An RF probe

I camee across N5ESE‘s site and started browsing around some of his projects. He’s got quite the list of them. In his Gizmos section is an RF probe which looked pretty easy to build.

I had a set of those springyy hook-y grab-y DMM leads that were broken, so I cut off the spring-y hook-y grab-y part to use for the DMM side.

On the work bench was a perfectly sized piece of PCB from when I was experimenting with cutting and scoring PCB. From the RXTX build, I just happened to have some extra 0.01 μF SMD capacitors which were perfect for the job. A 1N34A diode and 4.7MΩ resistor finished off the parts.

I used a small hacksaw to score the PCB and divide it into three sections, cut out a notch for the diode and cut the head off a brass nail to serve as the probe. Soldering everything in place was pretty easy.

RF probe
RF probe

Soldered on the leads and a ground wire with alligator clip and put everything into a shrink wrap tube.

RF probe covered with heat shrink tubing
RF probe covered with heat shrink tubing

The full probe, with all the leads ready for some RF to measure. I still need to find a decent enclosure to shield the probe with.

Completed RF probe
Completed RF probe

Dummy load enclosure

A little Sucrets tin turned out to be a good size for my QRP dummy load.

Dummy load enclosure
Dummy load enclosure
Dummy load enclosure
Dummy load enclosure

A short section of 1×4 sliced in half to about 1 cm thick serves as the base. The dummy load is screwed onto the board and the board glued into the tin. For extra security, I put three screws through the sides of the tin into the board.

On the antenna analyzer, it reads pretty close to 51Ω and 1.0 SWR between 3-10 MHz. Above that the impedance creeps up to as high as 55 Ω and 1.5 SWR below 28 MHz. SWR shoots up pretty high in the VHF range.

I don’t have an external connection for the power measurement yet. Still need to figure out how I’m going to do that. For now it will be an “open the lid” measurement.

A QRP dummy load

Every ham needs a dummy load, right? I’ve been wanting to build a small little dummy load for QRP for a while so it became one of my day-off projects today. The dummy load kit from Hendricks QRP Kits served as my model for this build.

I emptied my inventory of 1kΩ 1/2W resistors and scrounged up 5 more 1kΩ 1/4W resistors out of the bag of goodies from Jason/NT7S (saved me a trip to Radio Shack). A total of 20 resistors went into the dummy load: 15 1/2W and 5 1/4W for a total theoretical power dissipation of 8.25W.

The platform for my dummy load is an Adafruit quarter size Perma-Proto board which I’ve really enjoyed working with. Here it is with half of the resistors placed on the board, but not soldered in yet.

Dummy load half complete
Dummy load half complete

Just under 100Ω with half the resistors in, so my layout is working.

Almost 100 ohms!
Almost 100 ohms!

It was a bit of a squeeze to get those 1/2W resistors in, but it worked. The 1/4W resistors fit easier into each row.

Tight fit
Tight fit

With everything soldered into place, I measure just under 50Ω. Pretty good.

Pretty close to 50 ohms
Pretty close to 50 ohms

The Hendricks dummy load also lets you use measure power output using a volt meter so I added that part in as well. A 1N5711 diode (again from the bag of goodies from Jason), a 10 nF cap and some header pins finishes the dummy load.

Completed dummy load
Completed dummy load

The two pins at the lower right of the board are for the connection to the radio, and the single pin on the left side is the voltage/power measurement point. All that remains now is to find an enclosure to put it in and add a convenient way to connect to the radio and access the measuring points.