The most simple radiomodem for CW, RTTY, SSTV, AMTOR

Recently availability of personal computers became the reason of revolution also in amateur radiocomunication. It is enough to hear amateur ranges HF or UHF that on the certain frequencies to hear not clear signals. These strange signals (similar to gurgle or pozvanivanie) it only digital communication(connection) carried out by means of a computer. Among digital ways of transfer to which concerns also CW, are available RTTY, AMTOR, SSTV, and recently also FAX and PACKET RADIO. In this clause(article) we would like to prove to readers, that work with digital ways of transfer not necessarily should mean the big financial expenses.
To carry out(spend) a radio communication with application of digital ways of transfer, the following hardware is necessary:
- Any computer such as IBM PC 286/386/486 with komportom and appropriate software,
- transiver HF or UHF (enough a usual radio telephone on a range 2ě, having jack "microphone", "naushnik/dinamik" and managements - PTT).
- The modem interfacing computer with transiverom.

Radiomodems are made for a digital radio communication in the West by many firms. Recently modems allowing use the big popularity to work only in mode PACKET RADIO. Modems TNC-2 and also cheapest radiomodem Baycom/Digicom concern to this group. With modems it is delivered sootv. The software.

Except for special (license) programs for digital transfer, there is a plenty of programs Shareware.

It seems to us, that would be to acquaint readers Elektroniki Praktycznej with the description of a design of very simple and cheap modem not bad. The most simple modem mozhna to collect from popular parts literally in half an hour and about use popular software it will be possible to look on the screen for example weather maps.


In a Fig. 1 the circuit of the most simple radiomodem which can be connected to RS232 to port of computer PC XT/AT is shown. At reception the modem receives a feed(meal) directly from jack V.24 and does not demand an additional pressure(voltage) of a feed(meal) of operational amplifier US1 (741, TL061...). The positive pressure(voltage) provides signal RTS (request to send) and negative - signal DTR (data terminal ready). On input(entrance) OU signal NCH from output(exit) NCH of the receiver or transivera moves. The minimum level of this signal should not be less than 100 mV. The target signal with OU acts on pin DSR (data set ready).

Fig. 1.

Vsvjazi with simplicity, the whole device can be made even without the printed-circuit-board by soldering necessary details to contacts of socket CANON such as HD25. If this modem will be used with program JVFAX, it is necessary to change polarity of inclusion of diodes.


In a Fig. 2 the circuit some more complex(difficult) radiomodem giving an opportunity to carry out(spend) bilaterial communication(connection) (reception / transfer) with use of set of programs shareware, including PCFAX, JVFAX, EASYFAX, SSTVFAX4, PKTMON, HamComm is shown.

Fig. 2.

At reception OU executes the same role as in the previous case. Connection of two additional diodes allows zapityvat' the modem irrespective of potentials on lines DTR and RTS.

By transfer on sootv. The input(entrance) transivera moves signal PTT. The positive pressure(voltage) translates transistor T1 from line RTS (BC107...) in the sated condition. At this time on a microphonic input(entrance) of the transmitter signal TD (transmitted data) moves, that becomes the reason of modulation bearing(carrying). Depending on the software, modulating signal can undertake also with usual PC Speaker'a. In any case the digital signal with help RC of the filter varies on sine wave, and potentiometer PT1 helps to establish on an output(exit) the appropriate amplitude of a signal depending on sensitivity of a microphonic input(entrance) of the transmitter.

All radiomodem can be mounted inside the plastic case of adapter HD25/HD9 (mum - daddy) with use of the printed-circuit-board submitted on a Fig. 3.

Fig. 3.

In a case if the modem will be used only on reception, we do not put transistor T1 and diode D5 and also elements RC used at transfer.

Correctly collected radiomodem does not require adjustment, only by means of potentiometer PT1 it is necessary to expose a target level of modulating signal.

Resistors: R1, R2 82 KOm, R3 1,5 KOm, R4, R5 4,7 KOm, PT1 22 KOm. Condensers: Ń1, C2 100nF, C3, C4 2,2 uF/16V, C5, C6 10 nF. Semiconductors: US1: ULY7741 or analogue. D1, D2, D3, D4, D5 1N4148 or analogue. T1 BC107 or another npn.

Translation of clause(article) from magazine
" Elektronika Praktyczna "
Dmitry Lyoushkin 

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