Tuesday, May 31, 2016

Antenna Analyzer

Doing my version of k6bez's antenna analyzer here (warning, pdf).

First, please notice that there are far more resistors on this board than there should be.  I could not find 50 ohm resistors anywhere on my workbench.  I ended up putting four 200 ohm in parallel.  (Also used two 10k in parallel for the 5k resistors, and used a 680 ohm instead of the 648(?!?) ohm he calls for.)

As a test, I just put the output of my VFO into this, measured the voltages out of the opamp with my DVM and calculated the VSWR in a spreadsheet against a couple of different resistor values.  Immediately noticed that where I should be getting 1:1 on a 50 ohm load, I was getting like 3.4:1.  Started poking around and found some cold joints in my bridge circuit.

Testing again, I get happier results.  However, they still aren't what I'm expecting.  I would expect a 50 ohm load to have a 1:1 reading, but is either 1.3 or 1.1 depending on what method I use to calculate (it should be (fwd + rev)/(fwd - rev), but I've seen people use the square of the individual values as well).

I saw some discussion on the Yahoo Group where someone was asking about the accuracy of the numbers.  They talk about the nonlinearity of the diode in there, and I also noticed in his slides that he talks about needing more power for accurate swr readings.  Maybe I'll make an amplifier next.

As a side note, something in my setup seems to be sucking down power.  Nothing is getting hot, but my setup browned out while I was testing.  Found the 9v battery I was powering this all off of was down to 7-ish volts.  I put in a new one, and in a few minutes it was down a volt, too.

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