rf probe 

This project came out of necessity, I needed the ability to quickly measure RF voltage for tuning filters and cavities.  After searching the web and looking over a bunch of designs I put them all together and here is the circuit I decided to build.  Click on the image for a full size version.

Most of the components came from my junk boxes.  A scrap of 3/4" copper pipe, capacitors, a resistor, coax connector.  All I had to buy was some diodes and two pipe caps.  I also chose to buy an analog volt meter to measure the RF voltage, an analog meter makes it easier to adjust for a peak or dip then using a digital meter.  I chose a 1N34A diode, its a bit harder to find and there are other substitute germanium diodes that could be used for this. 

Here are some of the parts as I started to collect them.  (My apologies for the picture quality all I had time to do was take some quick snapshots with my camera phone in case I decided to document the build later)  The copper pipe was just cut with a tubing cutter to a length that would give me enough room to house a small piece of perf board in and enough clearance for the caps.  The coax connector is just a regular PL-259, I had a special hollow center bolt that would screw into the back of the connector, but you can substitute an RG-58/59 reducer to accomplish the same thing.

First thing you need to do is take one cap and drill a hole in the center for the size of the reducer.  Clean the hole on both sides so its free of burs from the drilling, a good deburring tool, a knife blade, or in my case I use a UniBit for drilling so the next step in size clears the burrs for me.  Once you have done that clean the top of the cap and the back edge of the connector with steel wool or a plumbers brush, whatever you have handy, even fine grit sandpaper would be fine.  Then fasten the connector to the cap with the reducer, don't forget to put the barrel on the connector you won't be able to later, the reducer has just enough of a shoulder to make a mechanical connection and hold it in place.  Once you have it tight, I used a needle nose vise grip for the connector and a needle nose pliers for the reducer, you can then move on to soldering.  If you have a small vise then use it to hold the front end of the connector and heat the copper cap until solder will wick in around the base of the connector.  In my case I was able to accomplish this using my butane powered micro-torch, if you use a larger torch try to work fast and have a damp rag to cool it after you flow the solder in, you don't want to melt or damage the center insulator in the PL-259.

Now go cut a piece of perf board or use some copper clad PCB and make your circuit board so it fits inside the piece of copper pipe.  Layout your components keeping all leads as short as possible, you can see I had to stand up one resistor to make room but everything else was short.  Then I stripped one end of a piece of insulated 12awg solid copper wire and soldered it to the input of the circuit on the board and then stripped enough from the other end to solder to the center pin of the PL-259.  Slide that down into the connector than you can solder it to the center pin of the PL-259.  Just remember do it quick, you don't want to reflow the solder on the other end on the board and have it come off.

Now we need to test it and make sure the circuit is working.  I just used some clip lead jumpers for now, I will make a more permanent connection before sliding on the copper pipe and the other end cap.  You can also see why I used the PL-259 now.  I used a T-Connector to connect a dummy load to one side and my probe to the other, this is needed to provide a load for the rf source.  An HT or low power transmitter works great for this if you don't have a signal generator for this purpose.  And lets face it if you have a signal generator you probably have access to a spectrum analyzer to right?  Or at least a service monitor with both.

Here you can see the full setup.  In this example the output of the radio is connected to one side of the T-Connector, the RF Probe on the other, and the center goes to my dummy load.  Of course if you were testing something like a filter or notch you would have that between the RF source and the dummy load/probe setup.  Also if your tuning a notch your going to need to pad the input to the notch so the radio generating the signal doesn't see a high SWR as you start to notch out the frequency.  I will probably do a follow up article eventually showing how to construct a home made pad for that.  The other way, for example on 2m VHF, is to use a cheap used 2m MFJ Tuner or construct your own.  As you adjust the notch you can readjust the tuner to provide a match for the radio, its a cheap and dirty way to do it.

And here you can see the result on a voltmeter with 5w applied to the dummy load and probe.  Now that I can see its working its time to finish enclosing the probe and move on to building an enclosure for the analog meter and finish the wiring. 

I finished the construction today! It's a pretty simple build, a dual gang PVC electrical box and cover, and a boat cleat that I had laying in the junk box. I cut out the cover for the Meter and mounted it using the supplied hardware, drilled a hole in the top of the box and installed a grommet and used epoxy to glue in the grommet and provide strain relief for the wire.

Wrap the wire around the cleat and secure the probe with a Velcro cable tie.

That's it!  A useful devices for cheap.  Total cost of this project was about $25, the most expensive items being the panel mount meter for $12 from All Electronics and the box for $6 from the local Lowe's.  Everything else was pretty much junk box parts.

copyright © 2013 Scott Lichtsinn | design by dcarter

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