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40m CW Transmitter Design Guide

The document describes a low-power 40-metre CW transmitter that operates on the amateur radio band of 7.0-7.1MHz, utilizing two FETs and two bipolar transistors to achieve about 1 watt of power. It outlines the circuit's four stages: variable frequency oscillator (VFO), buffer amplifier, RF preamplifier, and RF power amplifier, emphasizing the importance of VFO stability and component selection. Additionally, it details a temperature-controlled Ni-Cd battery charger designed to prevent overcharging by switching to trickle charge mode when the battery pack warms up.

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86 views1 page

40m CW Transmitter Design Guide

The document describes a low-power 40-metre CW transmitter that operates on the amateur radio band of 7.0-7.1MHz, utilizing two FETs and two bipolar transistors to achieve about 1 watt of power. It outlines the circuit's four stages: variable frequency oscillator (VFO), buffer amplifier, RF preamplifier, and RF power amplifier, emphasizing the importance of VFO stability and component selection. Additionally, it details a temperature-controlled Ni-Cd battery charger designed to prevent overcharging by switching to trickle charge mode when the battery pack warms up.

Uploaded by

taufiq
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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40-metre CW transmitter

D. Prabakaran

T
his 7.0-7.1MHz (40-metre) amateur stem of the ferrite core used in TV balun T3 (2N2222) supplies sufficient drive to
radio band low-power transmitter core. the final power amplifier stage that is built
using two FETs and two bipolar The VFO’s stability is important for around transistor BD139 (T4). Morse key
transistors produces sufficient power of proper operation and hence you should can be connected in the emitter circuit of
about 1 watt for CW operation, to pro- use styroflex/polystyrene capacitors in the transistor T4.
vide adequate coverage if propagation oscillator stage. The entire VFO should
conditions are favourable. The circuit be housed in a mechanically strong alu- RF output is coupled to a half-wave
consists of four stages, namely, vari-
able frequency oscillator (VFO), buffer
amplifier, RF preamplifier, and RF
power amplifier.
The transmitter transmits in the
40-metre band. It is a fully solidstate
unit built using easily available compo-
nents. The variable frequency oscillator
(VFO) stage is built around BFW10 (or
BFW 11). Another BFW10 is used as
the buffer stage.
VFO circuit is a conventional
one, used by many amateur radio op-
erators in India. A 50pF variable ca-
pacitor is used for adjusting the VFO
to ham frequency band of 7.0-7.1
MHz. VFO tuning coil L1 is made by
winding eleven turns of 28 SWG enam- minium box to avoid any stray interfer-
eled copper wire on a PVC pipe former ence. It has an excellent short-/long-term dipole antenna through coaxial feeder.
having a diameter of 2 cm. The length frequency stability. Since the power output is less, there is no
of the coil should be about 1 cm. The carrier output from the VFO is need of antenna tuning network.
RF chokes (RFC1 and RFC2) have an fed to the next buffer stage built around Note. Transistor T4 should be mount-
inductance of 1 mH. One can make these transistor T2 (FET BFW 10), in order to ed with a heat-sink to avoid thermal
by winding 30 turns of 36 SWG on central avoid overloading of the VFO. Transistor runaway.

Temperature-Controlled
Ni-Cd Battery Charger
Shibashish S. Patel

O
vercharging is one of the main charged. Once fully charged, the cells starts charging the battery pack with a
causes of low cell life, which this start warming up. This charger senses preset constant current. When the pack
charger will help prevent. that warming and flips to the trickle becomes warmer than the reference
Overcharging of Ni-Cd cells causes charge mode. temperature (plus a small temperature
heating of cells, resulting in popping The temperature is sensed by a delta, set by the potentiometer), the
of their internal seals and venting of differential thermistor that compensates charger switches to the trickle charge
electrolyte. The Ni-Cd cells on a charger for changes in room temperature. On mode, so the pack can remain connected
stay cool until these are almost fully pressing the reset button, the charger to the charger without harm.

154 ELECTRONICS PROJECTS Vol. 23

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