The Tale of Two Meters

Well, I’ve been mucking around with my RF probe circuitry a bit tonight, and encountered something pretty strange. I have two digital multimeters lying around, and my newly rebuilt RF probe/dummy load that I described earlier. I hook one multimeter to it, and I get a reading of 14 volts. I get my other cheapie meter, and I get a reading of about four volts. Yet, both are probably “correct”. How can that be?

Well, first, let’s review the circuit that I have revised to:


I have used a different diode (a 1N914, which is probably better than the 1N4001 that I had in there previously, but not as good as the germanium diode you see in the schematic) and didn’t have any 4.7M ohm resistors, so I hooked two 2.2M ohm resistors in series. The voltage drop for the 1N914 is probably about 0.7 volts, but haven’t measured it. The purpose of the 4.7M ohm resistor is to convert the peak voltage (as gathered in the capacitor) to RMS voltage. How does this work? Well, if you imagine that the meter has some input resistance, then we basically have a voltage divider: the voltage that is measured by the voltmeter is the peak voltage, multiplied by the resistance of the meter divided by the sum of the meter resistance and the 4.7M ohm resistor. In years past, where this design was first put out, the common impedance for meters was around 11M ohms. So, the voltage we get out is the peak voltage * 11 / (11+4.7) = .7006 * the peak voltage. The peak voltage is sqrt(2) times the RMS voltage, so the RMS is the peak divided by sqrt(2). 1.0 / sqrt(2), which is .7071 or so, so the 4.7 Mohm resistor would be about right for a voltmeter with 11M ohm resistance.

Reading up on the subject, modern digital volt meters have an input impedence of 10Mohms. To figure out what size of a resistor we need for that, we solve 10 / (10 + R) = 1/sqrt(2) for R, and we end up with about 4.1M ohm resistance. I ended up approximating this by two 2.2M ohm resistors, which is closer, but still not perfect.

Okay, so that’s my thinking. So, I jumper the thing to one of two meters, and get a reading of about 14 volts. This indicates an output power of 14*14/50 or 3.92 watts (this was my FT-817 in 5 watt mode). The number is small, but I expected that. I haven’t accounted for two things: either the voltage drop of the diode, nor the problem with using the wrong series resistors. Instead of dividing the peak by the sqrt(2), I am multipling by about .694. Still assuming a 10M ohm resistance, my 14 volts should have been about 14.17 volts, plus the voltage drop (say .7 volts). This works out to about 4.42 watts. In the right ballpark, given that we didn’t really measure the impedence of the meter.

But here’s the odd thing! I hooked it to another, cheap meter of mine, and got only 3.92 volts! This is a budget DVM, which probably cost me about $10 that I have used mostly to check to see if my car battery is dead, but the difference is startling. The only conclusion I can draw is that the cheaper meter has a dramatically different impedence, probably around 0.5 Mohms. To test this idea, I measured the peak voltage at the junction between the cap and the diode, and both meters were in agreement. Interesting!

Through a coincidence, WB8ICN has been examining the same issue on QRPedia. He suggests using a potentiometer hooked to a regulated power supply, and measuring the voltage as you adjust the potentiometer until it reaches half the supply voltage, then the potentiometer is set to the impedence of your meter. Of course, with a little algebra, you don’t need to do that: you can just measure the voltage drop of any resistor with a resistance of a 1M ohm or so, and work it out. I’ll probably do that for both my meters, and then stick a little note on them so I remember.

Oh well, that’s my electronics tinkering for the day.

Aircraft reflection? Or something else?

This duplicates a posting I made on the website.

Aircraft reflection? Or something else? | WSPRnet
On Saturday, Jan 10th, I had my transmit-only beacon that I wrote sending both a visual MEPT indicator (the MV that you can see in the attached image) and the WSPR beacon, and tried out a simple program that simultaneously records, and later makes an FFT display of that portion of the band.

It seems to work!

I’m curious though about the brighter of the wspr messages visible here, which seems to have a shadow which arcs down to meet it. Is this the result of an aircraft reflection? There are some fairly close stations here (such as NN6RF) which conceivably could be reflecting off aircraft landing at Oakland airport, but I’m surprised that the signal level could be so high on the 30m band. I believe that such reflections can only occur if the source is reasonably close to the aircraft, correct?


I see a couple of other, similar traces in the 2 hour recording I did, which you can peruse here.

Diodes for RF Probes

My dummy load experiment still has a few unanswered questions, but I found that the ARRL Handbook has had a circuit which is basically what I built, minus the one series resistor. It looks like this:


Okay, they use a germanium diode with considerably lower voltage drop, and include a 4.7M ohm resistor in series to the DVM. This circuit also appears on the QRPEDIA posting on RF probes, and apparently is constructed as part of the assembly of the Elecraft K2.

Here’s a posting talking about how the different diodes make for different performance.

Diodes for RF Probes.

Dummy Load/Watt Meter Experiment

Okay, this is pretty basic stuff really, but it’s part of my trip toward additional homebrewing, and it might be of some vague interest, so here we go. If anyone spots anything horribly wrong here, please feel free to help me learn by leaving me a comment here.

Before Thanksgiving, I was having quite a bit of fun with my FT-817. I was getting five or six good 40m/30m PSK31 contacts per night, and was getting pretty regular spots on my 30m WSPR beacon. I had a bit of a hiatus, and only recently got back on the air, but was having much more limited success in PSK31, so much so that I was wondering if my antenna had failed or my coax had gone bad.

The coax seemed a likely culprit. It’s that crappy stuff that Radio Shack sells, and I hadn’t gone to any effort to waterproof the junction where it joins the balun on my 40m dipole. Water could conceivably have gotten into the coax during some of our recent damp weather, and could have caused some additional loss. How could I tell?

Well, the answer is pretty simple: get a watt-meter, and measure the power at the back of the transmitter, and then at the far end of the coax, and see what the difference is. There was only one problem: I didn’t have a watt meter.

So, I built a (crude) one.

The basic QRP watt meter is a combination of a dummy load, and a simple rectifier/capacitor circuit in parallel. The dummy load provides the appropriate 50 ohm load, and the diode rectifies the code, converting it into a DC current, which is then used to charge a capacitor. If you measure the voltage across the capacitor, you can compute the power pretty simply. Power in watts is just Voltage squared divided by resistance, so if you square the voltage and divide by 50, you get the power.

Yes, yes, this isn’t quite right. I haven’t taken into account the voltage drop of the diode, but let’s run with this.

So, I went into the junk box (which I’ve been trying to add stuff too, dug out a rectifier diode, one of my larger 8ufd caps, and a pair of 10w, 100 ohm resistors (more on this later). I also dug out an SO-239 socket that I picked up from Radio Shack. Here’s the resulting gadget.


Using my cheap old Radio Shack digital multimeter, I clipped some test leads to measure the voltage between the diode and the ground. I took a 1 foot cable, hooked the load to my FT-817, and tested it out. Setting the FT-817 to its max power level, I turned on the transmitter and measured the voltage. I got a reading of 19.2 volts with this (essentially non-existant) cable run. If you multiply this out, we get 19.2*19.2/50 yielding about 7.372 watts (a bit higher than the nominal 5 watts that the FT-817 lists, but we’ll get back to that later). On its low setting, the voltage read right around 6 volts, yielding about 720mw.

Allright, that’s my baseline. I then took the dummy load outside to the far end of my cable run, and hooked it up there. The results were 17.8 volts (yielding 6.337 watts) and 5.5 volts (yielding 605mw). How does that compare with the nominal result we’d expect from RG-58 over a 50 foot run on 40m? The coax calculator can tell us. Plugging in 50 feet of RG-58 with an SWR of 1:1 on 7.07Mhz and an input power of 7.37 watts, we find we get about 0.5db of loss, and an output power of 6.587 watts. Similarly, if our output power was 720mw, we’d find about 643mw as the output power. That’s pretty darned close to what we expect from RG-58.

So, my conclusion is that my coax, while not the greatest stuff in the universe, isn’t a complete dummy load. If I’m going to blame my relatively poor recent performance, I’ll have to look elsewhere for a scape goat.

Okay, here’s some open questions. First of all, the resistors I chose. They were $0.99 at Fry’s, but their body seems to be made out of some kind of ceramic. There is a possibility (even a good one) that these resistors are internally wirewound, which means they aren’t necessarily entirely resistive: they could have an inductive component. Does anyone have any experience with this sort of resistor? Should I find some others?

Secondly, I didn’t account for the voltage drop for the rectifier diodes. The diodes were something that I dug out of my junk box, and I’m not even sure what type they are. I imagine they have somewhere between 0.5v and 0.7v drop. Technically, this means that the voltage measured at the cap is low by that amount, so the power should be a bit more.

Thirdly, I am confused by the values that I got. Especially considering the above, these power values seem much higher than the nominal values that we are supposed to get from the FT-817. The nominal maximum power should be 5w. I am suspicious that we are mixing peak and RMS voltages/powers here, since the values that I got are almost exactly sqrt(2) off from the values that I might expect, but I haven’t worked that out entirely in my head yet.

Anyway, that’s enough for today.

Addendum: This site has a circuit which is essentially what I built (different diode, and I am using a volt meter, rather than a current meter). It does indeed seem to indicate that the voltage that I’m reading out is the peak, not the RMS voltage. I’ll have to think about it some more.


Addendum2: Wes Hayward, W7ZOI has a writeup based upon the exposition in EMRFD which would be good for me to understand.

Addendum3: Yes, the resistors are probably wirewound (these cement power resistors apparently almost all are). I’ll have to go find some 200 ohm, 2 watt resistors to gang up in parallel.

Addendum4: I can’t stop! Another potentially useful link for measuring low powers, might be useful when I get to my QRPP beacon project.

K6HX beaconing on 30m with WSPR/MEPT

Okay, I hadn’t been doing any WSPR beacon operations since before Thanksgiving (which was also before my new callsign) and I was kind of bored today, so I dug out my power supply, tuner and interface and set my computer beaconing again on 30m. It’s a combination beacon: using WSPR above 10.140100, and a visual “MV” written as part of a sawtooth in the visual MEPT portion of the band (between 10.140000 and 10.140100 Mhz). Output power is about 4 watts, split between both signals.

I’d be interested in any reception reports.

Addendum: Click this link to examine the reception reports I’m getting via the automated WSPR logging.


Today, on Jan 2, I got a reception report from W1BW, and I could faintly see my “MV” appearing on his grabber:

MV on the W1BW grabber

You can see his “flying W” very strongly, and if you stare really close (click on the smaller image) you can see my MV which looks like part of a sawtooth around 23:22 (and other places).

Addendum2: Alan, VA3STL in Ontario also noticed my signal on the 2nd. Here is his screen grab, showing both my MV and my MEPT signal.


K1EL mounted in a case…

Well, this morning I decided I wanted to get the K1EL keyer that I put together mounted in a proper aluminum case. The sad thing was, I didn’t really pay enough attention to internal clearances, with the net result that I did a pretty crappy job. I’ll probably try again sometime soon, but in the mean time, it’s at least functional, and is no longer at risk of being yanked apart.

BT878 Audio Capture Trials

I subscribe to a number of different mailing lists related to radio topics. Something that has intrigued me for quite some time is VLF or Very Low Frequency radio, especially so-called Part 15 or “lowfer” operation. There has lately been some overlap with my interest in software defined radio. Some people have recognized that there is a family of fairly inexpensive video capture cards based upon the Brooktree 878 chipset which can be retargeted to do high A/D conversion of raw signals. These cards are quite inexpensive (the last one I got was about $40 or so), so they might make a reasonable platform to create a SDR platform.

Check out BT878 Audio Capture Trials for some information about what is possible.