George R.

Steber, WB9LVI
11105 N. Riverland Ct., Mequon, WI, 53092; steber@execpc.com

A Tunable RF Preamplifier Using

a Variable Capacitance Diode
Variable capacitance diodes can replace expensive
mechanically adjustable capacitors in an RF project.

Substituting a variable capacitance diode

for an expensive mechanically adjustable QX1711-Steber01 QX1711-Steber02
Antenna Antenna
capacitor can be done in many RF projects.
But there are a few tricks that need to be
learned. Take a look at how it’s done with L2
0.6 μH
this versatile RF preamplifier that you can
build yourself.
RF Filter HF Receiver
Working with home brew ham receivers,
C
inexpensive commercial short-wave sets L1 365 pF
R
and software defined receiver (SDR) based 0.25 μH
“dongle” radios can be fun and challenging. Figure 1 — An RF filter between the antenna
In some cases, however, the benefits of good and a low cost radio or experimental HF SDR
front-end RF selectivity may have been “dongle” receiver can significantly reduce
noise and interference problems
overlooked. For these cases adding a RF Figure 2 — Classic tunable RF band-pass
filter between the antenna and receiver as filter with variable capacitor C. Resistor R
shown in Figure 1 can help. Filtering signals But we’ll show you how to change some represents input to next stage, usually a FET
from the antenna provides rejection of strong component values to cover the frequency amplifier.
out of band signals, which otherwise might band of your own interest. To ease
overload the input. The filter can also prove construction, through-hole components are
used exclusively — no surface mount parts. nearby stations. WSPR and JT9 signals by
to be highly beneficial in situations where
It runs on a 12 V dc supply and requires less definition are low power signals and this
there is a lot of noise or interference. interference was making it more difficult
If your application requires just a single than 100 mA. And, it doesn’t require any
fancy equipment except for your antenna and to copy them with my computer software.
frequency or narrow band of frequencies The interference from the computer wasn’t
then a fixed input filter can be used. But if receiver to verify its operation.
So, if you want to get started experimenting helping either. Adding this filter made it a
your application requires a number of bands,
with a tunable RF preamp, tame your lot easier to reduce this noise — but it does
as in short wave listening, you will need an require turning a knob to peak the filter on the
input filter that is tunable to the desired band interference problems and at the same time
learn a bit about the application of a varicap desired frequency.
of interest. Figure 2 shows a classic tunable circuit
Presented here is an easy to build diode, read on. Home brewing your own
tunable preamp might be the way to go. that is often used to provide RF selectivity. It
tunable preamplifier that should find use in
is not the same as an antenna tuner, which is
many interference situations. It has good used to match the impedance of your antenna/
performance and can be built for a fraction of RF Tuner Background feedline to your receiver or transmitter. The
the cost of a commercial unit. An interesting This project began when I found that the tuner shown here is basically an adjustable
aspect of this project is that it uses a variable low cost short-wave radio I used for receiving band-pass filter. The inductors L1 and L2
capacitance diode (VCD), sometimes WSPR and JT9 signals on various bands was provide matching to the antenna in the range
referred to as a varicap or varactor in the suffering from severe interference because of 25 to 100 W and form a tuned circuit with
literature. of its wide input RF stage. My dongle- C. Resistor R is shown to represent the high
The design offered here covers the based SDR receiver was also suffering the impedance input of the next stage. R must
frequency range of 6 MHz to 23 MHz. same fate, being overloaded from strong be very high — in the megohm range — to

QEX January/February 2018 11

avoid loading down the tuned LC circuit. 12
Normally a FET is used here. The peak in
8 Figure 3 — Four
the RF response is moved over the frequency
4 frequency response
range by adjusting the variable capacitor C. curves of the tuned
Figure 3 shows response curves for various 0 preamplifier as
values of C. They were taken from a SPICE –4 capacitor C is varied.

Gain (dB)
simulation and were found to closely follow –8
those seen with a spectrum analyzer on the
–12 Cmax
actual circuit.
Finding a mechanically adjustable –16
capacitor to use in the tuner would have –20 Cmin
been a piece of cake a few years ago. This –24
once common part is now hard to find —
–28
even at hamfests! When found, the cost is
skyrocketing. I found a few on an auction 2 5 8 11 14 17 23
site but the prices were quite high. This could Frequency (MHz)
be a special hardship to those in clubs or QX1711-Steber03
outreach programs that wish to build your
project using this component.
600
Fortunately there is another way to do
the tuning. Use a varicap! In the next section Figure 4 — Varicap
we’ll talk about the varicap and how it can be capacitance C versus
500 applied dc voltage for the
used in this application.
1SV149 diode used in the
project. Data was taken
Using A Varicap 400 from the manufacturer’s
data sheet.
Capacitance (pF)

A varicap is essentially a voltage-

controlled capacitor. They have been around
since the 1960s and are commonly used in 300
voltage-controlled oscillators, parametric
amplifiers and frequency multipliers.
Here’s how it works. When a diode 200

is operated in a reverse-biased state very

little current flows in the device. The effect
100
of applying the reverse bias voltage is to
control the thickness of the depletion zone
and therefore its p-n junction capacitance. 0
The greater the applied voltage, the greater
0 2 4 6 8 10 12
is the depletion zone and the smaller the
Voltage (V)
capacitance. Most diodes exhibit this
characteristic to some extent but varicaps QX1711-Steber04
are manufactured to exploit this effect and
increase the capacitance over a larger range.
The varicap used in this project (1SV149) 24
has a very large capacitance variation and Figure 5 — Variation of
22 peak frequency with
was designed to replace the tuning capacitor varicap voltage. Data is
in AM radios. It has high Q (at least 200), a 20 taken for the 1SV149 used
small package, high capacitance ratio, and in the project. Frequency
low voltage operation. It can be found on 18 range is about 6 to 23 MHz.
Peak Frequency (MHz)

the internet for under a dollar! The varicap

capacitance variation for the 1SV149 with 16
respect to applied dc voltage is shown in
14
Figure 4.
Substituting a varicap for a mechanical 12
capacitor requires a bit of planning. We
need to know the capacitance range covered 10
and the tuning voltage required. The tuning
curves shown in Figure 3 covered a range 8
of 30 pF to 365 pF. So we would like to
6
find a varicap that covers that range with a
reasonable bias voltage range. The 1SV149 4
varicap covers 25 pF to 500 pF with a voltage 4
0 2 6 8 10 12
range of less than 10 V.
Voltage (V)
When the 1SV149 varicap is substituted
QX1711-Steber05
for the mechanical capacitor using an

12 QEX January/February 2018

 Optional L1 12 VDC R8
Gain Control 0.6 μH 10 Ω

J1
C5 C6
Coax
0.1 μF R5 0.1 μF
VR2 15 kΩ
Antenna L2
1 kΩ
0.25 μH
R3 Q2
D C3 2N2222
120 Ω
G 0.1 μF
C4 J2
0.1 μF Coax
C2
12 VDC 0.01 μF S
Q1 Output
J310 R4 R6
R7
R2 1 kΩ 15 kΩ
470 Ω
1 kΩ R1 D1
1 MΩ 1SV149
VR1
10 kΩ
C1
1 μF
RF Tune
(put on front panel) QX1711-Steber06

Figure 6 — Schematic diagram of the tunable RF preamplifier. The component values are shown in Table 1.

appropriate circuit, the tuning law shown Table 1.

in Figure 5 is obtained. As is seen, the
Components values for the RF tunable preamplifier.
varicap easily covers the same range as the
mechanical capacitor and more — in this Qty label value description
case 6 to nearly 23 MHz. 1 C1 1 mF electrolytic capacitor
1 C2 0.01 mF ceramic capacitor
Tunable RF Preamplifier Design 4 C3, C4, C5, C6 0.1 mF ceramic capacitor
Notes 1 L1 0.60 mH inductor, 11 turns T50-6 core
1 L2 0.25 mH inductor, 5 turns T50-6 core
Figure 6 shows the complete schematic 1 R1 1 MW 1/4 W 5% resistor
of the RF preamp. This circuit was designed 2 R2, R4 1 kW 1/4 W 5% resistor
using a SPICE simulator, LTspice.1 After the 1 R3 120 W 1/4 W 5% resistor
breadboard circuit was built, it was analyzed 2 R5, R6 15 kW 1/4 W 5% resistor
using a spectrum analyzer with tracking 1 R7 470 W 1/4 W 5% resistor
1 R8 10 W 1/4 W 5% resistor
generator. Very close agreement was found 1 VR1 10 kW potentiometer
between the Spice simulation and the actual 1 VR2 1 kW potentiometer (optional)
hardware. 1 D1 1SV149 varicap diode
This tunable RF preamp can be built 1 Q1 J310 FET
for about US $25, depending on your parts 1 Q2 2N2222 bipolar transistor
inventory. It does not require extraordinary 2 J1, J2 connector coax connector

skill with RF circuits. In fact it makes a nice
weekend project once the materials are on
hand. It is unique in some aspects, such as series. R1 feeds the dc voltage to D1 and is BNC connectors were used for J1
manual tuning the peak frequency. It will not very large to avoid loading the LC circuit. (antenna) and J2 (output) as they matched
compete with an expensive transceiver. But it VR1, R2 and C1 are there to provide a filtered, my equipment. Other connector types may
does produce good results in many situations. adjustable dc voltage to control the varicap. be used. A standard 12 V dc power supply is
It made my radios perform significantly better. VR1 should be mounted on the front panel used. If there is noise from the power supply,
Here’s how the circuit works. Starting at of your unit. Try to obtain a VR1 that rotates add an electrolytic capacitor between the
the antenna there is an optional gain control over a large angle (320 degrees) as this will 12 V dc line and ground.
VR2. Its purpose is to reduce the signal level provide finer control over your tuning. Various techniques may be used to build
to the preamp if necessary. The preamp does Q1 (n-channel JFET J310) is used as a this preamp. Use your own ingenuity! Some
provide RF gain of 6 to 9 dB, which helps high impedance buffer for the LC circuit. It good construction methods may be found in
with digging out weak signals. But if the has a resistor R3 at the gate to reduce possible previous QST articles or one of the ARRL
signals are too strong, just reduce them with instability. It must have a bypass capacitor Handbooks.2 I constructed a breadboard on
this control. C5 close to the drain D of the device. The a small solderable copper-pad perf-board.
Next is the tuned circuit consisting of L1, last stage is a standard bipolar transistor Q2 A simple PCB inside a metal case would
L2, C2 and varicap D1. It looks very much that provides a low impedance output. Q2 is probably be the best approach.
like that of Figure 2 and operates the same a 2N2222 type, but if a higher noise figure
way. Capacitor C2 blocks the dc that is on can be tolerated a 2N3904 may be used. For
the varicap D1. If C2 is made much higher Tunable Preamplifier Modifications
a lower noise figure try a 2N5109 — but this
than the capacitance of varicap, the varicap The tuning range measured for this
one may be hard to find. Also be careful of
capacitance dominates since they are in circuit was 6 to 23 MHz. This range can be
the P2N2222, which has reversed pin order.

QEX January/February 2018 13

changed easily. It is mainly dependent on the Using the preamp is straightforward.
inductance of L1, L2 and capacitance of C. Simply adjust the front panel-tuning resistor
It is difficult to predict the range accurately. VR1 until the received signal is the loudest.
This is because inductors may be constructed If you are using decoding software such as
slightly differently, the stray capacitance of JT9 or Fldigi that have a spectrum analyzer
the circuit is unknown, and the varicaps have display you will see a pronounced increase
a large range of tolerance. in the signal as you approach the peak.
First, lets take a look at the problem Remember to tune slowly.
theoretically. The peak frequency f occurs at
resonance
1 . Conclusion
f = I enjoyed constructing and using this
2π ( L1 + L 2)C preamp. Hopefully you will find that to be the
case as well. While building projects like this
The tuning range for (L1+L2) equal is fun, they can also be educational. Consider
to 0.85 mH and two values of C (30 pF demonstrating this application of a varicap
and 500 pF) yields a range of 7.7 MHz to in a teaching situation such as a presentation
31.5 MHz. This is quite different from the at your local ham club or possibly as an
measured values — most likely because the outreach tool at your local high school
inductor values are slightly different and science class.
there is stray capacitance. When you get your RF tuner working,
To accurately calibrate the tuner, you please let me know about it. With the radio
must measure the response after building interference reduced, you might now be
it. A suitable RF generator would prove able to copy those weak signals from the far
useful in this case. With that caveat, here are distant regions of the world.
some ranges for my circuit that I obtained
experimentally. George R. Steber, Ph.D., is Emeritus
Choosing L1 = 2 mH and L2 = 0.68 mH Professor of Electrical Engineering and
covers 4.3 MHz to 11.9 MHz. Going even Computer Science at the University Of
lower choose L1=10 mH and L2=2 mH which Wisconsin-Milwaukee. He is now semi-retired
covers 2.1 MHz to 8.0 MHz. You can play having served over 35 years. George, WB9LVI,
with inductor values to get many ranges. has an Advanced class license, is a life member
You may want to consider putting in a range of ARRL and IEEE and is a Professional
selector switch to cover the ranges you want. Engineer. His last article for QST was “An
Generally speaking, L2 is chosen to Easy WSPR 30 Meter Transmitter” in the
provide 25 to 100 W reactance over the January 2015 issue. George has worked for
frequencies of interest to match the antenna. NASA and the USAF and keeps busy working
But that is not a strict rule as the tuner can and lecturing on various subjects at the
University. He is currently involved in cosmic
be used with many different antennas, even
ray research and is designing equipment to
long wires. study them on a global basis. In his spare time
he enjoys WSPR-X mode JT9, racquetball,
Final Tuning astronomy, and jazz.
The RF tunable preamplifier can prove
helpful in a variety of interference situations. Notes
Even though its design is simple, the varicap 1
LTspice, www.linear.com/designtools/
works well and is a good substitute for the software/.
mechanical capacitor. One nice benefit of 2
The ARRL Handbook for Radio
using the varicap is that the variable resistor Communications, 2017 Edition. Available
from your ARRL dealer or the ARRL
VR1, controlling the dc voltage, has a larger Bookstore, ARRL item no. 0628. Telephone
angle of rotation (320 degrees) than the 860-594-0355, or toll-free in the US 888-
mechanical capacitor (180 degrees) and 277-5289; www.arrl.org/shop; pubsales@
therefore has finer control. The varicap also arrl.org.
has a larger range.

14 QEX January/February 2018