[SOLVED] How to measure directivity of a directional coupler?Beginners question

[themaccabee]

Hi all,

I ve got a dual directional coupler Narda 3020A which has a minimum directivity of 35dB.But can somebody tell me how to measure the same?

I was following the readings at
Power dividers and directional couplers - Wikipedia, the free encyclopedia
Using directional couplers
The second one details a method for measuring directivity
But when i do so
Input power =0dBm at 500MHz
Coupling at 500MHzat REV Port=20.11dB
Power measured at REV Port for 0dBm input=-41.72dBm
So according to the webpage the directivity is 41.72-20.11= 27.61dB only
The other ports are terminated by 50Ohms as said.
the datasheet guarentees minimum 35dB directivity
Im i wrong in calculation/measurements? Can somebody correct me??

Regards
Steve.

 

[FvM]

You calculation assumes an ideal termination. I guess, you don't have it, as far as I know, it hasn't been yet invented.

P.S.: The best terminators, I could identify are Huber&Suhner N-precision series with VWSR < 1.03 below 4 GHz. They would nearly allow to verify the said specification. What's the specification of your terminator?

 

[themaccabee]

Thank you ..FvM you are right.., my termination wasnt ideal..i switched to a better termination (a standard one) then the measurement improved greatly..
Thanks again for the help
Steve

---------- Post added at 18:56 ---------- Previous post was at 18:48 ----------

By the way..Can you tell me how exactly these standard terminations differ from ordinary 50Ohms resistor terminations?
Do they internally have an ordinary 0603 or 0805 resistor?At high frequency i see in the standard terminations u mentioned ie in a Huber & Suhner series ..that they have mentioned VSWR 1.05 upto a frequency and 1.07 at another band..
Is this is the exact difference of these standard terminations from an ordinary resistor? ie..their special construction that improves vswr at higher frequencies??
Thanks
steve

 

[FvM]

I assume, that high performance terminations are using special shaped (e.g. circular) resistors to minimize parasitic elements like series inductance.

 

[themaccabee]

thanks Fvm... i think its clear to me..after some more reading though..