Wednesday, March 14, 2018

A Simple Field Strength Meter

A Communicator Reprise...

Summer 2011

Anyone use a field strength meter anymore?  It’s kind-of like a radiometer for RF energy.  Remember the radiometer?  It’s those little black and white squares that spin inside a glass ball when light shines at it; the brighter [or hotter] the light, the faster it spins. Cool “instrument” from the 1870’s.

Well a field strength meter is sort of like that, in that in its heyday, it was used by Hams and CB’ers to measure the transmitted signal strength of any antenna - from a distance usually  1, 3, or 30 metres or whatever measured distance you had.  As long as the meter was “calibrated”, one could set up the antenna, mount a FS meter X number of feet or metres away, pump 1, 5, 10 or 100 watts out of it and measure the “strength” of the RF field at that measured distance.  It was simple, you could tune for maximum meter deflection, usually meant your SWR was at its lowest. An OK tool if you didn’t have a sophisticated watt meter or new-fangled SWR bridge.

Today, it can be used by the Ham antenna experimenter to measure the gain of the antenna – in RF volts or Db or whatever scale you had labeled on your meter, even S-units.  A sensitive FS meter can pick up low power bugs, or any source of RF energy – guess what those ghost hunters use?   More useful if you spent big dollars and put a tuned circuit, attenuators or a pre-amp in the circuit, and of course lots of LEDs.
But of course, good RF meters are expensive and somewhat hard to find, not many at the swap meets these days… and they are usually combined with other types of measuring devices, watt or SWR meters, thus more money than the typical cheap Ham wants to dish out.

Solution, make your own!!  OMG! What a concept!  A simple FS meter is the simplest thing to make and is good enough to see if the antenna under test is radiating more power than your old ground plane, old mobile vertical or just radiating at all in a particular direction or in all directions.

Here is what you need:
  1. A digital voltmeter with a dc millivolt scale – every Ham should have a few in their shack.
  2. A Germanium diode, just about any one, as long as it’s Germanium, like 1N34, 1N270, 1N914 or 1N100. The best one, a non-North American standard. The super-sensitive OA91 from down under or Europe/UK  – Great for your crystal radio project too.
  3. A 3.3MΩ 1% resistor, 1/8 or ¼ watt.
  4. A 100 picofarad capacitor
  5. And a hand-made inductor [L1] of 7 turns on a ¼ inch coil form with a ferrite slug (some experimentation required to cover the North American FM Band) 24 to 28 AWG lacquered wire.
  6. Some miscellaneous parts like an antenna or antenna connection, a tiny box to put it all in, and some jacks that your DVM leads will insert into.
Using a digital multimeter, as opposed to an analogue meter has a few advantages in this circuit.

First, the impedance of a DVM is very high, around 10MΩ per volt on most meters.  This will not shunt or load down the tank circuit.  Second, compared to an analogue meter, very slight differences in signal strength can me more easily observed.  An third, a digital meter will have better linearity responding well to both weak and stronger signals.
All you want to see is the numbers, the higher the number, the more signal strength.  Just remember a few basic rules.  Keep the distance and power out the same for all your experiments, and turn off all your APRS trackers and digipeaters as they will want to add their 2-cents worth to your measurements.

If you have it in a hand-held configuration, you can “see” lobes, minimum and maximum RF fields as you walk around your test antenna. Oh, and then put a set of crystal ear plugs in place of your DVM and you might just hear the nearest AM broadcast station… well, until they all go digital.


The original article appeared in the Communicator - Summer 2011 edition

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