Active Antenna Module

Short-wave listeners
often are not able to, or allowed to, install a long-wire antenna or
other large dimension antenna in or around the home. In such cases, the
present active antenna, intended for the frequency range 3–30 MHz, may
be found useful. The author used a 1-metre long rod or brass tube with a
diameter of 2–6 mm. The circuit consists of two parts, one to be
located close to the antenna, while the other should be placed in the
associated power supply of the receiver.

Picture of the project:

The two sections may be connected by a coaxial cable of up to 20 m
long without causing any discernible attenuation. The antenna signal is
pre-amplified by a two-stage combination, T1-T3. The main amplification
is provided by the input transformer, formed by L3, L4, and L5, in the
receiver section. This is followed by a switch that enables the
frequency range to be selected (3–10 MHz in position LOW, and 9–30 MHz in position HIGH). The signal strength may be adjusted to suit the receiver with potentiometer P1.

The active antenna is readily constructed with the aid of the two
printed circuit boards shown. Since we are concerned with only
relatively low radio frequencies, the choice of components is not too
crucial. Various types of FET may be used: BF245, BF246, BF256, or the SMD
variants of these, but do mind their connections! The same applies for
the transistors: BFW16, BFY90, BFR91, BFR96; any of these will do.

Circuit diagram:

A few hints for readers who conduct their own experiments. A lower
value of capacitor C1 results in a somewhat looser coupling to the
antenna, but also in lower signal strength. It may be worthwhile to
replace the capacitor with a variable type. Inductor L6 ensures that the
output voltage at higher frequencies (30 MHz) is not much higher than
at lower frequencies (3 MHz). This is because the Q factor of coils L4
and L5 increases at higher frequencies, which leads to higher
amplitudes. This is compensated by L6.

Parts and PCB Layout

This inductor may be omitted and replaced by a wire bridge, but then
the output voltages at higher frequencies increases. Aim at obtaining
as tight a coupling as feasible between L4 and L5. Because of this, it
is better to wind the two coils as one, that is, 30 turns with a tap
than to wind two separate coils (see photograph).

*Components List”
Resistors
R1 = 470k
R2 = 220k
R3 = 1k
R4 = 100R
R5 = 150R
R6 = 39R
R7 = 1M
P1= 470R logarithmic

Capacitors
C1 = 1nF
C2,C3,C8,C9 = 100nF
C4,C10 = 47µF 25V radial
C5,C7 = 47pF
C6 = 500pF tuning capacitor

Inductors
L1,L2,L6 = 10 turns 0.7mm ECW, 4mm
diameter (wind on 3.5mm drill bit)
L3 = 1 turn 0.7mm ECW, around L4
L4 = 8 turns 0.7mm ECW, 12mm
diameter (wind on 10 mm drill bit)
L5 = 22 turns 0.7mm ECW, 12mm
diameter (wind on 10mm drill bit)
L7,L8 = 1mH miniature choke

Semiconductors
T1 = BF247B
T2 = BF245A
T3 = 2N5109

Author: Stefan Delleman – Copyright: Elektor Electronics

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