplied. The thicker core, E680025 is used for the 30 dB coupler and the thinner
core, E680008 is used for the 20 dB coupler.
Using the photograph as a guide, wind the required number of turns onto the toroid cores. Make sure
that the direction of the winding is the same as in the photograph. Turns are counted by the number
of times the red enamel wire passes through the inside of the toroid. The photograph shows a 10-turn
inductor; the 31 turn inductor is identical except for the number of turns. The wire should be evenly
spaced around the core and will occupy about 90% of the core.
Remove the enamel insulation from the ends of the wires. A hot soldering iron, can be used or
sandpaper can be used. When the insulation is removed, the ends of the wire should be tinned with a