Directional Couplers with RF output and DC output for 28MHz to 435MHz
pa0nhc.  Ver. 20031003. / .20110306 /  20130509 / 20150217 / 20170929 / 20171231 
20180121 / 20210310 / 20211225.
This site is located at : www.pa0nhc.nl
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(C) The use, copy and modification of all info on this site is only permitted for non-commercial purposes, and
thereby explicitly mentioning my radio amateur call sign "PA0NHC" as the original writer / designer / photographer / publisher.

How to order factory made PCBs
PCB assembling.
usage, drilling and 
installing the BNC busses.
for DIY PCBs
(also for N busses)
details of these 

For an introduction see the text below.

Prototype coupler with DC-output in a small TEKO 371 box.
The BNC busses with PTFE insulation are directly soldered|
onto the OUTside of the box.

Tests on the prototype like shown on the photo shown above, showed nearly NO reflected power at 25W transmitter output,
when terminated with  good dummy loads. And it clearly showed differences in return loss between different brands of small 50/51 ohms terminating resistors, and a high power dummy load. Good terminators showed up to 30 dB return loss in the short wave range up to 30MHz. Nominal FWD output on 145 MHz is -32 dB in respect to power input.
Decoupling between RFL end FWD ports >= 20 dB.

Below the FWD and REFL coupling attenuations and the port VSWR, of the prototype coupler RF 372.
Measured with a calibrated NanoVNA Vplus4, on a grounded surface,
and 1m RG58 coax between VNA port0 and coupler main channel input.
All coupler outputs were terminated with 50 Ohms.
Red = coupler port output. Blue = coupler port VSWR. 

Forward port output 29MHz : -45.7 dB.

Forward port output 145MHz : -31.9 dB.

Forward port output 435MHz : -22.6 dB.

Reflected port output 29MHz : -70.4 dB.

Reflected port output 145MHz : -55.2 dB.

Reflected port output 435MHz : -42.3 dB.

Schematic of a coupler with DC output

R1 R101

50 Ohm 1%  SMD 2010 (2x 100 1W parallel)

R2 R102 1k  SMD 1206
R3 R103 100k  SMD 1206
C1 C101 1n  SMD 1206
C2 C102 10n  SMD 1206

Attention for the correct position of D1 (BAT62-03W).
The wide leg (= anode) is here at bottom right.


An indication of the DC output.
Measured on prototype DC type PCBs
with BAT62 skotky diodes.
(A=DC371 and B=DC372)
REM : DCoutput can vary with 
installed diode types.

A/B port 25W 2,5W 0,25W


(V) (V) (V)
A fwd. 1,91 0,52 0,070
B fwd. 1,90 0,52 0,065
A refl. 0,039 0,0038 0,0001


A fwd. 5,28 1,475 0,33
B refl. 0,300 0,0485 0,0043
A refl. 0,361 0,057 0,005

     Some details.
RF type PCBs
have RF output. Only two loading resistors of 50 Ohms 2W 1% tolerance SMD (or two 100 Ohm 1W on top of each other) have to be soldered on these PCBs. 

The max power handling of these couplers depends on the wattage of the terminating resistors R1 and R100. Soldering here 2W resistors, the coupler max power is 1.5 kW on 145 MHz, and 150W on 435 MHz.

For t
DC type PCBs more parts have to be soldered. See for details at the right side side of this page, and on page :

Assembling page.

Four PCB layouts with different dimensions are meant for three different TEKO boxes. The to be used TEKO box type is mentioned onto each PCB top silk. The smallest PCB is for not soldering into a box, but for soldering coax cables directly onto the PCB. 

Installing the DC type PCB into the TEKO boxes is identical to the RF type PCB.

        Applications :
The RF-output directional couplers + a portophone or signal generator + a step attenuator + a common receiver with the S-meter as indicator, can be used to measure with very low RF power the return loss of  :
- Small receiver filters,
- Large cavity filers,
- Large helix filters : 
    - Low pass,
    - High pass,
    - Band pass..
- Antenna systems.

(See PI3RTD and pa0nhc cavity filter).

          Applications for RF output couplers.
An RF output coupler can act as an attenuator which does NOT influence the measured circuit.
They can be used for instance to safely inject RF signals into a live operating
repeater, by inserting such an RFcoupler into the antenna cable. Then measure the following repeater properties without generating spurs, nor damaging the signal generator by the repeater output.
-  Its real practical sensitivity,
- The correct action of the CTCSS system,
- The squelch action,
-  Large signal handling (blocking).
-  The
duplex filter using a portophone at low power, and additional step attenuators 
    - Frequency characteristic of the duplez filter
    - Up to -120dBc suppression at the notch frequencies can this way be measured !!

          Applications for DC output couplers.
- RF transmitter output (145-435 MHz 1W - 800 W)
- Returned power from loads such as antenna's (1W - 800 W)
- The small "DC COAX'"  PCB build inside an RF power stage for VSWR level for shutdown.