September 1960 Popular Electronics
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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 bandwidth
for just $30, 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 Popular Electronics provides that opportunity.
Build a Field Strength Meter
By Russell Keller, K9CZO
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
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
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.
Wiring in Field Strength Meter is point-to-point,
with smaller parts supported by their own leads. Be sure to
insulate antenna jack J1 from chassis.
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
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.
Schematic diagram of field strength meter.
Exact values of C1 and M1 will depend on desired range and sensitivity
of unit; switch S1 can be omitted if antenna is unplugged whenever
meter is not in use.
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
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