Also see:  Receiving Antenna Design   

To see world QRN (delayed): Lightning world map

To see my local QRN (nearly real time): Lightning USA Map

My local noise level quiet night from NE is -127dBm, 350Hz BW, from pair of ~800ft broadside Beverages ~375ft spaced.  

The noise that limits our ability to hear a weak signal on the lower bands is almost always an accumulation of many signal sources. Below 18 MHz,  the noise we hear on our receivers ( even at the quietest sites) comes from terrestrial sources. Receiver 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 antennas. 

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. There is often no evidence of winter night 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 sources are utility distribution lines, because of the large amount of hardware required to serve multiple users. Other noise sources are switching power supplies, arcing signs, and other unintentional man-made noise transmitters. 

Suburban

Suburban locations average about 16 dB quieter than urban locations, and are 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 has added a new dimension to suburban noise. 

There is often a small increase in nighttime winter noise at exceptionally quiet suburban locations. This increase occurs when propagated terrestrial noise equals or exceeds local noise sources.

Rural

Rural locations, especially those miles from any population center, offer the quietest environment for low-band receiving. 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.

Primary local noise sources are electric fences, switching power supplies, and utility lines. I can measure a 3 to 5dB daytime noise increase in the direction of two population centers, Barnesville (population 7500, distance 6 miles) and Forsyth (population 10,000, distance 6 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 about 5 to 15 dB at night when the band "opens". As in the case of suburban systems, directional antennas reduce noise power.

Nighttime is the "big equalizer", reducing the advantage of location as noise propagated via the ionosphere from distant sources increase with improved nighttime propagation.

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. It has NO dominant field. It can either be electric or magnetic field dominant.

Noises arriving from ground wave sources some distance from the antenna are vertically polarized. The 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.

Electric (E-field) vs. Magnetic (H field) Field Impedance

We often hear things about high E-field response being bad and low E-field response being good. 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.

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. The 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 feedline conducted signals. The overall antenna pattern also has a large effect. It is possible either an electric field probe (very small dipole or monopole) or a magnetic loop will be "quieter". Which 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 causes loop antenna to appear to work better. It is much easier to build a "magnetic loop" that is decoupled from the feedline (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. 

To hear a demo of noise and directivity, go to the DX Sound page.    

©2003 W8JI