Also see:
Receiving Antenna Design
Power Line noise
To see world QRN (from
past NASA data):
Lightning
world map
To see USA
Lightning:
Lightning
To see my local QRN (nearly real time):
Lightning
USA Map
Noise
Noise primarily
limits our ability
to hear weak signals on
lower bands. The
noise on lower bands is
often an accumulation of many signal sources
that combine to
establish our noise
floor. Below 18 MHz, noise we hear on our receivers (even at the quietest sites) comes from
terrestrial sources.
This noise is generally a mixture of local groundwave
and ionosphere propagated noise sources, although some of us suffer with
dominant noise sources located very close to our antenna
systems.
Sometimes we
don’t do ourselves
any favors. We might
not pay attention to
common mode currents
on cables, or we
might not locate our
antennas in the best
locations
possible…away from
local noise sources.
Some of us are
fortunate enough to
live in quiet
locations, where the
dominant noise
propagates in from a
distance. My local noise level
on quiet night from
a northeast
direction is -127dBm, 350Hz BW.
This is from a pair of
~800ft long broadside Beverages
spaced around 375ft
between antennas.
This is pretty
quiet, actually
allowing me to hear
signals transmitted
in the microwatt
range at 1000 miles
on 160 meters!
Locations
Our locations fall into three basic “radio” categories that may or
may not be related to our actual communities:
Note: noise levels quoted in this text are the average of three
independent studies by Bell Labs, FCC Land Mobile Advisory committee, and the
Institute for Telecommunication Sciences. Rural data are actual measurements of
summer noontime and winter midnight noise at my location, several miles from
high voltage transmission lines and far from any industrial or suburban
populations.
Urban
In urban-type noise situations, noise arrives from multiple random sources
through direct and groundwave propagation from local sources. One or more sources can actually be
the induction-field zone of our antennas (in most cases the induction field dominates
at distances less than 1/2l).
Urban locations are the least desirable locations because typical noise floors
average 16dB higher than suburban locations.
High noise levels
are present both day
and night, under all
different
propagation
conditions. There is often no evidence of
a winter nighttime noise increase on 160 meters, since ionosphere-propagated noises
are swamped out by the combined noise power of multiple local noise sources.
Much of the noise
comes from
electrical
distribution lines,
because of the large
amount of hardware
required to serve
multiple users.
Other noise sources
are switching power
supplies, arcing
switch contacts or
loose electrical
connections, and other unintentional man-made noise
transmitters.
Suburban
Reports and
studies indicate suburban locations average about 16 dB quieter than urban locations, and are
typically about 20 dB noisier than rural locations. Noise generally is directional,
arriving mostly from areas of densest population or the most noise-offensive
power lines. Utility high-voltage transmission lines are often problematic at
distances greater than a mile, and occasionally distribution lines can be
problems. The recent influx of computers and switching power supplies have added
a new dimension to suburban noise.
There is often a small increase in nighttime winter noise,
when compared to
daytime noise
levels, at quieter suburban locations. This increase occurs when
the accumulation of
many ionospheric propagated terrestrial noise
sources equals or exceeds
the sum of multiple local
direct or ground
wave propagated noise sources.
Rural
Rural locations, especially those miles from any population center, offer the
quietest environment for low-band receiving.
Studies have show daytime 160 meter noise levels are
typically around 35-50 dB quieter than urban, more than 20 dB quieter than
suburban locations. Nighttime brings a dramatic increase in low-band noise, as
noise propagates in via the ionosphere from
multiple distant
sources.
Primarily, rural
electrical local noise
comes from electric fences, switching power supplies,
and utility lines.
During daytime on
160 meters, I can measure a 3 to 5dB noise
level increase in the
direction of two population centers;
Barnesville
(population 7500 and distance 6
miles) and Forsyth (population 10,000
and distance 7 miles) Georgia.
Typical daytime noise levels, measured on a 200-foot omni-directional
vertical, are around -130 dBm with a 350 Hz bandwidth (noise power is directly
proportional to receiver bandwidth).
On QRN-free winter
nights, noise power increases
from daytime levels
by about 5 to 15 dB when the band “opens”. As in the case of suburban systems,
directional antennas reduce noise power.
This noise power
reduction comes
because directional
antennas focus or
collect noise from a
smaller area of
propagated noise.
Nighttime is an “equalizer”
between suburban and
rural locations,
with the skywave
noise reducing the advantage of
quieter locations.
This is because noise
propagated via the
ionosphere from distant
sources increases
that largest amount
in naturally quiet
locations under improved
nighttime propagation.
Noise Polarization
Noise is generated by randomly polarized sources. Noise polarization is filtered
by the method of propagation.
Noise arriving from the ionosphere is randomly
polarized. It arrives at whatever polarization the ionosphere happens to favor
at the moment. It has the same ratio of electric to magnetic fields
(also called field
impedance) as a
“good” signal.
Sources within a few wavelengths of the
antenna combine and
produce a randomly polarized
noise. Local noise
generally has
no
particularly dominant field.
Very local noise, in
the nearfield of the
antenna, can either be
electric or magnetic field dominant.
Noises arriving from groundwave sources some distance from the
antenna are vertically polarized. This
relatively
fixed polarization
occurs because the earth “filters
out” horizontal components. Horizontal electric field components are
“short circuited” by the conductive earth as they propagate and are
eliminated, and
since removing the
electric field
attenuates the
magnetic field (they
are inseparable in
radiation) any
horizontally
polarized components
from distant
groundwave sources
are quickly
attenuated.
Electric
(E-field) vs.
Magnetic (H field)
Field Impedance
We often hear
things about high
E-field (electric
field) response
being bad and a low
E-field response
being good for
rejecting noise. Another
thing we might hear
is
that
loop antennas are
“magnetic”,
and the magnetic
field is good for
desired signals while
rejecting undesired noise.
Along the same lines,
we sometimes hear a
“shielded loop”
rejects noise while
good signals pass
right through the
shield walls. In fact none of
these explanations are
technically accurate.
Here is something
that might surprise
people, but is
absolutely true. At distances more
than 1/10th
wavelength, a magnetic
loop actually
responds better to
electric fields than
it does to magnetic
fields! As distances
increase to 1/2
wavelength and
beyond, the electric
and magnetic fields
even-out. At enough
distance, field
impedance becomes
fixed at the
impedance value (or
field ratio) of
freespace regardless
what the source or
receiving antenna
actually is.
The
graph below shows
the field ratio or
field impedance of a
small “magnetic”
loop and a very
small dipole:
The loop field impedance
shown in this graph is
unchanged by a
shield.
The
difference in noise
response between a
magnetic loop and a
small voltage probe
is actually caused by
the amount of common
mode rejection of
unwanted feed line conducted
signals. The overall
antenna pattern also
has a large effect.
At any given
location, it
is
possible either
an electric field
probe (very small
dipole or monopole) or
a
magnetic loop will
be “quieter”.
Which system works best
depends on local
near-field noise
field impedance and
how the antenna is
constructed. There
isn’t anything that
causes one field
to always be the dominant
field of noise
sources.
There is
something that
makes loop antenna
generally appear to work
better. It is much easier to
build a “magnetic
loop“
that is decoupled
from the feed line
(which connects to
noise sources) than
it is to build a
voltage probe that
is properly
decoupled.
Field impedance
noise rejection is
probably one of the
deepest rooted
falsehoods in
amateur and SWL
receiving.
Follow these rules for receiving antennas:
- With small magnetic loop antennas, make sure the antenna is properly
balanced
- Lay feed lines directly on earth or bury feed lines in the soil so earth losses
reduce shield current and limit feed line common mode impedance
- Be sure shield connections are properly made and snug
- If noise levels are high or antenna sensitivity is very low, isolate
the feed line from the antenna as it approaches the antenna by using choke
baluns
- Ground the feed line a few dozen feet away from the antenna
- Avoid autotransformers. Instead use isolated primary and secondary
isolated winding transformers
- Use an independent ground on the antenna. Never connect an antenna
signal ground to the
coaxial cable shield!
To hear a demo of noise and directivity, go to the
DX
Sound page.
©2003 W8JI
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