Build a Field Strength Meter
September 1960 Popular Electronics

September 1960 Popular Electronics Table of Contents Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, published October 1954 - April 1985. All copyrights are hereby acknowledged.

There is little incentive to build your own field strength meter these days when commercial instruments are readily available and relatively inexpensive. For instance, you can buy an MJF-801 FSM with a 100 kHz to 500 MHz spectrum coverage for just $45, brand new. More sophisticated, calibrated instruments are available for a lot more, but this basic unit is dirt cheap. However, if you want to read a little about the theory behind a field strength meter and see how one goes together, this article from a 1960 issue of Popular Electronics magazine provides that opportunity.

Build a Field Strength Meter

Are you curious about the radiation pattern of your CB or ham antenna? Here's a simple field strength meter (FSM) that will give you an indication of relative field strength on either the 6- or 10-meter ham bands or the 11-meter Citizens Band.

This little instrument is nothing more than a tiny receiver which drives a meter instead of headphones. The meter lets you read the relative signal strength of your signal at various points near your transmitting antenna. Parts should cost less than $10, and total construction time shouldn't exceed a few hours.

Construction

The unit should be housed in a 4 1/4" x 2 1/4" x 1 1/2" (or larger) metal box; unshielded plastic boxes are not suitable since inductive pickup by the FSM's coil will give a false meter reading. Mount the r.f. portion of the FSM (capacitors C1 and C2, coil L1, jack J1, and diode D1) in the upper half of the box as shown. Insulate antenna jack J1 from the box with a fiber washer. Keep all leads in the r.f. portion short, and use a heat sink when soldering diode D1 and transistor Q1.

A one- or three-band version of the FSM is possible, the only difference being in the choice of tuning capacitor C1. For a three-band model (the 6- and 10-meter ham bands and the Citizens Band), use a 75-uuf. unit (Hammarlund APC-75 or equivalent) for C1. If you want only a six-meter FSM, use a 25-uuf. unit (Hammarlund APC-25 or equivalent).

Coil L1 consists of six turns of No. 18 enameled wire, 1/2" in diameter. Solder L1 directly across the terminals of capacitor C1 and solder the negative lead of diode D1 to a tap 1 1/2 turns from the ground end of L1. Be sure to scrape the enamel from L1 in the area of the tap before soldering D1 in place. All other components except meter M1 are also soldered in place by their leads.

A battery holder is not used since zero-signal current drain is only a few microamperes and penlight cell B1 should last indefinitely. On-off switch B1 can also be dispensed with if desired, but the antenna should be unplugged when the FSM is not in use.

Mount meter Ml in the lower half of the box. For a more sensitive instrument, use a 500-ua. or 100-ua. meter instead of the 1-ma. unit specified; no circuit changes are needed for either of these meters. With one of the more sensitive meters in the circuit, you can operate the FSM with a shorter antenna and measure r.f. field strength at a greater distance from the transmitter.

Make a short whip antenna, as shown, by soldering a 1' or 2' length of No. 12 or No. 14 busbar to a banana plug. Jack J1 on the FSM is a banana jack and permits the antenna to be unplugged when the FSM is not in use.

Operation

You can use the FSM to check the radiation pattern around your antenna or to see if your transmitter is improperly shielded and radiating r.f. Before these checks can be made, however, the FSM must be tuned to the transmitter. Do this by inserting the FSM's whip antenna into J1 and placing it near the transmitter. Then rig a temporary short-wire antenna to the transmitter, and tune up the transmitter. If yours is a CB rig, just switch to "transmit" and use a clear channel. In any case, keep all experiments down to a minimum so that already burdened Citizens Band and ham frequencies are free of unnecessary interference.

Switch on the FSM and adjust capacitor C1 to the transmitter frequency. The meter will show a sharp rise from the zero mark at the transmitter's frequency. Adjust C1 for a maximum reading on the FSM. If the meter goes off scale, move the FSM further away from the transmitting antenna. At this point, you'll notice that the FSM pickup depends on its polarization with the transmitting antenna: maximum pickup results when the FSM antenna and the transmitting antenna are parallel to each other.

Once the FSM is tuned to the transmitter, disconnect the temporary antenna and connect your regular transmitting antenna. If your transmitter and coaxial transmission line are properly shielded and grounded, you should get no reading on the FSM no matter how close to the transmitter or coax the FSM antenna is placed.

When this check has been made, go outside to your transmitting antenna and turn the FSM until its antenna parallels the transmitter's. Walk around the transmitting antenna with the FSM, taking care to stay at least several wavelengths away from the antenna.

The r.f. field you detect should correspond with the type of antenna you have. If your antenna is directional, the r.f. field will be stronger in one location than in another; this is true of horizontal antennas. Vertical antennas, on the other hand, should exhibit a perfectly uniform field in a 360 degree sweep. Antennas with reflectors should be most effective on the side away from the reflector.

How It Works

Operation of the FSM is similar to that of a receiver using a diode detector followed by a one-transistor amplifier. In this case, the transistor feeds a milliammeter rather than headphones. When r.f. is picked up by the antenna, it is tuned by coil L1 and variable capacitor C1 . Diode D1, connected to a low-impedance tap on L1, rectifies the r.f. appearing across the L1-C1 tuned circuit. The rectified signal is filtered by capacitor C2 and fed to the base of transistor Q1, where it is amplified and fed to meter M1. Serving as a visual indicator, Ml measures the amplitude of the rectified signal, which is proportional to the r.f. field strength. Battery B1 powers Q1 through on-off switch S1.

Posted August 17, 2021
(updated from original post on 3/26/2013)