This article describes a receiver and directional antenna combination that is suitable for direction finding on the 80m band. The receiver is a single conversion SSB receiver and the antenna is of the ferrite rod or "loop-stick" style.
The Melbourne fox-hunting group are very active in vehicle based hidden transmitter events. We are now expanding this interest into pedestrian style events. (Our partners/families would probably describe our interest as an obsession rather
than an interest however). The pedestrian style fox-hunting is known as ARDF (amateur radio direction finding) and is very popular through-out the world, particularly in Japan, Korea, China and Germany.
Many of the Melbourne fox-hunting fraternity have been competing in our monthly vehicle based fox-hunts for 15 or more years and feel like a new challenge. The sudden interest in ARDF was precipitated by an ARDF competition held in Townsville last year. Australia was the host nation for an international ARDF event that spanned 5 days and attracted competitors from many countries. A group of VK3's went to
Townsville and had a fabulous time. The VK3 participants have spread the word south about what a great activity ARDF is. We have since held several ARDF style events which were well attended with up to 14 competitors taking part.
At the conclusion of the Townsville competition the VK4's offered a loan of a set of transmitting gear to the VK3's. This generous offer was quickly accepted by Mark Diggins VK3JMD. Four excellent transmitters built by Ron Graham VK4BRG
were transported down to Melbourne and they have helped us get this exciting radio sporting activity off the ground in VK3.
ARDF is conducted under a strict set of international rules. The rules describe the type of transmissions, course length and protocol for competitors. The frequencies used for ARDF are 2m and 80m. The 2m band is very well established as a fox-hunting band in VK3 however we are not very experienced at 80m fox-hunting. The aim of this article is to stimulate and facilitate the construction of 80m sniffers and increase the number of people able to participate in 80m ARDF style events. The performance of this sniffer with the loop-stick antenna in the uni-directional mode is not unlike that of a 2-metre
sniffer with a 3-element beam. When the antenna is switched to the bi-directional mode, very accurate DF's can be taken and this makes 80m a very attractive band for direction finding.
The receiver has one stage of RF amplification at 3.5MHz. The signal is then converted up to an intermediate frequency of 8.00MHz within the MC3362. Although intended for FM applications the Motorola MC3362 IC makes a very effective SSB receiver chip. Only the two mixers, two oscillators and varactor diodes are put to use in this receiver. The IF is selected by a simple crystal filter on the first mixer output.
This is followed by a product detector which gives an audio output.
The antenna is switchable from a "figure of eight" pattern (bi-directional) to a cardioid" pattern (uni-directional). The loop-stick and vertical sense antenna combination is based on a design in the ARRL handbook (refer to "Direction Finding" in index). Minor changes have been made to allow easy reproduction with locally available parts. The loop-stick antenna is made by winding 20 turns over the centre of a 10mm
diameter, 200mm long ferrite rod. The loop-stick is housed in an electrical tee junction box which has short lengths of conduit protruding from each end. The junction box is mounted directly on to the back of the metal housing box. The sense antenna is a straight piece of brazing rod soldered into a Bananna plug. A Bannana socket is mounted on top of the metal box to provide secure but removable connection. The cardioid pattern is achieved by switching the sense antenna "in" and then adjusting RV5 and L5 for a single null. This procedure is well documented in the ARRL handbook and is not repeated in this article.
The RF amplifier associated with Q1 provides a gain of approximately 20dB. The control voltage on gate-2 of Q1 also allows this stage to function as a signal attenuator, which is important when DF'ing up close to the transmitter. The 3.5MHz
output from the RF amplifier goes to the first active mixer in the MC3362 and combines with the 4.5MHz VFO. Potentiometer RV1 tunes the 4.5MHz VFO varying the bias on a pair of varactor diodes within the 3362 IC. The specified VFO tank circuit allows a tuning range of about 100kHz and I used a ten-turn pot to allow easy tuning of sideband signals. The tuning range can be reduced by increasing the value of R9. To increase the tuning range increase the number of turns on L4 and decrease C18. The value of the 10-turn pot. is not critical, any value between 10k and 100k will be fine. A single turn pot. may be used to cover only a small segment of the 80m band provided R9 is increased accordingly.
The 8.00MHz IF component of the first mixer output is selected by the simple ladder crystal network of X1-X3. Capacitors C21 to C24 set the filter bandwidth to approximately 4kHz when typical computer crystals are used. A fairly wide IF bandwidth has been chosen to facilitate tuning while on the run. The 8.00 MHz IF signal passes directly to the second mixer in the MC3362 which works as a product detector and gives an audio output. There is no IF amplification. The RF amplification of Q1 and the gain of the active mixers in the 3362 provide more than adequate gain for the 80m band. X1 sets the BFO frequency, which in this application is tuned to the high side of the IF pass-band to allow reception of LSB signals. The stage associated with IC2 is a gain variable audio amplifier. The gain is adjustable from zero to about 20dB. The RF gain of Q1 and the audio gain of IC2 are both controlled by RV2. The audio from IC2 may pass directly to the LM386 audio PA or it can be switched to the "whoopie" VCO. D3, D4 and the following RC network produces a DC signal on pin-9 of IC3 which is proportional to received signal strength. The VCO in IC3 is set to operate in the audio range and produces a "whooping" sound as the antenna is pointed towards or away from the signal being tracked. The "whoopie" mode is now widely used in ARDF and generally allows a bearing on transmitter direction to be more easily discerned than by judging received audio loudness. An "S-meter" could be used however the operator would need to stop and read the meter, where-as an ear piece together with "whoopie" mode can be used while on the run.
* The tuning range can be reduced by increasing the value of R9. To increase the tuning range increase the number of turns on L4 and decrease C18. Receive modes of LSB and "whoopie" signal strength.
Drilled circuit boards, pre-wound TOKO coils for L2/L3 and some of the other hard to get components are available from the author. Phone on +61 3 9876 3643 or write to Ian Stirling (VK3MZ), 169 Glenvale Rd, Ringwood Nth., Victoria, 3134.
Kits for all the parts and the antenna are available through Mark Diggins (VK3JMD), 134 Howard Rd, Dingley Village, Victoria, 3172, Australia. Phone (AH) +61 3 9558 2959.
