136kHz LF and 472kHz MF Pre-amplifiers. (2014)
This circuit is based on a design by Dave Pick G3YXM and has been redesigned
using SMD coils after the original parts were made obsolete.
Two versions have been designed, one for 136kHz and another for 472kHz
which uses the same PCB. The
boards are supplied built and aligned for mid band. Blank PCBs (48 x 31mm) are also
available should you want to construct a version for another area of the LF / MF
spectrum. - See kits page for details.
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Most commercial amateur receivers have a relatively poor performance when tuned
directly to the 136kHz LF band.
It is common for receivers to have an attenuator permanently in circuit below
1.8MHz to prevent overload from
high powered broadcast stations. Unfortunately this and a general 'lack of
performance' below 500kHz severely reduces the ability of the
receiver to pick up weak amateur transmissions. However, a pre-amp will overcome
these losses and restore your radio's sensitivity.
The pre-amp consists of a top coupled bandpass filter using a pair of Toko
coils followed by a FET
amplifier and a bipolar transistor buffer. The overall gain is apx 11dB on the
136kHz version, and 14dB on the 472kHz version. This is
enough to overcome the losses of most commercial receivers. The bandpass filter has a
narrow response which prevents the preamp being overloaded by broadcast and other high powered
transmitters operating in the LF / MF spectrum. The capacitors used are 5%
polypropylene and ceramics.
If you have an application on another on a different frequency,
then the blank board has space for 2 capacitors in parallel for each coil
allowing any frequency 100kHz to 1.8MHz to be covered using standard values. The
coils are adjustable over +/- 6% of their nominal value.
Input / Output impedances are suitable for 50 / 75 Ohm systems.
The circuit is mounted on a double sided 48 x 31mm printed circuit board
136kHz receive
pre-amplifier.
Blank PCB also available.
Typical Performance: 136kHz. - Plot 70kHz to 200kHz
Marker 1 @ 100kHz. Marker 2 @ 198kHz.
Typical Performance: 474kHz. - Plot 370kHz to 770kHz
The preamplifiers are available ready assembled tuned and tested for £16 + £1 post
within the UK or $28.00 / 23 Euros including post to US or Europe
Power required is a nominal 12V - 13.8V at 20mA.
Output 1dB compression point is apx. +4dBm.