Circuit : Andy Collinson
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Description:
A Tropical Band Receiver (up to 3000KHz) based on the MK484 (formerly ZN414). Tropical band stations transmit on less power than regular shortwave, so you may only hear a station if you live in that Country. Tropical band stations are common in South America and stations near the Equator.

Notes:
Error. In my original draft of this circuit I incorrectly pointed out that the ZN414 and its derivitives may work higher than 3MHz. This is not correct and the receiver will not work at frequencies higher than 3MHz.

The data sheet for the MK484 states a maximum working frequency of 3000kHz (3Mhz). This will receive tropical band stations although both sensitivity and selectivity will fall at these higher frequencies.

However if you live in Australia Northern Territories then there is a chance you may hear National Radio Australia on one of the frequencies below:-

These tropical band transmissions do reach Europe, although not on this receiver, you would need a very good aerial and a commercial short wave radio to receive in Europe as the transmitters only use 50kW.

Coil Details:
To make a tropical band receiver try a variable 100pF capacitor and a fixed 60uH inductor. Try about 24 turns of 22swg insulated wire around a 2.5 inch diameter cardboard tube spaced evenly over 1 inch length of the coil.

My Tuned Circuit page has some useful calculators for both the tuned circuit and to calculate inductance values.

Some experimentation will be necessary and you may find that you also need an external aerial to receive broadcasts. coil or via a series capacitor value between 10pF and 100pF.

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