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  • Author Author: kmikemoo
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Field Strength Meter

kmikemoo

Today, I shift my focus to RF.  As part of the November 2019 Radio Frequency challenge, I plan to test antenna radiation patterns.  But I'm also cheap.  So... I plan to do this with simple radio frequency detector circuits.  Why not?

There are many variations of the RF detector circuit.  I chose the one below.

image

I've build three iteration of this circuit.  The first is in a Harbor Freight free with purchase multimeter.  This build seems the most consistent.  The second build has a BNC connector and I tried a stock antenna (rubber duck).  I then built this one with the same banana plug setup as the Harbor Freight meter.  The only difference between version 2 and version 3 was version 2 was more selective of frequency and version 3 responds to... everything (fluorescent lights, CFL bulbs, etc.).

 

I drilled the hole for the banana plug jack.  I placed the parts and started soldering.  THANK YOU to everyone who has posted a video of soldering up proto board.  I always tried to complicate it.  Not any more.

imageimageimageimage

Now it was time to give it some output.  Arduino with  LCD.  I should also throw in a reset button since I want to display a running peak value.  Put it in a (que dramatic music)... project box.

Yes, they still make those.     The schematic colors don't match what I used.

imageimageimage

 

The only thing I forgot to add was the warning LED that the sensed voltage was approaching the 5V limit.  The warning and 2 second pause (to notice the light) are still in the sketch.  I'll add it later - when I find the hot glue gun.

I probably over-documented the Arduino sketch - but I'll thank myself later when I ask myself what I was thinking when I wrote it.

 

Time for a test run.  5w to 8w on the radio.  I started with the radio about 2 meters away.  There is no scaling of the 0 to 1023 value - since it isn't important for my antenna radiation pattern testing.

Yes.  Yes.  Licenced amateur.  Yes.  Used call and announced what I was doing.  Yes.  Checked frequency before use.  No.  It didn't make any difference if I was speaking into the microphone or not.

 

CONCLUSION:  This is a very simple circuit.  Unfortunately, it is inconsistent and will require multiple tests to insure somewhat congruent results.  The inconsistency makes it impossible to make multiple detectors give similar readings.  I'm going to try anyway.  One more project for Makevember!

Attachments:
RFPowerMeter.txt.zip
  • in reply to shabaz

      This is awesome!.  I did buy some AD8307 chips off of eBay.  I wonder what I really got.  Maybe that explains why the repair that I did with one of them did not change the output of the device I put it in.

  • in reply to three-phase

    Hi Donald,

     

    Ahh, you're right! It looks like it could be helpful for that use-case too. Perhaps the Maxim chip might be suitable, since it works to 1GHz.

  • in reply to shabaz

    They look interesting, I think I will take a look at these for my PD detector box, especially if I plan to make up a PCB for it.

     

    Kind regards

  • Hi Mike,

     

    It's great to see this RF project : ) The results look good, especially considering the desire to keep it low-cost with the diode detector.

     

    If you ever wanted to upgrade that part of the design, there's some chips (more expensive than the diodes unfortunately) that have very few connections, and are all-on-one diode detectors and log-output amplifier, with a voltage output that could go into the Arduino as you have it, or into any multimeter/panel meter. The AD8307 is quite popular, since it is in a easy-to-solder SOIC-8 package, and needs hardly any components. It's expensive though : ( Newark/Farnell has an alternative, MAX2014 but in a harder to solder package : ( In some ways it is better, due to higher frequency span, but the AD chip has a greater power measurement range.

    eBay always seemingly has plenty of AD8307 that are cheap, but it is a extremely common fake chip.. I got 20, all were fake (top sanded down and re-marked) so if it looks like a bargain for that chip, it probably isn't : )

    There's also a LT5507 chip, again hand-solderable (but not as easy as the AD8307 which is very easy to hand-solder). However it is a very basic chip, since it doesn't cover as large a power range and not very linear. It is cheap though. Photo below is of my crude LTC5507 meter. I've purchased a AD8307 however, since the LTC.. chip wasn't meeting my needs.

    image

  • in reply to jw0752

    Great question.  After a little simple testing tonight, I believe my consistency issue may be a $35 Baofeng UV-82 issue.  I put the Arduino meter and the HF meter up about 1 meter apart (half wavelength).  The readings were comparable between the two meters - although the range of measured values varied.  Then I tried my other UV-82.  I got consistent readings.  It may be time to "retire" a radio (disassemble and see what's inside).  I was planning on using the handhelds for my RF project for ease of portability and lower power output than my mobile (base) radio.  If the other handheld weirds out, I may be forced to change my plans - or buy a better handheld radio.