136kHz L.F. Pre-amplifier. (2003)
This circuit is based on a design by Dave Pick G3YXM and was
jointly redesigned in 2003 after the original coils and polystyrene capacitors
were made obsolete.
The circuit below is intended for 136kHz but can retuned by adjusting the 2 ferrite cores to any frequency between 130kHz and 200kHz.
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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 13dB. 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 gain and selectivity can be adjusted over a small range by the value of the top coupling capacitor. Increasing its value from 82pF will increase the gain but broaden the bandwidth
Input / Output impedances are suitable for 50 / 75 Ohm systems.
The circuit is mounted on a single sided 50 x 35mm printed circuit board.
136kHz receive pre-amplifier.
C1 47n C2 C5 4n7 C3 330pF
C4 82pF C6 1n C8 10uF
C9 220nF C7 22n TR1 BF245A TR2 BC237
L1 L2 Toko part number 707VXA016YKS
136.5kHz +13dB ( -3dB bandwidth 3.5kHz )
129 and 143 -20 dB Relative to 136.5kHz
118 and 168 -40 dB
100kHz (Loran ) -52 dB
When retuned to the following lowfer band frequencies, the following performance was measured:
160kHz Input 420mV Output 1470mV Gain = 3.5 times or 10.88dB
200kHz Input 395mV Output 1000mV Gain = 2.53 times or 8.07dB
The PCB is available ready assembled tuned and tested for £10 or $18.50
It requires a nominal 12V - 13.8V supply at about 10- 15mA.
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