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Thursday, February 13, 2014

Back to the W7ZOI/W7PUA Power Meter

Recent e-mails and Facebook postings from Jim (W8NSA) and Michael (AA1TJ) got me thinking about my old W7ZOI/W7PUA power meter.  The 15 inches (38 centimeters) of snow that fell last night gave me the day off -- and time to play with this very useful and interesting piece of gear. 

The last time I used it I remember thinking that a digital readout would be nice.  But I didn't feel like going back into the world of Arduinos and LCD screens.  So I came up with a real Kludge solution:  I had cheap little DVM that I wasn't using, so I just velcroed it to the side of the power meter.  That little connector above the BNC is the output for a DVM.  I might work on calibration later today.

Wes has some very interesting info follow-up info on the meter on his site: http://w7zoi.net/qststuff.html
I really like the part about how the meter is so sensitive that you can see the thermal noise in the input circuit and can actually measure the strength of signals from your antenna. 

I think I might need a low pass filter at the input of the meter.  There are strong FM broadcast transmitters in this area (some of you may have listened to them in the background of early episode of the SolderSmoke podcast!). I notice that just bringing my fingers close to the input causes the meter and the DVM readout to swing up.  That's not good.

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4 comments:

  1. But for digital, did you see the Project that Roger, KA7EXM did some years back...
    http://ka7exm.net/pic_power/index.htm

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  2. I'm curious about the thermal noise thing. I was crunching some numbers using the kTB formula for thermal noise (Boltsman's contstant * Temp (K) * Bandwidth) and at 30 kHz and room temp it's about -129 dBm. From experience working with AMPS cellular FM stuff that's pretty low, our test equipment couldn't always quite get down that low. If you go up by 100 deg C it only raises it to about -127 dBm. That's about 550 uV @ 30 kHz and assuming 50 ohm resistor.

    I looked at the noise floor stats from the QST review of the latest Kenwood and it can only get down about 10 dB worse than the thermal noise floor measured at a 500 Hz bandwidth (something like -135 dBm minimum signal vs. a -145 dBm thermal noise floor). I may well be missing something here as I'm not an expert in noise floors. Or I might have forgotten to carry the 1. But it's interesting to contemplate.

    I need to dust off my KA7EXM version and make some measurements, you keep making me curious about this stuff. At this rate my BITX20 will never be completed (between this and VSWR meter design/understanding)!

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  3. I checked the link and his meter has 500 MHz BW, much larger than 30 kHz. I got his same number of about -86 dBm using the formula (which he has on his page too).

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  4. Whoops I think the above # should have been about 0.08 uV not 550 (I thought that seemed high). I'll stop now.

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