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AMATEUR RADIO BY GARRY CRATT, VK2YBX Build a 6-metre handheld transceiver Amateurs looking to experience the 6-metre FM band might care to consider this project. Just buy an inexpensive walkie-talkie & fit it with crystals for the 6-metre amateur band. As many amateurs are no doubt aware, there exists in the electronics marketplace a device called a “headset communicator”. This item, usually sold in pairs, has featured in US mail order catalogs for at least 10 years and has been sold in Australia by at least half a dozen outlets. The original concept was a VOX operated headset, which controlled a low-power FM transceiver, operating on 49MHz. The antenna was formed by a piece of thin spring steel wire, which protruded from the headset, giving the operator a space age appearance. Of course, the frequency of 49MHz played havoc with Channel 0 television reception and several years ago the then Department of Communications produced an appropriate technical specifica­tion, removing the devices to 55MHz and reducing the operating power to a non-interfering level. The appropriate specification is now called ECR-60 and allows a transmitter output of 2mW EIRP, which equates to a field strength of 99dB µV/m measured at three metres. Given these limitations, such a device can not really be considered as a serious piece of communications equip­ ment and, as expected, has a quite low retail price – around $70. However, given the characteristics of the 6m FM band during the hot summer months, when temperature inversions can cause excellent signal propagation, such a device could form the basis of a simple FM handheld transceiver. Fortunately, Dick Smith Electronics carry a version of this style of FM transceiver which looks like any other handheld transceiver rather than the ridiculous looking headset version. R29 1k C34 .02 JUNCTION OF T1 AND R2 R27 15k C65 R26 6.8k +4.2V C36 0.1 T3 R50 4.7k BASE, Q2 Q3 9018G TO R51 R48 X3 68k C35 R28 680  +8V C57 0.2 Q12 9018G C56 C55 D10 Fig.1: this is the first local oscillator in the unit. Crystal X2 should be changed to 41.825MHz. 56  Silicon Chip R49 100  L1 C8 X2 Branded Digitor, the unit sells for just $69.95 (Cat. No. D 1095). Best of all, the unit is fitted with crystals for 55.150MHz, which is not too far from our target frequency of 52.525MHz, the national FM calling frequency. Our inspection of the workings of one of these units re­ vealed a dual conversion receiver using an MC3361 IF strip, an 8-pin DIL audio amplifier, a dual op amp for TX PTT and VOX func­tions, and some TX/RX switching diodes feeding what we assume is a loaded 55MHz helical antenna. The transmitter uses a 44MHz fundamental crystal, while the receiver uses an 18MHz third overtone crystal. The receiver first IF is 10.7MHz and the second IF is 455kHz, the expected conven­tional arrangement. We decided to put the unit on 52.525MHz, the national FM calling frequency, due to the difficulty in finding an accurate list of 6m FM repeaters. Whilst many repeaters are licensed to operate, few seem to exist. However, the unit would seem perfect­ly suitable for repeater operation. The mathematical calculation for 52.525MHz followed logi­cally: divide BASE, Q13 C54 L9 1.5uH Fig.2: this is the transmitter oscillator. Crystal X3 should be changed to 17.508MHz. X3 L1 T3 VR4 X2 L5 T1 T2 X1 VR1 Fig.3: this photo of the Digitor 55MHz transceiver shows the positions of the principal components, some of which may need to be adjusted for best performance after the crystals have been changed. 52.525 by three to obtain the Tx crystal frequency; ie, 17.508MHz. To obtain the Rx crystal frequency, subtract the first IF (10.7MHz) from the carrier frequency (52.525MHz) – in this case 41.825MHz. A call to Darren McCloud at HY-Q Crystals revealed that they had on file the exact specifications for both crystals, having been approached by various importers over the years for sample units. For the record, the transmitter specification is GG05S and the receiver specification is GG05Q. Both crystals are housed in the standard miniature wire in QC-49 holder. The next step was to replace the existing crystals with the HY-Q replacements and retune the transceiver. The photo of Fig.3 shows the PC board of the transceiver. Marked on the photo are crystal X2 (the receiver crystal) and X3 (the transmitter crystal). The remaining crystal, X1 has a frequency of 10.245MHz and is the second IF mix-down crystal. Also marked on the photo are VR1 (the preset mute trimpot), inductor T3 (the local oscillator coil) and other components which need to be adjusted during the align­ment procedure. Minor re-alignment is required to obtain optimum perfor­mance on the new operating frequency. Receiver alignment is best done with the receiver unmuted. Potentiometer VR1 adjusts the preset mute level and can be rotated anticlockwise (looking from the edge of the PC board) to unmute the receiver. A signal gen­erator should be used to re-align the receiver front end. The best connection point is where the existing helical antenna is connected to the PC board. It may be necessary to run a level of 10µV or so for initial alignment, backing the signal generator off as the receiver sensitivity improves with tuning. Inductor T3 adjusts the receiver local oscillator. The frequency can be measured at the base of transistor Q2, or the audio output can be monitored whilst adjusting T3. Receiver sensitivity can be improved by adjusting T1 and T2. As the transmitter only contains three devices, alignment is simple. Inductor L1 adjusts the output frequency and L5 adjusts antenna matching, best checked by monitoring field strength during adjustment. There is also a mysterious phase cancelling coil, having a few turns wrapped around the loaded helical anten­ n a. Dick Smith Electronics advises that the purpose of this is to reduce second and third harmonics when operating the unit with a headset. The headset wiring apparently changes the loading of the antenna, and subsequently the radiated harmonic output. A possible modification we considered was replacing the “mic sens” pot on the top panel of the unit with a fixed resistor, and using the redundant potentiometer to control the receiver mute. This would save running the receiver permanently either muted or unmuted, the only two options with the existing trimpot arrange­ment. The transmitter audio has two adjustments, VR2 adjusts the deviation, whilst front panel control VR3, a 10kΩ potentiometer, adjusts the microphone sensitivity. This adjustment is really designed to have the effect of changing the VOX sensitivity. We also considered replacing the existing helical antenna with a coaxial “tail” terminated with a BNC socket. This would allow connection of an external antenna. No doubt there are other modifications which can be made to the unit to further improve performance. In any case, relocating what might otherwise have been dismissed as a “toy” transceiver to the 6m band will prove to be a worthwhile exercise for many amateurs. Finally, HY-Q Crystals has advised that they can supply crystals at around $30 each. They can be contacted by SC phone on (03) 562 8222. February 1994  57