Directivity of a Directional Coupler
Directivity of a Directional Coupler
(OP)
I guess this is a similar question/problem to my last question on waveguide spacers.
(These are factual statements not academic thought experiments) I have a 10dB directional coupler with a claimed directivity of 30dB. I have four nominally identical terminations each claiming to have a VSWR of better than 1.06 (p=0.029). The terminations and coupler are from the same manufacturer. I tried to verify the directional coupler at a spot frequency using a power source and yet another directional coupler to give me constant incident power.
So I did the experiment as it says in my text books, just swap the coupler around and take 10*log10 of the power ratio. Easy. The trouble is the results don’t seem to tell me much at all. All the power ratios were above 28dB, which is consistent with vector summation of the error terms (it could be around 24dB worse case). The trouble is that it seems that I cannot set a limit on the directivity of the coupler at all. For example a 15dB directivity coupler and 15dB return loss terminations could give the same result. Certainly if the coupler directivity is poor the termination would have to be similar in order to get a good overall result, but that doesn’t give me a limit.
It seems that I need something else to separate the variables. Perhaps a sliding termination?
How do engineers normally handle this situation? (Other than getting someone else to calibrate the loads/couplers of course)
(These are factual statements not academic thought experiments) I have a 10dB directional coupler with a claimed directivity of 30dB. I have four nominally identical terminations each claiming to have a VSWR of better than 1.06 (p=0.029). The terminations and coupler are from the same manufacturer. I tried to verify the directional coupler at a spot frequency using a power source and yet another directional coupler to give me constant incident power.
So I did the experiment as it says in my text books, just swap the coupler around and take 10*log10 of the power ratio. Easy. The trouble is the results don’t seem to tell me much at all. All the power ratios were above 28dB, which is consistent with vector summation of the error terms (it could be around 24dB worse case). The trouble is that it seems that I cannot set a limit on the directivity of the coupler at all. For example a 15dB directivity coupler and 15dB return loss terminations could give the same result. Certainly if the coupler directivity is poor the termination would have to be similar in order to get a good overall result, but that doesn’t give me a limit.
It seems that I need something else to separate the variables. Perhaps a sliding termination?
How do engineers normally handle this situation? (Other than getting someone else to calibrate the loads/couplers of course)





RE: Directivity of a Directional Coupler
Real life coupler performance for both coupling and directivity depends on your load quality at your connectors. Hence, you may not be able to get the results they state in real life without ultra low rtn loss loads.
kch
RE: Directivity of a Directional Coupler
I just found a VNA that will do the job. I hate to think how much it is going to cost but that is someone else’s problem!
RE: Directivity of a Directional Coupler
kch
RE: Directivity of a Directional Coupler
I think I must be missing out on some fundamental metrology considerations. I know standards labs use air lines as calculable impedance standards. This must get tricky for waveguides as the impedance changes with frequency ratio from cutoff. I suppose a standard straight guide is an impedance standard, like an air line, but the termination can't be checked at DC and its inductance calculated as you could with a coaxial standard.