Active antenna preamps]

[Andre' Kesteloot]

James Moritz wrote:

> Dear LF Group
>
> Congratulations to Laurie & Larry on their QSO - for those in the
> Antipodes wanting to participate in G to VK and ZL tests, perhaps
> now would be a good time to switch on the spectrogram software -
> by the time you have successfully obtained a 136kHz allocation,
> and built new transmitters and antennas, enough time might have
> elapsed to receive a CQ call :-)
>
> Thanks to PA0SE for the info on the active antenna - this preamp
> circuit is a high frequency application of the "charge amplifier"
> circuit configuration, widely used with other capacitive signal
> sources, such as condenser microphones and piezoelectric
> transducers. Resistors of up to 5Gigohms are listed by RS
> components, if you don't mind paying several pounds each. Not
> sure what an FD300 diode is, but it must have very low leakage!
>
> For those interested in using op-amps at LF frequencies, I have
> been experimenting with the Burr-Brown OPA604. Although
> primarily an audio device, this is quite well suited to tuned loop pre-
> amps and other high impedance, moderate bandwidth applications.
> It has fairly low voltage noise, 10nV/rootHz, which is less than the
> thermal noise produced by most tuned loops, and negligible current
> noise. It is reasonably fast - 20MHz GBP, 25V/us slew rate, but not
> really fast enough for wideband antennas. It works OK from a
> single 12V supply rail. It is fairly cheap at a bit over 1GBP and
> seems to resist destruction quite effectively.
>
> Cheers, Jim Moritz
> 73 de M0BMU