Gloucester Amateur Radio and Electronics Society

Club Project - Direction Finding Receiver

Aerial Amplifier



This design is aimed at club members who have a receiver that does not have a directional amplifier. For example, some members have direct conversion receivers that they have built from kits. In our experience, these receivers are rather deaf and so our amplifier has some 22dB of gain. It is built on a small piece of Veroboard.

Like the other aerial designs in this section, it is based on a medium wave ferrite rod aerial ((LB12N from Maplins) that has been wrapped in tinfoil to provide directionability. Click here for full constructional details. The aerial is tuned with a 5.5pF to 65pF trimmer (WL72P from Maplins).

A 50 ohm coil on the aerial provides the input to an MAR6 monolithic amplifier (MMIC), which also has a 50 ohm output. DC blocking is provided by 100nF capacitors. Power is provided to the MMIC via a 100 microhenry choke and a resistor. An optional 1N4001 diode is also provided to prevent reverse polarity connections. The power rail is decoupled with a 100nF capacitor.

The MMIC requires 3.5 volts at 16mA. Therefore, the value of the resistor can be calculated using the formula (V-4.1) * 62.5 if the diode is used or (V-3.5) * 62.5 without a diode. V is the supply voltage and the result is the resistor value in ohms. Round this to the nearest off-the-shelf value.

Construction

The amplifier is constructed on a small piece of Veroboard.



The MMIC is soldered onto the track side of the board and that the pin that is cut at 45° should be connected to the track that goes towards the 65pF trimmer. It does not matter which way up the IC is, as long as the angled pin is towards the trimmer.




Wire links are fitted either side of the MMIC. Bend a piece of wire to form a 0.2” wide “U” shape and insert it from the top of the board so that it links the tracks either side of track. Now bend one leg of the link so that it connects to the adjacent track as well. Solder.

The best way to align the aerial is to use a grid (or gate) dip oscillator (GDO). Set the GDO to the required frequency by tuning it for zero beat against another receiver. Then tune the trimmer on the aerial board for a dip on the GDO's meter.

So, where to now.

On to the aerial construction details
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