Directional Couplers

[Mikebits]

Hello, I have a few questions on couplers. I want to sample both forward and return signal. In one website I visited concerning couplers, they said you should use 2 couplers back to back, but this adds insertion loss, and I don't like that. Can't I just use one? Also, I assume all ports should be terminated into 50Ω, am I correct? So which way is correct in my attached image?
Thanks

 

[MikeMl]

Most of the SWR sensors used for HF up to VHF use just one coax section for the power path, with two sampling links. The classic Bird RF wattmeter uses a thru-line with a single, rotatable sampling link.

 

[JimB]

I am not as au fait with directional couplers as I would like to be, but in the spirit of self learning I found this, have a look here:

https://www.microwaves101.com/encyclopedias/directional-couplers

From this I think that your scheme with the single coupler module will not work as you hope for.
The first scheme with the two modules should be OK within its limitations.

When I first saw the datasheet for the MiniCircuit coupler module, my thoughts were that the directivity was not very good. This is (sort of) confirmed by end of the Microwaves 101 article in the section about reflectometers.

JimB

 

[Mikebits]

Thanks Jim, Nice website I bookmarked that. I like the comment on the page.
"You can build a reflectometer using a single directional coupler to form a reflectometer and two power sensors, but it is not recommended (use the dual-directional coupler you cheapskate!)"

Guess I will go with 2 couplers. I will see if I can find a coupler with better directivity (Is directivity the isolation between the 2 coupled ports?). I think internal terminations will also be better.

This part on the website confuses as it pertains to terminations. The part in red, is that to say the coupled port should be un-terminated? Im confused.

Bi-directional property
Any directional coupler is bi-directional, that is, it performs equally well when the signal is incident on port 2 versus port 1, but the coupled and isolated ports flip. All direction couplers are bi-directional, unless you terminate one of the ports. Consider the coupled-line coupler below. Port 4 is the coupled port when a signal is incident on port 1, and port 3 is the coupled port when a signal is incident on port 2.

Thanks

 

[Mikebits]

Okay found the answer on directivity.

Directivity is defined as
the difference in dB of the power output at a coupled port, when power is transmitted in the
desired direction, to the power output at the same coupled port when the same amount of power is transmitted in the opposite direction.
Coupling factor = 10 log ,
where P1 is the input power, and P2 is the coupled power.
Directivity = 10 log ,
where P2 is the coupled power for the forward signal,
and P2R is the coupled power for the reverse signal.
Directivity (dB) = Isolation (dB) – Coupling (dB)
Finite isolation is the reason for limited directivity.

 

[Mikebits]

Most of the SWR sensors used for HF up to VHF use just one coax section for the power path, with two sampling links. The classic Bird RF wattmeter uses a thru-line with a single, rotatable sampling link.

Mike, not quite sure I understand what your saying.

 

[JimB]

Mike, not quite sure I understand what your saying.

What the other Mike is getting at is that most (V)SWR meters have a coaxial section into which are built two coupling lines.
One line to sense the forward power, the other line to sense the reverse (reflected) power.

He goes on to say that in the Bird 43(?) type wattmeter, in the "slug" is a sampling line, turn the slug in one direction and it measures forward power, turn the slug through 180 degrees and it measures reflected power.

The slug is an easily exchangeable insert, many different slugs are available, covering different frequency and power ranges.

JimB

 

[Mikebits]

The only directional couplers that I could find with better directivity of about 30 dB were about $380.0 I don't think SMT versions get much better than 20 dB

 

[Mikebits]

I am still unclear as to why two back to back couplers is better than a single coupler used as a 4 port device. Much of the coupler theory is like magic to me.

I found this interesting article on coupler directivty.
https://www.google.com/url?sa=t&rct...LAsynI3xw&sig2=XZnvyF1b3LWG3lxZokl1sg&cad=rja