Message-ID: <2850.200102191729@gemini>
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Mon, 19 Feb 2001 17:34:08 +0000
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Subject: LF: Trans-atlantic QSO/Active antenna preamps
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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