PWR METER Wishing to replace my good old trusty BIRD 43 Watt-meter, I prospected a little on the Internet and found the description of an open source project for a Watt-meter which corresponded perfectly to my needs.
This is a fork of the version made by TF3LJ, and developed by PD0LEW. A new hardware version was subsequently developed by WA2T. I chose the version with an OLED display, but there are versions with touch-sensitive TFT displays.



The main characteristics are:
- 1-50 MHz range
- measurement from a few mW to several kW
- instantaneous, PEP, direct and reflected power measurement
- measurement in W and dBm
- fast digital display and 2 bargraphs using an OLED display.
- automatic caliber change
- configurable audible SWR alarm
- coupler directivity > 30 dB up to 30 MHz and 20 dB at 50 MHz
- etc.

The assembly consists of a coupler installed in a shielded metal box and the Watt-meter with the digital display in another box.
The connections between the 2 are made using 2 coaxial cables connecting the FWD and REF ports.

Coupler

The coupler is made using a printed circuit and a HAMMOND 1590BS box.
It integrates a Sontheimer - Frederik type coupler followed by attenuators in order to make the levels compatible with the input levels of the CPU board.

The directivity is very good, the slightest mismatch is detectable. The use of SO239 sockets already introduces a measurable mismatch !
To test the effectiveness of the directivity, it is therefore necessary to solder a non-inductive resistor of 50 Ohm (for me 2 x 100 Ohm 1% CMS, in parallel) directly on the printed circuit at the output of the coupler. I thus measured between 30 to 40 dB up to 30 MHz, which is excellent and more than enough for our amateur needs!

Once put in a box with the SO239 connectors, we already measure a mismatch. Same as soon as you connect a piece of coaxial cable.
The transformation ratio of the 2 transformers is 24:1, so it provides a decoupling of 27.6 dB. Two attenuators are providing an additional loss of 9.63 dB on each port. The total attenuation is therefore 37.23 dB, which is confirmed by the VNA measurements.
In order to improve the frequency linearity of the coupler, an 82 nH inductor and a 15 pF capacitor have been added. I found that the linearity was better WITHOUT these components!

Coupleur 2 sur PCB <