e001020.pdf

RADIO, TELEVISION & VIDEO

active loop antenna

wideband

for general-coverage receivers

There’s nothing better

than a long-wire

antenna for short-

wave reception, but

not everyone has the

necessary space. The

active loop antenna

described in this arti-

cle can be an excel-

lent alternative to a

long-wire antenna

where space is

restricted. Not only is

it easy to build, it also

provides outstanding

performance and is

very inexpensive.

Anyone who wants to use a general

coverage receiver for more than just lis-

tening to the BBC World Service or the

Voice of America (VOA), soon discov-

ers that a simple telescopic antenna has

its limitations. The efficiency of such an

antenna is very low, and it also picks

up a lot of interference when used

indoors. If you take long-distance

short-wave reception a bit more seri-

ously, you will soon find that you need

something better.

What then should it be? Whole

books have been written about

antenna technology, and there are

countless types and varieties of anten-

nas. Before plunging into the forest of

possibilities, it’s a good idea to first con-

sider what requirements the antenna

in question should meet.

For a short-wave or general-coverage

receiver, it is important that the band-

width of the antenna is sufficient to

cover a range of (say) 3 to 30 MHz, and

that it has a reasonable efficiency

within that range. If you are looking

for the simplest solution, then the

‘good old’ long-wire antenna is an

excellent choice. Of course, there are

numerous other types of antennas that

are also suitable, but their construction

is in most cases considerably more

complicated.

The common feature of the these

types of antennas is that they take up a

considerable amount of space, which is

reason enough to regard them as typi-

cal outdoor antennas. What can you do

if you have nowhere near enough

space for such an antenna? Must you

content yourself with the telescopic

antenna, or are there other affordable

possibilities?

design by G. Baars

T AKE A LOOK AT THE

WINDOW

A loop antenna is a good indoor alter-

native for a long-wire antenna. Some

people may respond to the term ‘loop

antenna’ with aversion, since they have

the idea that the only place for such

antennas is a museum. This is

Elektor Electronics

1/2000

absolutely not true! The basic loop

antenna design can still be used to con-

struct quite useful antennas, and these

antennas have the significant advan-

tage that they are excellent for indoor

use, due to their form and characteris-

tics. For example, if such an antenna is

wound around a window frame, it

takes up hardly any space and is also

practically invisible. In addition, a loop

antenna has the desirable characteris-

tic of reacting only to the magnetic

component of the transmitted signal,

which means that it rejects a large

number of electrical disturbances.

These features of the loop antenna —

compactness and interfer-

ence rejection — should not

be underestimated.

10mA

100n

2n2

6V3

BF494

BF451

BF494

4V9

2n2

BF451

BF494

100n

Figure 1. The schematic

diagram of the active

loop antenna is remark-

able for its simplicity.

L ET ’ SGO

ACTIVE !

In terms of effective surface

area, the antenna described here can

be roughly compared to an average

long-wire antenna, since it is made

from 10 to 15 metres of wire. However,

since it is folded into a loop and

installed indoors, the loop antenna has

different characteristics and its effi-

ciency is significantly lower.

To deal with this, there is actually no

other choice than to implement an

‘active’ version of the loop antenna,

which simply means providing it with

a built-in amplifier. At the same time,

the amplifier also allows the impedance

of the antenna to be matched to the

standard 75-ohms cable impedance.

The schematic diagram in Figure 1

shows that such an amplifier need not

be all that complicated. As can be seen,

the loop antenna is connected to the

inputs of a differential amplifier built

using discrete components. It employs

the well-known high-frequency tran-

sistor BF494 and its PNP equivalent,

the BF451. The differential gain stage

provides an amplification of around 10

and has a bandwidth of more than

30 MHz, which thus covers the entire

short-wave band.

Transistor T3 acts as a buffer and

impedance converter. The amplified

signal passes to the output connector

K1 via capacitor C3. A coaxial cable can

be used to carry the signal from the

output connector to the radio receiver.

000005 - 11

possibly even easier. Assuming that the

dimensions of the window frame are 1

by 1.5 metres, you should wind two to

four loops of insulated hook-up wire

around small nails located at the cor-

ners of the frame. The core diameter of

the wire is not particularly important.

Be sure not to use a metallic window

frame, since the antenna will not work

at all with a metallic frame.

For the prototype, the window

frame measured 82 by 133 cm and

three turns were used. This gave out-

standing results. The amplifier was fit-

ted in a small box located at the bottom

of the window frame. Since the circuit

does not draw more than around

10 mA, a 9-V battery is fully adequate

for the power supply. However, a sta-

bilised (and well filtered!) mains

adapter can naturally be used instead,

if desired.

extensively tested in combination with

the short-wave receiver described in

the January 1999 issue of Elektor Elec-

tronics . In a one-to-one comparison, the

active loop antenna proved to perform

just as well as a long-wire antenna.

Both types of antenna produced essen-

tially the same S-meter readings over

the full range of the receiver. By com-

parison, the telescopic antenna was

significantly inferior; its S-meter read-

ings remained at around one third of

the level of the other two types.

(000005-1)

Text (Dutch original): S. van Rooij

Design editing: K. Walraven

Figure 2. A simple design

means that the printed cir-

cuit board can be delight-

fully compact.

P ERFORMANCE

With an antenna, a practical test says a

lot more than a whole list of numbers.

The active loop antenna was thus

+9V

COMPONENTS LIST

Resistors:

R1,R2 = 82k Ω

R3 = 560 Ω

R4 = 220 Ω

000005-1

C ONSTRUCTION

Figure 2 shows the track layout and

component layout of a printed circuit

board design that is suitable for con-

structing the wideband loop antenna.

This board is unfortunately not avail-

able through our Readers Services, so

you will have to etch it yourself. After

this is done, assembling the board

should not take more than around half

an hour, given the small number of

components used.

Constructing the antenna itself is

Capacitors:

C1,C2 = 2nF2 ceramic, raster 5mm

C3 = 100nF ceramic, raster 5mm

C4 = 100nF, raster 5mm or 7.5mm

Semiconductors:

T1,T3 = BF494

T2 = BF451 (BF450)

Miscellaneous:

K1 = BNC or cinch socket

10 to 15 metres enamelled copper or

hook-up wire

Elektor Electronics

1/2000

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