Directional Coupler directivity Improving..Beginners questions

Hi ,

I was trying to build a VSWR measurement system.. using a directional coupler Narda 3020A which has a 40dB directivity and AD8302 evaluation module..

I understand now from a previous thread that VSWR measurement accuracy depends upon directivity of directional coupler used.

Ive got two proposed connections for taking out incident and reflected coupled waves as shown below...

Can somebody tell me which will yield best results and why?(considering only the coupler connections)

Also will the directivity of the coupler will be enhanced / improved in any of these systems?
1)

2)

Thanking You
Steve
Please help

I can't see your image, and don't know why?
I think you should use coupled port, don't use isolation port. Because coupled port have higher power, and that is easy to measure. If you think coupled power is too much, use some attenuators.
Good luck.

I ve added the images as attachements..
Regards
Steve

I think the second is better, because the VSWR of INPB has less impact on the other input. Since your freq is only 500MHz, very low, you can use some 0603 package SMT resistor, 49.9R, 1%. Make sure the resistor PADs are the same width with the line. And you can use some Pi resistors attenuators between the 50R and the Narda port. Such as you insert 5 3dB resistors Pi attenuators between the 50R and narda port, you can improve VSWR about 30dB.
Good luck.

May be im asking a stupid question...but considering the second method as a complete system(as a whole)..Can we tell the directivity of that system is better than the first system?
Or is it simply better because both inputs INPB & INPA are isolated now?
Thanks
Steve.

Directivity is a measure of how the coupler isolates forward and reverse signals.

Using one coupler and if you inject power in Port_A[dBm], directivity in dB is:
Directivity[dB] = Power_D(dBm) – Power_C(dBm)

Using second method with two couplers and doing the same thing, you get the same directivity, because forward and reverse signals are the same (plus some small insertion loss due to second coupler).

As in your previous thread, the directivity is rely on the matched load. In the 1st method, the INPA or INPB VSWR is not so good, so the directivity will be discounted. But in the second method, that's better.

Thank you vfone & thank you tony_lth i thinks its now clear for me :)