* MO^202 FSK DATA MODULATOR, Run up to

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* COR*2 KIT With audio miner local speaker ampiifier, tail & time-out tamers

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* AUTOPATCH KITS, Provide repeater auto-paichP reverse patch, phone tine remote control of repeater secondary control.


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65-D MOUL ROAD • HILTON NY 14468 Phone: 716-392-9430 Hamttonfcs1 ts J f^gist^nd trademift

Helical Bandpass Filter

R144/R220 FM RCVRS for 2M or 220 MHz. Ot5uV sens .3 uolextal filter & ceramic Miter in M. helical resonator front end for exceptional selectivity. > lOQdBat ^ 12kHzh best available today. Flutter-proof squelch. AFC tracks drifting xmtrs. Xtal oven avail. Kit only S138. R451 FM RCVR Same but for uhf. Tuned line front end, 0.3 uV sens. Kitonly S138. R76 FM RCVR for 10M. 6Mr 2M. or 220. As above, but wio AFC or heL res. Krts only S118, Aisoavart wr - pole filter, only S9&kil, R110 VHF AM RECEIVER kit for VHF aircraft or ham bands or Space Shuttle, Only $98.

TASI VHF FM EXCITER fof 10M. 6M 2M, or 220 MHz. 2 Watts continuous, up to 3W intermittent. Kit onty $68 TA451 UHF FM EXCITER 2W cont,, up to 3W intermittent. Kits only $68. Xtal oven avail. VHF & UHF LINEAR AMPLIFIERS. For either FM or or SSB. Power levels from 10 to 45 Watts to go with esc iters & *mtg converters Several models. Kits from $ 78.

Gre% Swam

Kit Corner: The DSE Radio Direction Finder

Excerpted hith permission from the February*. !986t issue ¿¿/Electronics Australia.

Most readers will be broadly familiar with the concept of a radio direction finder iRDF). A basic RDF consists of a receiver and an antenna which can be rotated on [is own axis. The direction of the transmitter is found by rotating the antenna for a signal peak or null.

You can easily demonstrate the effect for yourself using a portable transistor radio Fitted with a ferrite rod antenna. By tuning the radio to a station and rotating the radio about tts vertical axis, a null will be found in the signal strength. The ferrite rod antenna will then point in the direction of the station.

Of course, this method requires that "fixes*' be taken at two or more widely spaced locations in order to find the true location of the transmitter, The exact Location of the transmitter is determined b> simple triangulation.

The classic application of this radio direction finding technique was in World War \L Many war movies showed how it was possible to track down enemy transmitters using special vans fitted with RDF equipment. Typicall) , these vans were fitted with a large external loop antenna which could be manu ally rotated. An operator inside the van listened in on headphones for peaks and dips in the signal strength- Provided the transmitter remained in the one location for long enough. its location could eventually be pinpointed.

The Dick Smith Radio Direction Finder is just the ticket for tracking down illegal transmitters and antisocial radio operators. Depending on the antenna system, it can operate on any band from 50 to 500 MHz and will work with FM receivers ranging from pocket scanners to amateur radio and CB transceivers.

Physically, the radio direction finder consists of tuo separate units. One contain^ the control and display electronics, and the other is a special antenna-switching unit (ASU) which is connected to the control unit via a 4-conductor cable.

An electronic "compass " display consisting of 32 LEDs indicates the transmitter bearing. When a signal is received, its relative bearing to the antenna system is indicated by whichever of the 32 LEDs illuminates,

In fixed installations, this allows the compass bearing of the signal io he directly indicated to within ±5,6degrees. When an RDF unit is installed in a car, successive readings allow you to pinpoint the exact location ot the transmitter.

How It Works

The theory ot operation is reasonably simple, Radio signals received on a rapidly moving antenna undergo a frequency shift due to the Doppier effect, an effect well known to anyone who has observed a moving car with its hom blowing.

Consider a single antenna mounted on the edge of a rapidly spinning disc (Fig, 1). As the antenna moves towards the source of the rf carrier, the apparent frequency will increase due to the Doppier effect (Fig, 2». Conversely, as the antenna moves away, the frequency will decrease.

Thus, the rotating antenna causes frequency modulation of the received carrier. When this type of antenna is connected to an FM receiver, a tone is heard. By analyzing the phase of this tone, the direction of the transmitter can be determined.

To avoid the obvious drawback of a me-chamcalK rotated system, the Dick Smith RDF simulates a rotating antenna electronically. Four vertical whip antennas are arranged around a circle with a diameter of 0,07-0 4 wavelengths The antennas are electronically switched clockwise in sequence such that all four antennas are scanned once even ! i 250th of a second.

Fig. I. Signals received by an antenna mounted on the edge of a Fig. 2 This graph illustrates the frequency shift its the antenna moves rotating disc are frequency modulated due to the Doppier effect. towards and away from the transmitter.

The Dick Smith Electronics Radio Direction Finder>
Ferrite Antenna Direction Finder
Fig. 3. Block diagram of the Radio Direction Finder. Signals from the antenna-switching unit are fed to an FM receiver and the output is compared to a reference phase.

This situation is equivalent to one vertical antenna mounted on the perimeter of a disc spinning at 1250 revolutions per second. A diameter of, say, 800 mm (for the 2-meter band) results in a tangential velocity of 3140 meters per second.

II the carrier frequency is 144 MHz, the carrier will deviate 1.5 kHz at a rate of 1250 Hz For lower carrier frequencies, the deviation will be proportionally lower. Note, however, that the 1250-Hz modulating tone remains constant, as it is a function of the antenna switching rate only.

The output from the FM receiver is applied to the signal input of the RLH: adapter and compared with ¿in internal reference phase. The resultant phase angle appears as a 5-bii binary code which is decoded to a one-of-32 output to drive the appropriate LED indicator.

In addition, the detected audio tone can be monitored on an internal loudspeaker. This provides an audible indication that the receiver is correctly tuned to die transmitter frequency.

The Circuit

Antenna switching is accomplished hy first deriving a 2-bit binary code from a 1-MHz master oscillator. Here's how it s done:

Inverter stages iC2a, bt and c (4069) form the I-MHz oscillator, with buffering provided by IC2d. This clocks decade counters IC4 and 1C7. both of which divide by five to produce a 40 kHz signal on pin 1 (CKi oflCIO,

IC10 is a 4024 7-stage binary counter. Its QJ-Q5 outputs directly drive the DJ D5 inputs of IC 12, a 40174 hex latch, while Q4 and Q5 also drive IC9. uhich is a 4555 one-of-fourdecoder

What happens is that IC9 accepts a 2-bit binary code from ICI0 and provides the quadrature antenna-switching signals. These signals are interlaced by a 1488 line driver (IC6). The outputs of 1C6 swing positive and negative in sequence to provide bias for the matrix diodes (D201-D208) in the antenna-switching unit (ASU).

i he diode matrix is arranged so that* at any given instant, three of the antennas are effectively shorted and only one is coupled to the receiver. Far example, when pin 11 of IC6 is low (-9 V), D205-D207 are forward-biased and short out antennas 2 to 4, At the same time. D201 will also be forward-biased while D202-D204are turned off. Antenna 1 will thus be connected to the receiver.

The detected audio tone from the FM receiver is applied to the input of the RDF adapter, limited by D1 and D2, and filtered by a single-pole active low-pass filter stage (ICS), This chip is described by National Semiconductor as an MF5 Universal Monolithic Switched Capacitor Filter. Basicallyt it is a general-purpose active-filter building block,

The rest of :C5 is configured as a second-order bandpass :11ter to remove unwanted audio modulation from the 1250-Hz tone. The center frequency of the filter is set to

1250 Hz by the clock signal applied to pin 8. This clock signal is derived via IC3T which divides the 1-MHz master oscillator signal by eight.

Note that the clock for the bandpass fdter is derived from the same source as thai used to switch the antennas. This means that the filter is automatically centered or the scanning tone, even when there is some frequency drift.

The output of IC5 (pin 3) is a sine wave \Mth a nominal frequency of 1250 Hz. This signal is applied to op amp JClla. which functions as a phase shifter, Adjustment of the phase shifter is by means of VRI.

The job of the phase shifter is to allow calibration of the circuit and to compensate for any audio phase shifts in the receiver.

From there, the signal is further processed by a 4046 phase-locked loop (PLL). The function of this stage is to average out any modulation present in the passband of ICS and to produce a 1250- Hz square wave which is essentially free of noise and jitter.

Et is this signal that is used to latch JC12. The output of the PLL (pins 3 and 4) is firsi inverted by IC2f and applied to D-type flipflop IC13a. Subsequently, when the flip-flop's D input goes high, ICl3a latches IC12 on the first positive-going clock pulse from pin 10 of IC4.

The result of all this is that IC12 is latched with a 5-bit code that is directly related to the transmitter direction. A phase-comparator function is thus performed.

Note that IC13a is necessary to prevent the latching signal from coinciding with a change of data on IC12%s inputs.

A pair of 74LSI54 one-of-16 decoders UC101 and IC 102} on the display board converts the 5-bit code to a one-of-32 output, These decoders directly drive the 32 display LEDs to indicate that transmitter position.

Switch SW102 allows the display to be held or "frozen" by resetting ICI3a. SWiOt serves as a power on/off switch, while SW 103 allows the display to be dimmed by switching a 330-0hm resistor into the common anode circuit of the LED display.

To make the unit as easy as possible to use. the audio output from the FM receiver is also fed loan internal loudspeaker. The volume is adjusted by means of potentiometer VR102, which is mounted on the front panel.

Power Supply

Power for the RDF unit is derived from an external 12-V source which connects to a 2-conductor socket on the rear panel. This supplies -f 12 V direct to several ICs and to the input of 3-termin;i! regulator IC1. ICIT in turn, supplies a regulated +5-V rail to the remaining ICs.

Op amp iCllb provides a buffered +6-V rail to !C5 and also to the phase-calibration control (VR101).

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