Friday, May 1, 2015

Testing the Return Loss Bridge

One of the functions available in the software for the Chinese SNA is the ability to measure SWR using a Return Loss Bridge.  When I was laying out the attenuator boards I had room to include a RLB.  

The layout uses edge mount  SMA connectors and surface mount resistors.  I allowed space for 2 parallel resistors to get the correct value if you need.  The plan for the RLB I used had 51 ohm resistors.  You can get 51.5 ohm by using a 47 ohm  and 56 ohm in parallel.   I had some 100 ohm resistors so used 2 of them in parallel to get 50 ohms.

PCB layout

Finished RLB
I had just put up a new multi-band fan dipole, thought that would be a good test of the RLB and software.  I used my FoxDelta antenna analyzer to align the dipole. Doing a full scan from 1-40 Mhz., I got the following SWR Plot.
FoxDelta Analyzer Plot

Using the SNA software in the SWR mode I got the following.  Since one display uses a log  scale  and the other linear for the vertical axis they look a little different, but after checking the values they are within a couple percent of each other.

Just for the heck of it I hooked the RLB and antenna to the SNA Jr. and came up with this as a result.
Since I do not normalize the data on the SNA Jr. I had to manually compute the normalized values and use a Return Loss- SWR table to find the SWR at selected frequencies.  The values were also within 10% of those on the FoxDelta Analyzer.  Looks like the SNA Jr. with the RLB would be useable as a portable instrument for tuning antennas.
SNA JR and Return Loss Bridge

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