October 1958 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Commercially available test equipment for hobbyists in the 21st
century is pretty high quality at a relatively low price, so
the motivation for buying parts and building your own RF power
meter has to be driven not by cost, but by a desire to gain
the experience. This article presents a simple RF power meter
that can be built for about $25 worth of parts. All the parts
should be readily available except for the Raytheon CK-721 (or
CK-722) germanium PNP transistor. The
2N3906 and the
2N2907 have been suggested as replacements, but some changes
in the biasing resistors might be required.
See all articles from
Simple R.F. Meter
By Charles J. Schauers
Measure r.f. output of the transmitter in your ham
shack ... or use this meter in -a dozen other ways
This inexpensive r.f. indicator has a wide variety of uses
around the ham shack or mobile radio installation. It can act
as an absorption frequency meter (if calibrated), a field strength
meter, neutralization indicator, or modulation monitor with
phones. However, the main job of the model shown is to indicate
proper antenna loading for my "minified" mobile transmitter.
It is relatively simple to put together, and nearly any low-priced
transistor will work well. However, for maximum sensitivity,
a transistor with a beta (current amplification) of .between
25 and 45 should be used.
Construction. A 3 3/4" x 3" x 2 1/8" aluminum
chassis houses all parts. The coil (L1) is tapped and connected.
to switch S1 before installation in the box to facilitate soldering.
Then the coil is cemented to the box by its plastic support.
Care should be taken in soldering the crystal diode (CR1)
into the circuit by "heat sinking" the connections with a pair
of long-nose pliers. A socket should be used for the transistor.
Fasten the 1 1/2-volt cell to the chassis with household
cement. With normal use, it should last almost its shelf-life.
After the unit is turned on, zero the meter with potentiometer
R3 in the collector circuit. Attach a small wire to the input
binding post on the rear of the box which feeds r.f. to the
tuned circuit, and you are in business.
Applications. If the device is to be used
as an absorption frequency meter, it can be calibrated with
a heterodyne frequency meter coupled to the input post through
a small (about 500-μμfd.) capacitor.
The simplicity of the r.f. meter circuit
(see schematic below) makes for ease of layout on the chassis
(above). Number of turns tapped on L1 for each band is indicated
on the schematic. Finished meter is shown at top.
A 2-1/2" length of wire is sufficient for r.f. pickup when
checking oscillator, doubler or buffer and final amplifier stages
of a transmitter.
When using the indicator as a field strength meter to adjust
a beam antenna, the pickup wire length will depend upon the
distance from the antenna and how much power is being applied
from the transmitter's final amplifier. Usually, a 2" piece
of wire will afford sufficient pickup at 100 feet with the average
low-power transmitter when the device is hand-held.
To provide some attenuation of very strong signals, the indicator
can be used harmonically. Set the bandswitch to 40 meters when
you want to measure carrier strength on 80 meters.
As a means for tuning mobile or fixed transmitters (especially
those employing pi-output-networks), this unit enables one to
determine very quickly if the antenna and not the pi-network
is taking the load. For mobile operation, the regular auto broadcast
antenna can be used for signal pickup. However, the device should
be harmonically operated as described above because of the strong
signal present. If the auto antenna is not used, try a small
piece of insulated wire taped to the inside of the front bumper,
connected with shielded wire to the indicator.
If you are interested in monitoring your modulation, a pair
of magnetic phones can be connected in the collector circuit
of the transistor. The meter is disconnected (as well as the
top of the balancing potentiometer) and the phones are connected
between battery minus and collector.
B1 - 1.5·volt D cell
C1 - 140·μμfd. miniature variable
C2 - 0.002·μμfd. mica capacitor
CR1 - Crystal
diode (Sylvania 1N34 or equivalent)
L1 - #24·wire, 1"·diameter
coil (Airdux 832T or B & W 3016 - 32 turns per inch, tapped
as shown in schematic)
M1 - 0-1 ma. d.c. meter
- 680·ohm, 1/2·watt resistor
R3 - 6500-10,000 ohm slotted
S1 - 1-p., 4-pos. rotary switch
S2 - S.p.s.t. toggle switch
TR1 - Transistor (General Transistor
GT·87 or GT-88
or Raytheon CK·721)
1-Aluminum chassis box (LMB·135)
HOW IT WORKS
Radio frequency energy tuned by L1-C1 is applied to diode
CR1. The rectified current then takes a path through the base-emitter
circuit of transistor TR1.
Current amplification occurs and is read by the 0.1 milliammeter.
Capacitor C2, connected between the transistor base and ground.
bypasses the radio frequency. The greater the strength of the
r.f. signal picked up, the higher the reading on the meter.
Posted September 19, 2011