A VLF Active Antenna for SoundBlaster Cards

This is a simple high impedance converter that plugs into the microphone input of a SoundBlaster card. For the circuit to be useful the sound card microphone input must be compatible with electret microphones. Such cards provide not only ground and input terminals but also a supply terminal with a voltage source of a few volts in series with a resistor of a few kiloohms. Most SoundBlaster cards use 5V in series with a resistor of 2.2k. My card is a SoundBlaster PCI 128.

The soundcard connection resembles the "super simple" version for connecting an electret microphone found here . The field effect transistor used is a BF245A. It is an n-channel type with a low pinch-off voltage. This is important in this application because of the low bias voltage (5V). 
     ^
     |    1m Whip Antenna or Wire
     |
     |
     |                      +-----------------------------+
     |                      |                             |
     |                      +-------------------------+   |
     |                      |                         |   |
     |                      |             +--------+ +-+ +-\/\
     |                      |                      | | | |    >
     |      10M        G |--+ D           +--------+ +-+ +-/\/
     +-----#####---+---->|                    |
                   |     |--+ S               |    3.5mm Jack
                   #        |                 |    to SB mic input
                   # 10M    | BF245A          |
                   #        |                 |
                   |        |                 |
                   |        |                 |
                   +--------+-----------------+

A short whip antenna at low frequencies resembles a voltage source in series with a small capacitance (approx. 10pF/m). This capacitance, together with the input capacitance of the FET (approx. 10pF) and the two 10M resistors form a bandpass filter with a center frequency of 1.6kHz. The FET then just converts the extemely high impedance to the relatively low impedance required by the SB card.

I use this antenna for monitoring vlf stations below 24KHz (48kHz sample rate) as well as for listening to whistlers.

Gerhard Kircher