A Homebrew Probe. No, this is not some device seen in one of those cheesy Science Fiction movies where the poor humans are subjected to probing in some unmentionable places. It is an RF Probe!
But first a true story, A ham emailed me about getting into homebrewing. He had just passed his Extra Class License and had no radios, had never been on the air and his whole ham shack consisted of a $6, 1000 Ohms per volt VOM he bought at a Drug Store and a Crystal Set he bought at a garage sale. His first question was: "is this enough to get started?" That was followed by a statement that he wanted to build one of my SSB Transceivers and needed help.
This email really irritated me not because of the content or the request but because the ARRL set this ham up to buy hundreds if not thousands of dollars of gear to get on the air. You know, you bought a set of 1940's NIB Cadillac hubcaps at a garage sale for $10 and for another $80K you can buy the rest of the parts to replicate that car.
I spent some time in my response by saying "start small, but start wisely". The first thing I did was to tell him what he had was not enough for a start but with just a few bucks could at least move in the right direction.
First on my list was a general coverage receiver with a BFO. It did not have be pristine but had to work. Many radios can be found on auction sites and even places like Good Will. But this radio even though not accurate to 1 Hz will let you hear things like if a VFO or crystal oscillator is putting out RF and importantly what frequency range. It will also give a comparative result like A is louder than B.
We could go on about linear (non-wall wart) power supplies and some basic tools but now to some test gear. If you just want to measure things and see that there is some value of voltage or current then your $6 VOM is OK. But if you want something with greater accuracy then you need a 20K per volt VOM or a Digital Voltmeter (DVM). I have both and use the VOM (about $25) for first run through and my $100 DVM for more precise work and for its other features.
The 100 MHz two channel Digital Storage Scope is up the pole at about $400. But if you want to sense if there is RF coming out the pipeline, then an Standing Wave Ratio (SWR) Bridge and a Homebrew RF Probe are your best friends.
I found a SWR Bridge at a Ham Fest where the sensitive meter part was smashed! I gave the guy $1.50 and just smiled as what I really wanted was the sensing circuit. Another $5 on eBay and I bought a new 25 uA meter and for less than $10, I had a really nifty and sensitive SWR Bridge. Of course I made a case for it and now it is one of my best instruments. The case for this project is made from PC Board soldered up like a box. I am surprised at how many hams do not have an SWR Bridge in line at all times.
Now to our RF Probe which is today's subject. It has three electronic components and the 1N34 is the most expensive of the three. For QRP work the 100pF Cap can be rated at 50 Volts. 5 watts is 45 Volts Peak To Peak across 50 Ohms*. But if you are anal retentive, then make the cap a 500 Volt rated unit. (*45^2*2.5/1000 = 5.0625 watts)
The other parts are from the scrap pile like an RCA Jack and the probe which was a piece of Brass rod sharpened to a point and insulated from the case with a fiber washer. You could even use an insulated Banana Jack and the Probe plugs into that jack. I even built a version of this inside of a 3/4 in in diameter two inch long copper nipple with end caps soldered to the body.
This will not give RF readings with high accuracy but what it will give you are two things. The 1st is a go no go. Is there RF Output --it will tell you if it is more than ZERO. The second piece of info is what provides the greatest relative output such as adjusting the bias on the IRF510.
Back to our ham. I suggested that once he found a general coverage receiver and built this probe that he should undertake building the Michigan Mighty Mite Transmitter. The Big Caution be sure to have a 50 Ohm load on the output and put the probe on the load for measurements and second if he is itchy to put the Mighty Mite into an antenna have a W3NQN Low Pass Filter in line.
Start Small and Start Wisely and this RF Probe is a one and done project (one evening and its complete) that will be useful no matter how much commercial gear you own. This is also a direct connection to our founding roots over a hundred years ago. There was no plastic and no DSO's just hams who improvised, adapted and overcame the problem. (Just like the US Marine Corps)
73's
Pete N6QW