[Amps] Power metering in linears

[Thomas Hoeppe]

Hello technicians,

I was adding a power meter to a new homemade linear. I used the directional coupler of the "Digital PEP Wattmeter and SWR calculator" from an old ARRL handbook (1987). I did not get a too deep null on calibrating, so the directivity was only 20 dB. As I only wanted to monitor the output of a linear with that coupler, the directivity was sufficient. 

The problem: The output voltage for a given power is the same from 40-10m, but nearly null on 80m and different on 160m.

Then I tried the design from the linears in the W6SAI handbook. They use Amidon 50-6 toroids. The output voltage ón 160 is about 1/2 of the voltage on 10m. 

Has anyone successfully built a directional coupler with constant frequency behaviour from 160-10m?

I would like to have proven designs.

Thanks!

Tom, DJ5RE