Handy-Dandy Super Simple Cheap and Dirty Field Strength Meter

by

Eric P. Nichols, KL7AJ

One of the most useful instruments one can have around the ham shack is a simple R.F. field strength meter. Most of the time we aren't looking for National Bureau of Standards accuracy, we just want a quick and simple method of tuning our system for "maximum smoke." The field strength meter I describe is just such a device; so simple and cheap, no ham should be without one, and sensitive enough to be truly useful. It will give a useable indication or relative field strength at power levels from QRP to full legal output.

You can deploy this instrument just about anywhere. You can just set the thing on your operating position, or if you really want to get picky, you can put it on a pedestal below your dipole and use a pair of binoculars to read the thing.

There is no sensitivity adjustment pot included, to make things even simpler. Sensitivity is adjusted by tweaking the length of a collapsible antenna.

The circuit is extremely simple, as you can see by the schematic. The most expensive part is the microammeter. (The last time I went to our local Radio Shack and asked for a microammeter, the genius behind the counter came back with some sort of battery tester and said, "This is the smallest ammeter we've got." Needless to say, you will have to find a more apt source for your microammeter, such as Ye Olde Junque Box). The real "guts" of the device is the MPF 102 J-FET which acts as an R.F. detector and DC amplifier. Incidentally, this beastie is so sensitive, it will detect DC fields as well, such as approaching lightning storms….a handy little side-effect you should be aware of.

You can install the whole shooting match inside a small plastic project box, or you can build a box out of double clad circuit board, which gives you that rich coppery appearance. Mount the collapsible antenna securely to the top of the box. (You might want to add some dead weight to the bottom of your box, so the thing wont fall over with the antenna fully extended).

This device is also handy for tracking down excessive R.F. in the shack. Theoretically, you should have more R.F. near your actual antenna than you do in your shack…if not, probably some investigation is in order!

By the way, you may find that the D.C. field strength capabilities of this are even more interesting than the r.f. aspect. A slight modification of the circuit, replacing the junction FET with a MOSFET, will make this a true static field instrument. (A J-FET requires a finite, if minuscule amount of gate current, while a MOSFET does not). The MOSFET version, with a long, vertical wire antenna, will also allow you to investigate phenomena such as electron precipitation events, if you happen to live in high latitudes. Electron precipitation is one of the major cause of sudden R.F. blackouts…and can be easily read with such simple instruments.

You will find this handy little instrument useful, educational, and fun. It's a great weapon in your radio arsenal.

73 de KL7AJ