swr meter couplers

[p.larner]

ive looked at a few hf swr coupler/bridges,some only use 1 torroid and some use 2 or 3 why is that?.

[jwet]

Measuring SWR requires you to measure forward and reverse power.  Forward power is usually measured as current with a 1:N transformer formed with a single turn signal going through a toroid with a secondary to give you a measurable voltage out- 10 turns is common.  Reverse power can be sensed as voltage with N:1 transformer across the output using a toroid with N turns (enough to give an impedance >>50 ohms) to a N turn secondary (N usually 10-100 turns).  Alternatively, voltage can be measured with capacitive divider on the output side- this saves a toroid.  The two should be equivalent though the two toroid design is usually more broadband if done correctly.  The ARRL handbook has good explanations of this as well as some projects.  The name for the most common of these bridges is usually attributed to Breune- good Google search term.

[p.larner]

i was more interested to know how its done with 1 rather than a few.

[jwet]

It can be done with none if you sense current without a transformer.  It will be quite lossy and probably only suitable for QRP application.

[geggi1]

You should check out LA8AK (sk) web. It is still operational after more than 10 years of passing.
He have made a description of a directional made of coaxial cable. https://www.noding.com/la8ak/m12.htm
The design is pretty genius with a low component count and no hard to get toroides,