Much information about couplers are found here, but I just had to show you this extreme construction!
Jigsawing and mechanics
The tube with the coupler is made of copper. The inner-diameter of the outer tube is 98mm and this type of feeder is normally called a “RL98”. From the beginning this tube was about a metre long and quite heavy. Robert at work convinced me to cut it down to about half a metre and the pictures are from this jigsaw cutting.
It's no point having this type of coupler if you can't adjust and calibrate it! Since it was hard getting less than 50dB coupling I decided to go for this. It's a little high for reflection-measurements since a 60dBm output and a 30dB returnloss would give you -20dBm on the reflected coupler port. This could be hard to measure, but maybe I just have to buy a new and more sensitive sensor to my power-meter...
Since the RL98 is made for quite some powerlevels I have never been worried about the attenuation, but I decided to measure it. This first measurement show the attenuation with a double 7/16-female connector. (I had to do this way since I had no calibration-normals for 7/16-connectors.) The attenuation is 0.005dB. The next measurement show the attenuation with the coupler and it's 0.01dB. This means that the total attenuation of the coupler is in the neighbourhood of 0.005dB at 144MHz. You can also clearly see that the measurement show that the coupler can be used up to 23cm but NOT higher! The dimension of the tube is so large it'll act as a waveguide.
Coupling factor adjustments
I began to adjust the sensor to about 50dB coupling-factor. After this I reconnect the network analyser to measure the other way to optimize the isolation. The isolation is optimized by rotating the sensor.
I decided to mount the coupler on the wall over my desk. In this way it's easy to connect the TX-feeder when it comes down from the ceiling and the amplifier from the 19-inch rack at the right on this picture. On the left I have a power-meter to measure the output or reflected power.