500kHz to 4 MHz Converter

Posted on Aug 17, 2012

This converter allows reception of signals below 500 kHz on a 3.5 ? 4 MHz HF receiver. It should therefore be useful for those with receivers that do not receive the lower frequencies. Again the converter uses the popular NE602 mixer/oscillator chip. A cheap 4 MHz crystal provides the local oscillator tuning. Note that tuning is backwards ? 100 kHz corresponds with 3.9 MHz and 500 kHz with 3.5 MHz. If your receiver tunes above 4 MHz, the 4 ? 4.5 MHz range will provide forwards tuning, making it easier to ascertain the frequency of an unknown station. The mixer is preceded by a low pass filter to attenuate strong local broadcast stations above 500 kHz.

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An unknown number of turns on an unknown ferrite toroid was used for this filter. As a starting point, I would suggest an inductance of between 10 and 50 uH. Build the converter first and then experiment with the filter if it is found necessary. Those very near broadcast stations may use two or three sections instead of the single section used here. The final tuned circuit resonates near 3.7 MHz. Again an unknown number of turns on an unknown toroid was used. However a toroid such as the 9mm and 13mm purple Philips types should work OK in this part of the circuit. The purist may want this circuit to be resonant at the receiver's exact frequency ? this can be arranged by making the 100pF capacitor variable. In Australia the main signals below 500 kHz are LF Aircraft beacons. Details of these are provided in the Australian Amateur Callbook. If VK amateurs succeed in obtaining a LF allocation, the converter could be a starting point towards a full transverter for these frequencies.

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