# Forward and Backward Wave Couplers - RF Cafe Forums

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Jeanalmira
Post subject: Forward and Backward wave couplers Posted: Wed Jun 28, 2006 9:51 pm

General

Joined: Tue Mar 15, 2005 11:43 pm
Posts: 65
Location: Singapore
Hi All :

I am confused about backward and forward wave couplers. I read that :

1. Waveguide couplers couple in the forward direction (forward wave couplers). So that the coupled port is closest to the input port.

2. Microstrip/ stripline couplers are backward couplers, so that, the coupled port is closest to output port.

I understand that it's called rule of thumb. But I do not understand how to differ between forward and backward wave couplers. What are their main differences?
And why waveguide coupler couple in the forward direction, while microstrip coupler couple in the backward direction?

I hope somebody outthere can clear my doubt

Thanks so very much.

Regards,
Jean (Eudyna Devices Asia Singapore)

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Stephen
Post subject: Posted: Thu Jun 29, 2006 1:24 pm

Captain

Joined: Wed Jun 21, 2006 8:33 pm
Posts: 21
Location: Queen Creek, Arizona
The primary concept is a parameter that can be generated from 4 port S-Paramteter measurements.

this paramter is called Directivity and is defined as:

Directivity D = 10*log(P4/P3)=-20*log(S31/S41)
if port 1 is your input port.

Below is a pathetical; graphic of a 4 port black box.
1---[ ]---2
4---[ ]---3

Directivity can be explained as a ratio of coupling factors between the input port and port 4 (backward coupling) and port 3 (forward coupling).
Thus you See the S31 and the S41 in the equation.

When you analyze a microstrip, it is noted that most of the coupled energy from port 1 appears at port 4 (S41 > S31). In a waveguide, most of the coupled energy appears at port 3 (S31 > S41). Note that I am talking about the coupled energy, not the incident and reflected power which is what is going on between ports 1 and ports 2.

Does this help?

_________________
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Jeanalmira
Post subject: Posted: Fri Jun 30, 2006 12:09 am

General

Joined: Tue Mar 15, 2005 11:43 pm
Posts: 65
Location: Singapore
Hi Stephen :

Thanks so much for your explanation.

But I'm wondering why : in a microstrip, the most of the coupled energy from port 1 appears at port 4 (S41 > S31) while in a waveguide, most of the coupled energy appears at port 3 (S31 > S41) ?

I am not sure about this. I'm sorry that I do not have enough background/ knowledge on waveguide structure.

Hope somebody out there could help me clear my doubt.

Thanks sooo much.

With best regards,
Jean (Eudyna Devices Asia Singapore)

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yendori
Post subject: Posted: Tue Jul 04, 2006 12:32 am

General

Joined: Thu Sep 25, 2003 1:19 am
Posts: 50
Location: texarcana
All I can offer is my understand of microstrip operation and why 1-4 > 1-3. referring to:

1---[ ]---2
4---[ ]---3

Imagine the input "1" = 0°
The will make "2" 90° (quarterwave line)
and "4" 180° (electromagnetic coupling)

Due to odd mode operation, "4" will be 270°, but due to even mode operation "4" will also be 90°. Therefore 270° and 90° cancel at port 4.

Port 2 and 3 are 90 degrees apart, hence quadrature couplers.

Now that my curiousity is up, I'll look into waveguide operation.

Rod

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Stephen
Post subject: Posted: Tue Jul 04, 2006 1:35 am

Captain

Joined: Wed Jun 21, 2006 8:33 pm
Posts: 21
Location: Queen Creek, Arizona
An easy way of explaining it with physics is simply this:

For a microstrip, following on yendori's comments:

Even mode impedance is Zo_even = Zo*Sqrt((1+Cf)/(1-Cf))
Odd mode impedance is Zo_odd = Zo*Sqrt((1-Cf)/(1+Cf))

When you calcuylate Voltage (or current) components of the network, you do see as Yendori pointed out that the components cancel out at port 4 and port 3 (they cancel out because they are of opposite polaties and equivalent magnitudes).

The question remains: so why do you get coupled energy at port 4 and not port 3?

This is because the reflections. Reflections of the even and odd components are of equal magnitude, but again opposite polarity which now places the reflected energy at port 4 in constructive interference.

Another reason microstrips are called Backward wave couplers is because the energy at port 4 is generated by reflected waves as incident waves cancel out.

I have not done a mathematical analysis of waveguides before so I can not speak with authority on then, but I imagine it is a similar yet opposite effect.

Reflections and constructive/destructive interference are the cause of seeing energy at port 4 and not port 3.

_________________
CMOS RF and Analog ESD Specialist!
www.srftechnologies.com

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yendori
Post subject: Posted: Tue Jul 04, 2006 4:30 pm

General

Joined: Thu Sep 25, 2003 1:19 am
Posts: 50
Location: texarcana
OK, I am mildly reatrded.

I typed "4" where there should be "3" . Let me try again.

referring to:

1---[ ]---2
4---[ ]---3

Imagine the input "1" = 0°
The will make "2" 90° (quarterwave line)
and "4" 180° (electromagnetic coupling)

Due to odd mode operation, "3" will be 270°, but due to even mode operation "3" will also be 90°. Therefore 270° and 90° cancel at port 3.

---------->
0° 1---[ ]---2 90°
-
-
->
180° 4---[ ]---3 90°/270°
<----------

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