Active Ferrite Rod Antenna.pdf

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An Active Ferrite Rod Antenna
with Remote Tuning
by
Chris Trask / N7ZWY
Sonoran Radio Research
P.O. Box 25240
Tempe, AZ 85285-5240
Senior Member IEEE
Email: christrask@earthlink.net
17 February 2008
Trask, “Active Ferrite Rod Antenna”
1
17 February 2008
Introduction
Active antennas are of interest to a wide
range of users, from shortwave listeners
(SWLs) and radio amateurs to designers of air-
craft radios. SWLs and radio amateurs living
in confined areas such as apartments or in
communities having antenna restrictions find
small antennas and especially active antennas
to be a practical solution. Antennas that incor-
porate ferrite rods are of particular interest and
practical usefulness as they have the potential
of offering reasonably good performance for a
very small physical size.
However, many commercial ferrite rod an-
tennas and published hobby articles fail to take
advantage of the performance capabilities that
are possible with a thorough design, and these
antennas are generally untuned wideband de-
signs which, when the wide bandwidth of sig-
nals are introduced to the active portion of the
antenna, result in often unacceptable inter-
modulation distortion (IMD) performance.
The purpose of this design is to demon-
strate that an active ferrite rod antenna can be
designed that incorporates high-Q remote tun-
ing prior to the active portion of the antenna,
and which also has sufficient sensitivity so as
to be part of a receiver system that has good
inherent signal-to-noise (SNR) performance.
Ferrite Rod Antennas
There is more than enough literature avail-
able about ferrite rod antennas that the basic
theory really does not need to be repeated here,
and very thorough treatments are available from
Snelling (1), Burrows (2), and others (3, 4, 5).
Since ferrite rod antennas respond primarily to
the magnetic field component of a signal, they
offer a good degree of immunity from electro-
static noise sources such as flourescent light-
ing, faulty mains transformers, and lightning,
which is highly desireable and which makes the
pursuit of this design worthwhile.
Trask, “Active Ferrite Rod Antenna”
2
Ferrite rod antennas work best when they
are unloaded, meaning they are conducted to
a high impedance load. When so configured,
the generation of IMD products has proven to
be satisfactory, as the various losses associ-
ated with the ferrite material are minimized due
to the fact that there is little or no signal current
in the antenna winding(s).
For the prototype to be described here,
the antenna consists of an RMX 1000 ferrite
antenna rod (1.0”D x 7.5”L, Fair-Rite 61 mate-
rial, available from ByteMark) with 200 turns of
#34 wire, closely spaced at the centre of the
rod. The overall performance of the antenna
can be improved by spacing the windings and
adjusting the overall length along the rod (6, 7),
however the approach used here has proven
to be adequate for the purpose of this design.
Other antenna configurations of ferrite
rods and windings are usable and will have lit-
tle effect on the remote tuning as that portion of
the design is dependent upon a small inductor
that is in parallel with the antenna, which will be
discussed in the next section.
Amplifier Topology
and Description
The schematic diagramme of the active
ferrite rod antenna amplifier is shown in Fig. 1.
Here, the antenna consists of a single winding
on a ferrite rod, which is connected to a tuning
network consisting of inductor L1 and the
varactors D2 and D3. This is followed by a high
input impedance voltage-to-current converter,
which in turn is followed by a transimpedance
current amplifier. Biasing is stabilized by way
of a voltage regulator IC.
Remote tuning is applied as a voltage
across resistor R1, which is derived from the
amplifier power and tuning voltage that varies
from 8V to 22V from a control unit. The zener
diode D1 provides a voltage drop of 7.5V so
that the supply voltage to the amplifier is always
17 February 2008
greater than 8V while the tuning control voltage
varies from 0.5V to 14.5V.
The resonant frequency of the tuning net-
work is determined by the varactor diodes D2
and D3 and the parallel combination of inductor
L1 and the antenna winding. This arrangement
allows for placing a large number of turns on
the ferrite rod, which will then provide a higher
signal voltage than if the ferrite rod winding was
designed to accomodate the tuning varactors.
With the value of L1 being 3.5uH, the tuning
range for the network can be varied from
5.5MHz to almost 15MHz with 0.5V to 14.5V of
tuning voltage applied to the MVAM115’s. For
different tuning ranges, the value of L1 can be
adjusted as needed.
The varactor diodes D2 and D3 are de-
picted in Fig. 1 as being MVAM115’s, however
a variety of varactor diodes are available for
this design, such as the MVAM109 and the
Parts List
C1, C2, C3, C4, C6, C7, C9 - 0.1uF
C5 - 22uF 16V electrolytic
C9 - 0.33uF 50V electrolytic
D1 - 1N5236B
D2, D3 - MVAM115 (see text)
L1 - 3.5uH (see text)
Q1 - J309 or J310 (preferred)
Q2 - 2N2222 or MPS6521 (preferred)
Q3 - 2N2222
R1 - 10K
R2 - 1.0M
R3 - 120 ohms (see text)
R4 - 1.8K
R5 - 3.3K
R6 - 22 ohms
T1 - 1:2 Transformer (see text)
T2, T3 - 3:1 Autotransformer (see text)
U1 - 78L05
Fig. 1 - Active Ferrite Rod Antenna Amplifier with Remote Tuning
Trask, “Active Ferrite Rod Antenna”
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17 February 2008
NTE618 (available from Mouser). Hyperabrupt
varactors of this capacitance range tend to be
a bit expensive, and their availability is a bit lim-
ited, however they are necessary if electronic
tuning over a wide range at MF and HF frequen-
cies is desired.
At the resonant frequency of the tuning
network, the ferrite antenna sees the only the
high load impedance of the gate of Q1. To-
gether with transformer T1, transistor Q2, and
resistor R6, this portion of the amplifier circuitry
forms a voltage-to-current converter of approxi-
mately 20uA/uV, or 20 Siemens (20S). Since
the J310 source follower sees a rather large
load resistance (about 10K) and the MPS6521
has fairly high current gain, there is very little in
the way of IMD products.
Resistor R3 is depicted as being 82
ohms, however the value should be chosen such
that the bias current for Q1 is in the vicinity of
15mA to ensure good linear performance. This
value is not critical, but is rather a one-time
adjustment needed to accomodate the range
of biasing characteristics of the J310.
Transformers T2 and T3, together with
transistor Q3, form a transimpdeance current
amplifier having a power gain of 19dB, as each
transformer provides a current gain of three.
Amplifiers of this nature add very little in the way
of IMD products as the amplification is a result
of the current gain provided by the transform-
ers and the transistor is basically performing
the function of a current-controlled current
source with a gain of slightly less than unity.
The 5V regulator U1 provides a stable
bias voltage for the amplifier stages while the
supply voltage is varied from 8V to 22V for the
remote tuning. The small value for the electro-
lytic capacitor C8 (0.33uF) is prescribed in the
manufacturer’s datasheet for applications
where the device is located at an appreciable
distace from the power source.
Transistor Selection
There is little that needs to be said about
the use of a J309 or J310 (preferred) for the
input stage. This device provides the high im-
pedance needed to realize a usable signal volt-
age from the ferrite rod antenna and to provide
a high degree of tuning selectivity from the tun-
ing network. The J310 is known to provide a
Fig. 2 - J310 Characteristic Curves
(horizontal 1V/div, vertical 5mA/div,
0.2V/step)
Trask, “Active Ferrite Rod Antenna”
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Fig. 3 - MPS6521 Characteristic Curves
(horizontal 1V/div, vertical 2mA/div,
5µA/step)
17 February 2008
(T2 and T3). They need to be constructed in
such a manner as to minimize parasitics such
as leakage inductance and intrawinding ca-
pacitance which affect the high cutoff frequency,
but at the same time maximize the coupling co-
efficient, which affects the low cutoff frequency
and the overall amplifier gain. Commercially
available transformers such as those available
from Mini-Circuits are convenient, however their
overall performance is insufficient for high per-
formance applications such as this.
Despite some unfortunate remarks made
in the Technical Topics column of RF Commu-
nications some years ago (8), it is entirely pos-
sible for hobbyists of average ability to construct
wideband transformers that will easily have per-
formance equal to and even surpassing that of
most commercial offerings, provided that sim-
ple guidelines concerning the design and con-
struction as well as the selection of materials
are adhered to (9, 10, 11).
One of the leading causes of poor trans-
former performance is the construction of the
windings. Many designs, including most com-
mercial products, make use of monofilar
(meaning single-wire) windings, which results
in less than ideal coupling coefficients, regard-
less of how they are arranged. Twisted wires,
either bifilar (two wires) or trifilar (three wires)
offer the best possible coupling, and one only
has to learn how to design transformers using
combinations of wires with 1:1 or 1:1:1 ratios.
With twisted wires, coupling to the core is mini-
mized, which results in lower losses, lower
intrawinding capacitance, and lower IMD prod-
ucts that result from magnetic field nonlin-
earitites in the core material.
To that effect, all three transformers are
constructed using a trifilar twist of #34
enameled wire. Eight turns are wound through
the holes of a Fair-Rite 2843002402 binocular
(aka balun or multiaperature) core. For lower
frequencies, a core made from Fair-Rite 73
material should be used, and for higher frequen-
17 February 2008
Fig. 4 - 2N2222 Characteristic Curves
(horizontal 1V/div, vertical 2mA/div,
10µA/step)
slight edge in terms of IMD performance over
the J309 when used as a source follower, though
other devices such as the 2N3819 and 2N4416
will deliver better IMD performance in common-
source applications. Fig. 2 shows the transfer
characteristics of the J310.
The MPS6521 was chosen for the emit-
ter follower Q2 as it has acceptable character-
istics (primarily capacitances and f
T
) for HF
applications together with a fairly high gain (h
fe
).
The 2N2222 may also be used for Q2, though
the IMD products may be a bit higher due to
the lower h
fe
. Both devices have very good lin-
earity characteristics, especially in the satura-
tion region, as shown in Fig. 3 and Fig. 4.
The 2N2222 was chosen for Q3 as it is
better suited for the fairly high voltage that will
exist on the supply line when tuning the ampli-
fier to higher frequencies.
Transformer Construction
The transformers are key elements in the
overall performance of the amplifier as they pro-
vide linear voltage gain (T1) and current gain
Trask, “Active Ferrite Rod Antenna”
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