[Table of Contents]People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights (if any) are hereby acknowledged.
like the title: "Put PEP in Your Antenna Tuner." For those not familiar
with transmitter lingo, PEP is Peak Envelope Power, but f course the
word "pep," as in energy, is a clever double entendre. If you aren't
averse to building a tube circuit and have a 6AG5 in your parts box,
then here's a simple antenna tuner circuit.
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Put PEP in Your Antenna Tuner
By Joseph W. Doherty, K2SOO The coupler you built last
November grows up with just a few extra parts
you built the pi-section receiver antenna coupler described in November
POP'tronics ("Soup Up Your DX with an Antenna Tuner"), here is a worthwhile
addition you can make at slight expense and in little time.
If you have not yet built the coupler, you can put together the complete
unit in one evening. It is sure to pay extra dividends in improved reception.
As a matter of fact, this unit, which started out as a simple antenna
tuner and grew into a preselector and signal booster, can make your
old receiver practically jump right off the table.
HOW IT WORKS
The pi-network coupler
is the tuned grid circuit of the 6AG5 tube. An untuned plate circuit
is used to prevent oscillation. The output is resistancecapacity-coupled
and fed directly to the receiver input terminals where it may be
peaked by the receiver input circuits, depending on design.
Choke RFC1 maintains a high impedance to ground in the grid
circuit of the amplifier, and provides a grid return circuit. Capacitor
C8 provides an r.f, grounding path from the negative side of the
power supply. PARTS LIST
C2-140-μμfd. variable capacitor
disc ceramic capacitor
C4, C5, C8-0.001-μμfd. disc ceramic capacitor
C6-100-μμfd. disc ceramic capacitor
150-volt electrolytic capacitor
L1-3" x 1" coil (B & W Miniductor
#3015-48 turns, tapped as shown in schematic)
R2-12,000-ohm, 1/2-watt resistor
R4-100-ohm, 1-watt resistor
R6-2700-ohm, 1-watt resistor
resistance line cord (JFD #2190)
RFC1-2.5-mh. r.f. choke
S1-2-p., 5-pos. shorting switch (Centralab #1404)
SR1-Silicon diode Type 1N1084 (Sarkes Tarzian
It combines the impedance-matching properties of the pi-network coupler
with a signal booster amplifier. The power supply is self-contained
but, if desired, an external supply can be used. No expensive components
are required and the layout is extremely simple. Design
. This signal booster will provide amazing improvement
even in a well-matched antenna system. There is a band selector switch
position which allows you to bypass the coupler without having to disconnect
the antenna and reconnect it to the receiver.
The power supply
is of the a.c./d.c. type. However, neither leg of the line is connected
to the chassis, thus eliminating the shock hazard associated with power
supplies of this type. The rectifier is a silicon diode #1N1084 which
comes complete with mounting hardware and 10-ohm resistor in the Sarkes
Tarzian Replacement Kit #M150. Any other type rectifier with a 20-ma.
or higher rating may properly be substituted.
To conserve space,
a resistance-type line cord was used. Take care not to coil or bunch
this cord, since it must be allowed to give off heat. Also, do not attempt
to shorten it. The built-in resistor is 390 ohms "long."
. The original design was modified
to accommodate the r.f. amplifier with minimum expense and fewest circuit
changes. Follow this procedure.
First remove the original Bypass
jumper from S1a, S1b. Disconnect the lead to the junction of L1 and
C2. Then disconnect the antenna input lead from S1a.
the antenna input lead to the wiper contact of S1b. Connect a lead from
the 80-meter position on S1b to the S1a 80-meter position and S1a wiper.
Now connect the Bypass lead from S1b to the ungrounded output terminal
and, finally, connect the grid lead from C3 to the junction of L1 and
Since there is considerable gain in the amplifier, it is
recommended that the input and output leads be kept well separated or
oscillation may result. As an example of the gain obtainable, one station
heard in the bypass position was read on the receiver S-meter at "S6."
The preselector raised this same signal to 40 db over "S9," thus pulling
it well out of the noise.
The coupler should be grounded to
a cold water pipe or similar good ground. If the receiver used is of
the a.c./d.c. variety, connect the coupler output terminals to the receiver
antenna terminals only. Tuning the Preselector
Tuning is not difficult if you remember that C1 is the loading capacitor
and C2 is the frequencydetermining capacitor. There is some interaction
between the two but, to keep it simple, adjust both alternately for
the loudest signal. It is best to start out with C1 at maximum capacitance,
tuning C2 until the signal is loudest. Keep in mind that the bandswitch
must be in the proper position.
Decrease the capacitance of
C1 gradually until the signal starts to fade. Readjust C2 for the loudest
signal. If the signal has increased, decrease C1 still further and repeat
the process until the greatest signal strength is achieved.
In most cases, a point will be reached where advancing C1 will cut the
signal strength in spite of adjusting C2. This point is just beyond
optimum coupling, which means that the antenna is overcoupled. At this
point, rotate C1 in the other direction slowly while adjusting C2 through
resonance until the optimum adjustment is found.
NOTE: While we normally bar a.c./d.c. chassis projects
because of the possibility of shock hazard, in this case - since the
antenna preamp was built up from an earlier project on the same chassis
- using a transformer would have been difficult due to lack of space.
While all B -- points are raised above chassis ground, there is still
some possibility of leakage. Therefore, we advise using an isolation
transformer with this project. The Editors
The B -return shown on the schematic must be kept above chassis
ground; this can be done by using a tie strip as return point. Note
that R7 is the line cord dropping resistor. SR1 and R5 are included
in the replacement kit as indicated in the text. Photo on opposite page
shows completed under-chassis wiring. Photo at top left shows top of
tuner, while photo above indicates proper method of connecting tuner
to receiver's antenna input. Posted