A cautionary tale: Check your instruments before relying on them!

Hi there! I’d like to share an experience I had when I was conducting RF measurements. A few weeks ago, an acquaintance passed me a MFJ 259B antenna/SWR analyzer. I took it back home, and played around with it.

I found that I needed to measure the impedances of antennas and build some matching networks. Thus, I pulled the analyzer out and tried to do some measurements. Having some experience with a VNA like the Anritsu SiteMaster (two-port measurement mode), I decided to do a check by doing an open, short and load (50 ohms) test. I found something interesting during the test:

 

 

 

 

 

 

 

 

 

 

Really? A VSWR reading of greater than 25:1? On a 50 ohm load? Surely something is wrong here! After turning the knobs and changing the frequency bands, the reading dropped to 1:1 VSWR from HF to VHF. I was also convinced that the R and X values were inconsistent with the VSWR values. Why? I was measuring a load (RF matching transformer), and got these results:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

At 13.71 MHz, the VSWR reading is 2.2, and the R+jX reading is 13+j24 (I assume a positive (inductive) value here. The analyzer doesn’t tell you what kind of reactive component the load is). Really? Let us verify this by plotting the point on the Smith Chart:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I have turned the VSWR circles on, and as you can see, the VSWR reading should be approximately 5:1 for a reading of 13+j24. The analyzer’s VSWR and impedance readings are not consistent with each other.

Now, I do not claim that the quality of the products manufactured by MFJ Enterprises are not good. And yes, if I wanted a very accurate reading, I should be using a better tool like a proper VNA, something like an Anritsu or Rohde And Schwarz VNA. Of course, you get what you pay for. But this article is about why checking and calibrating your equipment is important.

Anyway, thanks for reading!

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