Hal wrote:
I assume the problem is noise. Is there any simple way to measure the noise
around 60 KHz? How about not so simple?
Extra credit for a way that others nuts can reproduce so we can compare the
noise at my location with other locations.
For any location near a city, the noise level (QRM and QRN -- mostly the
former unless there is storm activity within a few hundred km) is
shockingly high. High enough to be clearly seen and measured with a
good spectrum analyzer. So the *simplest* way (but not necessarily the
cheapest, depending on what is in your lab already) is to use a good
spec an with noise integration over the band of interest (e.g., HP 3585A
or B). You get noise density readings in volts per root Hz. Divide by
the antenna length and you have volts per root Hz per meter.
Lacking a suitable spec an, any receiver with a reasonably narrow rx B/W
and a calibrated, input-referred detector can be used. Wave analyzers
(frequency-selectable voltmeters, e.g., HP 3586) are good candidates, as
are some commercial receivers with calibrated "S" meters (e.g., Ten-Tec
RX340). It would also be pretty easy to design a simple "sniffer"-type
receiver (input op-amp, active filter, logarithmic detector feeding a
standard 1mA meter movement) that could be calibrated by design from
first principles and that everyone interested could build for, perhaps,
$25-30.
In the suburbs of a fairly large US city with aerial electric service, I
generally see noise densities measured in tens to hundreds of uV per
root Hz per meter below 100kHz. In other, similar locations I have seen
as much as hundreds of mV or more per root Hz per meter. It depends on
local factors (whether the electric service is buried or aerial, how
well the power utility maintains its equipment, how far away the nearest
industrial neighborhood is, how far between dwellings, how much noisy
technology the neighbors use, etc, etc.).
In order to compare with others, everyone needs to use the same antenna.
There are lots of possibilities, but for the sake of universality I
recommend a 1m vertical whip. Everyone can make one of those.
Note that this sort of antenna is NOT the best type to minimize received
noise and maximize received S/N ratio. For that, you generally want a
balanced, shielded loop.
Best regards,
Charles
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