pa0nhc directional couplers

(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.

These directional couplers have 20mm log coupling tracks, with abt. 30dB coupling loss on 145 MHz. This value ensures negligible influence from the output ports to the the 50 Ohms thru-line impedance, and less chance of damage to connected signal generators, or generating IMD. At 435 MHz the coupling loss is abt. 20dB. At 14 MHz the coupling loss is abt 50 dB, making it to insensitive for most uses.

The new developed PCB versions with DC output use modern SMD Skottky diodes with very low signal threshold, and a 51 Ohms 1W (or 2x 100Ohm 1W in parallel) SMD terminating resistors. This indicates a maximal power handling of abt. 800W. DC outputs have an internal resistance of abt. 100 kOhms.

Loaded with a hi-Z digital DC meter (>= 1Mohm), and RF power input of 1W @ 145 MHz, should indicate at least 100mV DC output.
With 800W input 9.4Vdc output should be expected.

        New versions.
The old 2003 versions of 200DPI DIY PCB masks were drawn with a drawing program, so no Gerber files of them exist.

To be able to order professional PCBs, i designed new versions with the same track dimensions but newer type SMD components, using PCB design software "FREEPCB"

One PCB with DC output but without BNC busses enables installment inside equipment, connect by on-soldered coax cables.

I advise you to use PTFE (Teflon (R)) insulated BNC busses. They can withstand temperatures up to 400C, so you can solder their rim directly onto the steel housing, without damaging the position of their central pins. To be sure, place a BNC plug on the bus before soldering.

Use two long ground lugs at each BNC, to connect them to the "ground" surfaces of the PCBs.

The whole PCB should make NO electrical contact to the steel housing, exept at the BNC busses.

        Possible uses.

Transmitter spectrum measurement, using a receiver or spectrum scope, with an attenuator and a standard dummy load.
2 Weak, influence-free coupling of a signal source onto a signal line.
3 Receiver sensitivity and transmitter sideband noise measurement of a fully operational full duplex repeater.
4 Return loss measurements, using microwatts from a signal generator, and a receiver as output indicator..
4 Power- and SWR measurements, connected to a DC metering circuit.
5 Automatic adjustment of output power and load match, using a version with DC output..

A directional coupler with HF-output can be used to measure the match of filters. Conveniently using only microwatt-power, from a simple signal generator. A powerful generator is not needed to measure filters (see "Band pass filters for 145MHz").

It also showed to be ideal in use for sensitivity measurements on our Rotterdam repeaters, which were working in full duplex mode on a dummy load or an antenna. The coupler was inserted between the antenna and the duplex filter, with the signal generator connected to the "return" port, via variable 50 Ohms attenuators. 

Tests on the older prototype directional couplers with HF-output, clearly showed differences in reflection damping, between different types of low power 50/51 ohms terminating resistors and dummy loads. Good terminators showed over 20dB return loss.