On 13 Sep 2005 14:21:59 -0700, "K7ITM" wrote:

But even so, even if you DID have a broadband antenna, you can find op
amps, and you can make amplifiers with discrete parts, that have
distortion products more than 120dB below the level of signals in
excess of a volt at the amplifier output, in the LF frequency range.
In other words, the distortion products will be less than a microvolt,
with one volt output signals. You don't need to run that preamp with
any appreciable voltage gain, so you're handling some pretty big input
signals.

With some kind of vertical antenna (possibly with some capacitance)
which is small compared to the wavelength on LF, it is going to have a
large capacitive reactance. There will also be some antenna input
capacitance from input to ground, so essentially there is a capacitive
voltage divider formed by the antenna capacitance and input
capacitance and the amplifier is trying to tap off that voltage. So
the amplifier really needs current gain, not voltage gain i.e. a high
input impedance and a manageable (50 ohm etc.) output impedance.

Paul OH3LWR