Bridge: RLB  VHF

Return loss bridge , target UHF but failed @9/11/2013

@update whole project , 2013/9/23

Wes Hayward, w7zoi, given a beautiful article   <Toward a Simple Return Loss Bridge>, which explain and test several implementation about RLB.

I try to built one, and hope it work up to 500Mhz,  even-through i failed to achieve this goal, but still get a bridge work up to 150Mhz.

Here  is the w7zoi's RLB:

T1: BN-73-2402   8T :    6 twister per inch.

T2: FT-37-61  (better UHF than 37-43)     11T:   6 twister per inch.


first version draft design of the PCB (FR-4)  

[note this is not the final version of PCB]

RLB sch

Here is hint how i get a pecice resistor from 5%  0805 resistor

parallel the 51R resistor with another 3.3k to 4.7k 0805 chip resistor, we got a good resistor  very close to 50R.

51|| 4.3k

51|| 6.2k

51||3.9k

result :  49.98~50.002  (use at least 4 1/2 half DVM).


And all trace use 2H type 50R Microstrip line.

finished version of the RLB

One concern is the 2 transformer is too close, and T1 is almost contact the transmission line, this will be a problem at UHF.  but i had to give it a try.

The final revise and test Result 

* i use some copper to isolation the input and output, which improve the directional about few dB.

* use hot glue gun fix the transformer, which improve about 8dB at above 100Mhz, i think the hot glue cut of some signal couple path.

Performance

Conclusion

Performance:

1-150Mhz,   directivity > 30dB

170Mhz, directivity> 20dB

400M?, unusable.

why UHF failed:

1.BNC connector is hard to get perfect 50R load, so the testing might not the actual performance of the RLB.

2. T1,T2 is too close and fly over the  input.

3. other i don't know yet.