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Reflection Bridge


I decided to build reflecion bridge with balun configuration according to Ivan Makarov VE3IVM, which reaches better directivity compared with a traditional transformer configuration.

The resistors value should be 50 Ohm. I used 0.5W Panasonic thick film resistor 100 Ohm, 1%, two in stack parallel in every section. Resistor size is 1210 (metric 3225), voltage rating 200V. Farnell part number is  238-0671. Bridge is DC blocked, ATC capacitors M1 size B are used. Transformer uses ferrite binocular core FB-43-2402 with 8 turns 0.25mm bifilar.

PCB is mounted to the Hammond case 1590LLB (50x50x 25mm). Distrelec part number is 30 02 71.

My N2PK VNA in transmision and reflection mode was used for the measurements. The following pictures show the basic measurement results. First I measured reference level for DUT open (blue curve). Then I terminated DUT port with precision 50 Ohms and measure isolation between Input and Output (red curve). The difference between both levels for each frequency gives Return Loss.  Return Loss varies from -28dB to -38dB in the whole frequency range 2 - 60MHz. It is good result. 

My bridge was primarily designed for intermodulation measurements. The most important quality parameter (except of Return Loss), is impedance matching between all ports and their terminations. Even when Output port termination is not critical for Return Loss measuring, when this port will be used for second signal generator connection during intermodulation measuring, its impedance matching will be very important for accurate measuring results.
That was the reason, why I added SWR measuring  for all three ports to diagrams, too. SWR at all three ports input was better than 1.1 : 1. That is also good result. This Return Loss Bridge can be used for IM measuring also.

I was interested in which frequency range will have this type of RLB good performance. Measurement was done in frequency range from 20MHz to 320MHz using the Signal Hound USB tracking generator and spectrum analyzer with results shown on the following pictures. Return Loss curve varies from -24dB to -38dB. Even if only short waves and 6 meters are matter of my interest, this type of RLB is usable with appropriate parameters in the VHF band, too.

In another chapter I described my kit for intermodulation  measuring,  using VNA  as a source of RF signals.  In the future I am planning to use double DDS with amplifier and LPF for this purpose.  I will work with the signal level  +30 dBm  or little more.  For this purpose I need corresponding combiner with good parameters.  Combiner with a larger ferrite core transformer (FT-82-43) and 9 turns of 0.4mm Cu bifilar was the first candidate.

Following pictures show the basic parameters of this combiner.  VSWR  (signed wrong as WSWR in the graph) was better than 1.07,   Insertion Loss  between Input and DUT  (as Input - Output in the graph)  is  little higher  than -6dB   (calibration error?). Isolation Input - Output  (as Input1 - Input2 in the graph)  as a most important  parameter varies from 30dB to 43dB  in  whole frequency range 2 - 60MHz. With the last parameter I was not satisfied, isolation has not good enough level.

Why isolation between inputs was not very good? I thing the problem is with unwanted coupling between the transformer and the board. Toroid core is big and placed very close to other parts. This solution will be much better in the bigger box with the transformer installed to the vertical position, I thing.

As a next solution I used ferrite beads 43006301 (two pcs parallel glued). Isolation level is much better (from 42dB to 56dB), rest of parameters are comparable to the previous type. This solution will suit me, I hope.

The following pictures show Reference level, Isolation and Directivity (Return Loss) of the HF Power Bridge in the frequency range from 20MHz to 210 MHz.

See the following pictures as a result of VNA Reflection Bridge basic parameters measuring (for comparison). VNA Reflection Bridge is usable  to abt. 200MHz (see VE3IVM directivity measuring results). My requirements is sufficient for the frequency range up to 60MHz  (Short Wave Bands+ 6 Meters). 

What is interesting here, is not good SWR at the output port (close to 1.3 : 1) in comparison with all other types of tested RLBs. What it means?  VNA Reflection bridge is not very good for IM measuring, because second input port would have unacceptably higher input SWR! Simplified solution can be insertion of 6dB attenuator between the signal source and the second input port... Why 6dB of attenuation? We remember that N2PK VNA has the possibility to provide two RF signals, but with 6 dB of level difference.....

Conclusion:  Both new types of Return Loss Bridges they have very good basic parameters in whole frequency range from 2MHz to 60MHz. Both of them are suitable to work in the VHF band with desired parameters, too. Winner in the short wave range is the "Power" Return Loss Bridge with transformer built from ferrite beads 43006301 (two pcs parallel glued) with 8 twisted turns of 0.4mm enamel. Its directivity is almost flat and reaches abt. 41dB in the range from 2 to 80 MHz, it is able to work with +33 dBm of input power. Reference level difference (OPEN vs SHORT) is max. 0.2dB in the  frequency range from 2 to 60 MHz.

I the office we are using close to 20 years old professional EAGLE RETURN LOSS BRIDGE (see the following pocture). I tried to check its parameters. On the label we see the frequency range from 5 to 1000 MHZ as usable. Measurements confirm that even after such a long time its parameters are still excellent.

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