Tuesday, July 26, 2016

Testing Out a Noise Source

Something I've craved for a long time here at ThePalmHQ is an antenna analyzer that can work all the way up into the microwave region.  I have an MFJ-259C and I have appreciated it for use on HF and 2 meter antennas.  But for 222 MHz and above something else is needed.

A video by Adam, 9A4QV inspired me to put together my own analyzer that will work well into the GHz range.  He's posted a video about it here and a more detailed presentation of the same thing has been posted by the folks at RTL-SDR.com here.  Basically, with a noise source, a directional coupler, an RTL-SDR dongle,and some free software we can analyze antennas and filters through all of our amateur bands up through 1296 MHz.  What's especially great is that the total cost of all of this should be around $50!

I tried out the first piece today, the noise source -- a simple device that just honks out an RF signal across a wide frequency range.  These are available for $20ish off of EBay (I got mine here, but there are a bunch of sellers -- just search for "noise source".)  A number of postings on-line note that earlier versions of these boards ran too hot and died.  The good news is that there's a redesigned board using SOT-89 MMICs instead of SOT-86s (be sure to get the 2016 version).  Mine runs hot, but not self-destruct hot.  An added advantage is that this redesign seems to put out a lot more power -- mine averages around -25 dBm across a pretty broad spectrum, whereas earlier versions seemed to put out more like -40 dBm.

I looked at the output on a spectrum analyzer.  It's putting out around -10 dBm at the very low end of the spectrum, has a pretty flat output between 200 - 1500 MHz where it's between -22 and -27 dBm and then it falls off to -40 dBm at 3 GHz.  There's probably useful power above that, but that's the upper limit of this analyzer.

I'm looking forward to sweeping a WA5VJB "cheap yagi" that I built for 1296 MHz as my first real-world application.  If this works out as well as I hope, I may just ditch that MFJ-259 entirely.  Stay tuned....