New Power Meter

In my last blog I talked about the way I had a power meter nearly build itself. (In retrospect I remember a SolderSmoke episode where Bill claimed the same sort of thing happening with a different version of a power meter.) Friday night I got the chance to build the power meter from my parts.

It only took an hour or so get it put together. The hole for the meter turned out to not be a big deal. The trickiest part was actually getting the BNC jack through the hole in the side. The dummy load was too big and the hole too small to angle the PC board in. I ended up filing one of the corners and put a little force to get it in.

Here is what I ended up with.

Here are some shots of the power meter with my K2. The power meter is connected to the K2 antenna jack and the K2 is keyed. The K2 reports the power from it’s internal power meter. So you can see the K2 calculated power with the deflection of the meter in the new power meter.

The readings are actually fairly interesting. By a serendipitous coincidence 0.5W reads as ~10uA, 2W reads 20uA, 5W reads 30uA and 10W reads 40uA. Couldn’t have gotten that sort of correspondence if I had tried.

And finally after building it I had to plug every transmitter I have into the power meter. The sw20+ showed around 2 watts. Here is a picture of the power meter measuring the output of my homebrew 30m CW crystal transmitter Reads about .5W, which is what I expected but now I have confirmation.

So, all in all I’d have to say I am very pleased. I am going to find some Avery labels and label this guy so when someone goes through the shack after I pass they will know what it is. Not to mention, keeping me from toasting it because I was sure I built it for 100W. Other than that I am ready to build another RF generator to plug in.

73

New Project falling together

So it has been a while since I’ve done anything actively in Amateur Radio. If you had seen my workbench before this weekend you would not have needed to be told that. At the same time you would have known what one of the big hurdles was. (I know my K2 is in there somewhere.)

Well, I checked EMRFD out from the library a few weeks ago. I hoped it would spark some interest. It did.

As an aside, if you are a homebrewer and don’t have access to Experimental Methods in Radio Frequency Design you should get access to the book. It is a great book to read and I hope to build out of.

One of the first projects recommended in EMRFD is an RF power meter. It will help with the question, “Is this thing doing anything?” I haven’t had one. I have the power meter in my antenna tuner, but normally I have used a NORCAL dummy load with a DVM or my trusty Simpson meter. But the “required hand count” goes up steeply with each probe that needs to be held in place. Pretty awkward.

As I was reading the section on power meters again, I realized that the only thing I use my dummy load for is testing circuits. So why not just build it into an enclosure and include a voltmeter in the box. Then I can just plug in my circuit and the meter just sits there telling me how many watts and providing a load. No hands required. Cool!

Well to keep this story short, I spent last night at my newly cleaned bench picking parts out of the parts bin to produce a power meter. Here is the list pf parts.

  1. The dummy load from NORCAL provides the 50 ohm load and has the peak detector on it. It also has a mountable BNC jack that adequately mounts the whole thing. The peak detector and dummy load is good for 1-10W in short bursts.
  2. The meter is a 0-50uA meter I got from my dad. It is brand new in the box.
  3. A couple of 1.5M ohm resistors in parallel give ~750 k Ohms of resistance in series with the meter to provide a roughly 0-36V volt meter.
  4. The plastic Hammond case was a surplus hand-me-down from a friend at a testing lab.

Calibration: It will be as accurate as my K2. I intend to plug my K2 into it and energize with varying power levels. Then I will probably just mark the meter face at 1/2 watt intervals.

So when I mount everything I will have a 1-10W power meter that is in a small (but big enough) box to stand on the bench while I am tinkering.

The box will also have emough room to put in a more sensitive milliW meter circuit that reuses the meter at a later date. That will take a little more soldering. I haven’t decided whether to just do another uncompensated meter or use something like my compensated RF probe, but that will come later.

So stay tuned and I will get some photos up when it is done.