The NM0S QRPoMeter

Some years ago, I purchased and assembled an Oak Hills Research WM-2 QRP Wattmeter from Milestone Technologies. As far as QRP wattmeter kits go, it was something of a classic at the time, and as such, I wanted one. I’m glad I made this purchase, as they are no longer available – at least, in this form. Another company is offering a very similar kit, but without the decals on the case. I was told that they acquired the rights to it from Milestone Technologies, so this would be appear to be a direct clone of the WM-2. The WM-2 is a great little wattmeter, with 3 ranges representing 100mW, 1W, and 10W full-scale – an ideal selection of ranges for the QRPer. You can read both forward and reflected power. While direct readout of SWR is not offered, it can be calculated from the forward and reverse power readings. The WM-2 has an analog meter and can be left inline while operating. Apart from the satisfaction of being able to see the needle bob up and down when transmitting, this type of indicator is very useful when peaking circuits for maximum output. It can certainly be done with a digital readout, but an extra stage of “translation” needs to happen in the brain, converting the number on the readout to a “level”. This process when looking at an analog meter is more immediately intuitive.

My trusty OHR WM-2 Wattmeter has served me faithfully for years – and continues to do so.

That was so many photos of my WM-2, that you might be thinking, “Hang on – isn’t this a post about the QRPoMeter? Well, it is – and we’ll get to that very soon. I don’t think I ever blogged about the WM-2 when I built mine years ago, so felt it was time to give it some air time on my blog.

For my purposes, the WM-2 meets my needs. However, I don’t have any other instruments with which to check the accuracy of it’s readings. A Bird wattmeter would be nice, but the expenditure is hard to justify. Another option is to use an oscilloscope to measure the peak to peak voltage a transmitter develops across a 50 ohm dummy load, and use that to calculate power. This is a definite possibility in the future, as I do intend to add a digital storage oscilloscope to the shack at some point. In the meantime, it would be good just to have another wattmeter of similar accuracy, simply to increase my confidence in the readings I am getting from either one. For the kind of operating us QRPers do, absolute accuracy is not essential. 5% of full scale is good enough which, on a 10W scale, means ±0.5W. If I claim to be transmitting with 5W, then the difference between 4.5 and 5.5 W is unlikely to even be noticed at the receiving end.

The QRPoMeter, designed by Dave Cripe NM0S, has been on my radar for a very long time. Originally offered by the 4SQRP group, it is a very affordable instrument for measuring power and SWR. It has a built-in dummy load, to make measuring the power into a 50 ohm load an easy task. Also, when measuring SWR, it uses a resistive bridge, so that the maximum SWR your transmitter will see is 2:1. I’ve long wanted to assemble this kit. A few times, I’ve waited too long to purchase a kit, only to find that it was no longer available (the SST, once offered by Wilderness Radio, was one example). With that in mind, and also because the QRPoMeter is so reasonably priced, I went ahead and placed an order with NM0S Electronics.

A few days later it arrived, in a small flat rate Priority Mail box. I love getting radio parts and kits in the mail. It’s exciting! The PCB pieces that, as well as forming the circuit board, also comprise the case, were slipped in between pages of the assembly manual to protect them. There was also a little bag of goodies. I love little bags of radio parts!

The bag of parts, emptied out into a styrofoam tray –

Also included was a piece of thin 2″ x 3.5″ PCB material, etched and silkscreened on both sides. On one side was the business card of NM0S Electronics. On the other side were these handy little band plans –

The pieces that form the sides of the case have to be broken off from the larger pieces of PCB material, and given a very light filing to remove the rough edges. It was immediately obvious how smart the final product was going to look. It was raining very, very lightly when I took this photo. If you look carefully, you might see some very small raindrops on the panels –

For anyone who has assembled a few kits, construction is uncomplicated. Dave’s instructions are clear and straightforward, consisting mainly of a checklist for populating the board, and instructions for constructing the included PCB case. The switches and input/output BNC connectors are all mounted directly on the board. The only wiring required is a 4-conductor ribbon cable that is used for the connections between the board and the LCD panel meter. Other than wiring up this meter, and soldering the case pieces together, construction of the QRPoMeter consists of populating the single PCB with the parts. This picture is of the finished board before the two switches were installed –

I only discovered two very slight issues during assembly. Neither will present a problem for anyone with a little experience, but they might slightly confuse a beginner. These were due to a change in sourcing parts, and Dave said he will take care of them in future versions of the construction manual. These were –

1. U4, the TLC2272, had no dot or u-shaped indentation to denote the correct orientation. I used the printing of the device number on the top of the IC as a guide instead, and this turned out to be right. See picture below, with U4 circled in red –

2. When using the 4-conductor ribbon cable to connect the LCD panel meter to the board, because the instructions refer to an earlier version of the meter, a beginner might experience some uncertainty as to which holes on the meter board to connect to which holes on the main board. On the main board, there are two pads next to each other marked +Vin and -Vin. These are connected to the pads on the meter board that are marked “IN” and “COM” respectively. The other two connections are more obvious. The pads on the main board, next to the schematic symbol for a 9V battery, marked + and -, are connected to the pads on the back of the meter board, next to the “DC 9V” text, that are marked + and respectively.

A portion of the board in greater detail, showing the 8 large surface mount resistors that form the dummy load/resistive bridge (or, to be more accurate, 7 of them, and a small part of the remaining one) –

Calibration is straightforward, and requires a fairly accurate DMM. I used my Brymen BM235 (the EEV Blog version). The only other piece of equipment needed is any HF QRP transmitter with an output of between 2 and 10 watts. The output power doesn’t need to be known, as long as it falls within that range. When the unit is calibrated, you have a very handsome and useful piece of QRP kit!

The QRPoMeter seems to be accurate enough for my purposes. Power measurements are in line with the ones reported by my WM-2, taking into account the accuracies of both instruments. SWR measurements are similar at the lower readings. They differ by fairly large amounts at higher SWR’s. This doesn’t concern me though, as once the SWR goes much above 2 or 3:1, it’s exact value is of little interest to me. I just know that I want to get it back down below 2:1! A useful feature of the resistive bridge in the QRPoMeter that is used to measure the values of forward and reflected power, is that when SWR is being measured, the transmitter never sees an SWR higher than 2:1. This was verified with the SWR indicator in my Elecraft K2.

Thanks Dave. A good-looking and worthy little piece of QRP test gear! The QRPoMeter is available from NM0S Electronics.