Dick Smith K-6345 Assembly Manual

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ssembly Manual for the
R DIO DIRECTION FINDER
K-634S
PL AS R AD DISCLAIM R CAR FULLY AS W
CAN ONLY GUARRANT PARTS AND NQT TH
LABOUR CONT NT YOU PROVID .
Reprinted in part by arrangement with Electronics
ustralia from their February, 1986 edition.
ant to find out where a radio signal is
coming from? Or locate the source of an
illegal transmitter? The radio direction finder
described here will track it down using an
electronically rotated antenna.
hysically, the radio direction finder
consists of two separate units. One con ­
tains the control and display electronics
and is located adjacent to an FM trans ­
ceiver or receiver; the other is a special
antenna switching unit (ASU) which is
connected to the control unit via a
4- way cable.
An electronic " compass " display con ­
sisting of 32 LEDs indicates the trans ­
mitter bearing. When a signal is re ­
ceived, its relative bearing to the an ­
tenna system is indicated by whichever
of the 32 LEDs illuminates.
In fixed installations, this allows the
compass bearing of the signal to be di ­
rectly indicated to within ±5.6 degrees.
When installed in a car, successive read ­
ings allow you to pinpoint the exact
location of the transmitter.
As such, the Dick Smith Radio Direc ­
tion Finder (or RDF for short) is just
the ticket for tracking down illegal
transmitters and anti-social radio opera ­
tors. Depending on the antenna system,
it can operate on any band within the
range 50-500MHz and will work with
FM receivers ranging from pocket scan ­
ners to amateur radio and CB transceiv ­
ers.
Radio direction finders of this type
can cost around $600 or more
We think it will be especially
popular with amateur radio operators.
How it works
The theory of operation is reasonably
simple. Radio signals received on a rap- determined.
idly moving antenna undergo a fre ­
quency shift due to the Doppler effect,
an effect well known to anyone who has
observed a moving car with its horn
blowing.
Consider a single antenna mounted on
the edge of a rapidly spinning disc. As
the antenna moves towards the source
of the RF carrier, the apparent fre ­
quency will increase due to the Doppler
effect. Conversely, as the antenna
moves away, the frequency will de ­
crease.
Thus, the rotating antenna causes fre ­
quency modulation of the received car ­
rier. When this type of antenna is con ­
nected to an FM receiver (the type most
often used on metres), a tone is
heard.
By analysing the phase of this tone,
the direction of the transmitter can be
To avoid the obvious drawback of a
mechanically rotated system, the Dick
Smith RDF simulates a rotating antenna
electronically. Four vertical whip anten ­
nas are arranged around a circle of
diameter .07-0.4 wavelengths. These are
electronically switched clockwise in se ­
quence such that all four antennas are
scanned once every l/1250th of a sec ­
ond.
This situation is equivalent to one
vertical antenna mounted on the perim ­
eter of a disc spinning at 1250 revolu ­
tions per second. For a diameter of say
800mm (for the 2-metre band), this re ­
sults in a tangential velocity of 3140
metres per second.
The deviation of the received carrier
is determined as follows. For V « C,
we can neglect relativistic effects and
write:
Text and Illustrations courtesy of Electronics ustralia