After the publication of the author’s articles “Suppression of Common-mode Interference to Radio Reception” and “Master-kit from “SELGA-404” Radio Receiver Parts: Anti-noise Ferrite Rod Antenna “Olusha-T”, the anti-noise ferrite rod antenna “Olusha-10” was developed, which, unlike the antenna “Olusha-T ”, is already an independent product, and not a rework from the parts of an old radio receiver: a printed circuit board was developed, etc. The antenna was made by order of radio amateurs in various versions, including installation on a photo tripod and with a friction vernier mechanism.
The brief technical characteristics of the antenna:
|Working frequency band||531..1611 kHz (medium waves)|
|Field strength sensitivity at a signal-to-noise ratio of 30 dB, at least*||0,092 mV/m|
|Sufficient sensitivity of the radio receiver at the antenna input with a signal-to-noise ratio of 30 dB*||9,80 μV|
|Output impedance at the antenna coaxial cable connection pins||50 Ohm ±10%|
|Power supply voltage||7,5..12,6 V|
|Maximum current consumption||18 mA|
|Dimensions||200 x 92 x 35 mm|
|Weight, no more||0,25 kg|
*at 1 MHz, see below for detailed specifications
Detailed characteristics of the antenna:
|Antenna tuning frequency:|
|550 kHz||1,0 MHz||1,45 MHz|
|Field strength sensitivity at a signal-to-noise ratio of 30 dB,
at least mV/m
|Sufficient sensitivity of the radio receiver at the antenna input with a signal-to-noise ratio of 30 dB||5,5 μV||9,8 μV||14,2 μV|
|Bandwidth by level -3dB||6 kHz||14 kHz||31 kHz|
|Quality factor, at least||90||70||45|
|Effective length, normalized to output signal level under conditions when Rload = 50 Ohm, at least||0,075 m||0,106 m||0,1 m|
The anti-noise ferrite rod antenna “Olusha-10” effectively suppresses industrial and domestic electrical interference, typical for modern urban radio reception conditions. This was achieved due to the most symmetrical design of the antenna, both in terms of constructive design and circuit design. Thus, it was possible to eliminate the so-called “antenna effect” characteristic of many magnetic antennas, when the conductive elements of the magnetic antenna work as a parasitic electrical antenna. Such a parasitic “antenna effect” leads to an increase in the level of received noise and interference, the appearance of parasitic reception channels and the “blurring” of a deep minimum in the magnetic antenna radiation pattern.
How exactly the nature of radio reception changes when using the active anti-noise ferrite rod antenna “Olusha-10” can be understood from comparative audio recordings:
The antenna amplifier of the anti-noise ferrite rod antenna is matched both in input and output. The input is matched with a high self-resistance of the signal source, which is a resonant parallel circuit. This matching made it possible to minimize the influence of the gate noise current of the antenna amplifier JFETs. The output impedance of the antenna amplifier at the antenna cable connection terminals is 50 Ohm ±10%, which allows you to connect a coaxial antenna cable with a characteristic impedance of 50 Ohm of any length, loaded at the opposite end to a matched load with the same impedance.
The schematic diagram of the active anti-noise ferrite rod magnetic antenna “Olusha-10” is shown in Fig.1. The signal to be fed to the antenna input of the radio receiver is taken from the secondary winding of a symmetrical matching high-frequency transformer T1:III. The two primary windings of this transformer T1:I and T1:II are connected to the sources of JFETs VT1 and VT2, on which a differential push-pull source follower is made. Resistor R1 sets the DC operating mode of the JFETs and serves as a load for the common-mode component of the output signal.
The ferrite rod magnetic antenna itself is an oscillatory circuit tuned to resonance with the received signal, the inductors of which, indicated in the diagram as WA1:(I-II), are located on a ferrite rod. To tune the circuit to the frequency of the received signal, a two-section variable capacitor C1 is used. Trimmer capacitors C6 and C7 are used to set the boundaries of working frequency band and setting the symmetry of both “shoulders” of the oscillatory circuit. For greater flexibility of this setting additional capacitors C2..C5 are also included in the oscillating circuit.
The signal selected by the antenna oscillatory circuit is fed to the input of a differential push-pull source follower, the gates of JFETs VT1 and VT2, through a symmetrical coupling coil WA1:(III-IV). Such a connection of the amplifying stage to the antenna oscillatory circuit makes the circuit insensitive to low-frequency interference, which can modulate the useful signal.
The antenna amplifier of the anti-noise ferrite rod antenna “Olusha-10” is assembled on a single-sided printed circuit board. Its dimensions are 87,5 x 36,2 mm. The trace of printed conductors is shown in Fig.2, and the placing of components on the board is shown in Fig.3. JFETs of the 2N4416A or J310 types are used with equals of zero-gate voltage drain current (IDSS) and gate to source cutoff voltage (VGS(OFF)).
Two halves of the antenna contour coil are symmetrically located on a ferrite rod Ø10x200mm, see photo. The number of turns depends on the magnetic permeability of the ferrite. The contour coil contains 47+47 turns of AWG29 wire in silk insulation for the ferrite with the magnetic permeability of 400. The coupling coil is placed in the middle of the ferrite rod and contains 18+18 turns of AWG33 wire twisted twice with a step of about 2-3 twists per centimeter of length.
In the photos below, you can see the design features of the active anti-noise ferrite rod antenna “Olusha-10”.
Photo 4..6 shows the active anti-noise ferrite rod antenna “Olusha-10” with the friction vernier mechanism. The antenna is fixed on a bracket with a central thread of 1/4″ UNC, which allows you to install the antenna on a photo tripod.
Copyright © Sergii Zadorozhnyi, 2010
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