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Active Antenna

The club active antenna kit was proposed and parts obtained by Kevin VK4WA. Kevin VK4ZR provided documentation and tested prototypes. It is described in the items below.


Frequency range: 10 kHz – 30 MHz
Power: 12 – 15 volts at 50 mA.
Second order output intercept point: > + 70 dBm.
Third order output intercept point: > + 30 dBm.
Maximum output power: in excess of – 15 dBm

Dimensions:Length: 100 mm, diameter: 40 mm

Power: Cinch, centre pin is V+

How it works

The proposed modification replaces the dynamic insert with a matching size homebrew module containing: an electret microphone insert, buffer amplifier An optional stabilised power supply taking advantage of the main 13.8 supply rail being available on the microphone socket.
The 12-14V supply is stabilized by an integrated 5V regulator circuit, with the two 0.1uF capacitors ensuring stability and removing any stray RF that might find its way into the circuit.
The regulator requires only 8V to operate correctly, and the overall supply current draw is only several mA which together lends itself nicely to add a light emitting diode in series for decoration  (no homebrew project is complete without at least one glowing LED) and as protection against accidental supply polarity reversal.
The electret insert is biased by a 10k resistor. Most electret inserts allow a constant bias current of 0.2mA, which results with the steady state voltage of around 3V on the microphone insert with the resistance and voltage used.
This is just sufficiently close to half the supply voltage, allowing us to use this potential to bias the buffer amplifier - no need for coupling capacitor and additional bias resistors thus reducing the parts count dramatically.
The AC signal voltage from electret microphone proved itself sufficient for Tait radios, so no additional amplification was required allowing the op - amp to work in a non inverting, unity gain follower configuration reducing the component count even further. Just one extra capacitor is required (at least 1uF) to AC couple the amplifier to the radio microphone input.

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date Updated 19 December, 2015

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