W8WWV - Lower Bands Background Noise Monitoring

Greg Ordy


Part of the charm and challenge of operating on the lower bands (160, 80, and 40 meters) is the unpredictable nature of DX (long distance) propagation. In recent years, many amateurs and professionals have been working on models and theories of how these bands operate (in a propagation sense).  The goal of this work, in general, is to be able to predict DX openings, so that operators can get on the air when conditions are ripe for DX contacts. Even with all of this work and progress, we are still currently at a point where all we can do is monitor and gather data, trying to pick out information that will be helpful in forming a useful model.

On this and related page, I would like to make a very small contribution to that effort.

In January of 2003, I added a data logging function to my S Meter Lite program. This program reads the radio receiver's S Meter through the computer, and in this application, saves the readings in a data file. The data file format is the popular comma separated value format, which can be imported by programs such as spreadsheets, where further data analysis and graphing can take place. In my case, I will use the popular Excel program, available from Microsoft.

I'm going to monitor the background noise on various frequencies, over a period of several days, scattered throughout the year. Who knows what this data  might reveal, but I have the ability to make the measurements, so I might as well see what happens. As they say, nothing ventured, nothing gained.


If one is at all familiar with these bands, and the background noise at their frequencies, it's easy to predict the general shape of the background noise curve. During the daylight hours, the D layer of the ionosphere is ionized by radiation from the sun, and effectively becomes an absorber of energy at these frequencies.  Background noise drops to a relatively low level. While low background noise is a good thing, that same ionized D layer prevents the long distance signals that we are interested in from being able to bounce off of higher layers, and travel around the world. Both signals and noise are absorbed. When the sun sets, the D layer looses its absorbing properties, and the background noise level rises, usually to high levels, especially in the local summer months. At the same time, desired radio signals can bounce longer distances, but they are now often masked by the noise.

So, during the day, the desired signals (and noise) are absorbed by the D layer, and during the night the signals can be present, but buried in noise. We expect a plot of background noise to show relatively low levels during the day, and relatively high levels at night.

The interesting parts of this daily cycle are the transitions, present at sunrise and sunset. For a number of reasons, not just related to background noise, enhanced long distance radio communication (at these frequencies) is possible at these times. Propagation at these twilight times is usually called Gray Line Propagation, since the area of enhanced propagation is located between the illuminated portion of the planet, and the dark portion of the planet. This thin band of twilight (the terminator) is actually a circle formed by the projection of the sun's rays onto the surface of the spherical Earth. As the Earth spins each day, and tilts across the seasons, the location of the circle wanders across the surface of the planet. When the Earth is displayed in a flat projection (such as Mercator), the circle shape is transformed into a sine-like wave shape. Many sites on the Internet display the current terminator. An example can be found at: qsl.net.

If daylight effectively prohibits long distance communication on the lower bands, then contacts can only take place when both radio stations are in darkness or twilight. Gray Line propagation enhancement can be very pronounced when both ends of the contact are in twilight. This mutual darkness/twilight requirement limits the times of the year when communication is possible between two selected points. For certain point pairs, there may only be a few days of the year when communication is possible, and then for only a few minutes each day. Consider the path between my location in the state of Ohio, and the island of Japan. There is a long path opening which occurs for a few weeks around the beginning of the year when my sunset occurs at nearly the same time as their sunrise. This puts us both in twilight. Except for these very small periods, at least one side of the contact will be in far too much daylight to permit communication (for that path). Signals will propagate from my location in the south-east direction, as opposed to the normal north-west direction. While this path is the long way around the world, the existence of mutual twilight makes communication possible.

I have worked this path a few times over the last several  years, and it's always a rush that leaves me exhilarated at the end of the contact. I'm sitting at the radio as the sun is setting on a cold winter day, which is approximately 5:00 PM local time. For a few brief minutes, signals from Japan may rise from the noise, as they experience sunrise. On my directional receiving antennas, the signal is coming from the south-east, the long path to Japan, but the path along the terminator which has been in darkness. It's the unpredictable and infrequent opportunities for this sort of contact which makes the chase and conquest worthwhile and exciting. As you move across the world, each region will require its own timing and strategy.

I looked in my QSL card folder, and found an example of this type of contact. Here is the QSL card I received from Fumiho, JR3GIY.

80 Meter Long Path QSO with JR3GIY, January 13, 1999

I used the Geoclock program to recreate the day/night view of the world at the time of the contact. Here is that image. [what appears to be smudges on the image are artifacts of conversion to the jpeg format to obtain higher file compression]

Day/Night State during QSO with JR3GIY
Day/Night Conditions at the time of the JR3GIY QSO

As the map shows, my location (Chesterland, OH, USA) is about to enter night, and JR3GIY is about to enter day.  Much of Japan is already in daylight, but Hyogo is in the south-west portion of the country, which is the last to see daylight. The usual path to Japan opens at our sunrise, and is to the north-west direction. This is the short-path direction of the great circle between Ohio and Japan. While that short-path may certainly be open at this time of year, only one side of the contact will be in twilight (mine). Japan will be several hours after sunset when I experience sunrise. The contact described here has both stations near twilight, which often times implies additional enhancement due to Gray Line propagation.

Even with the Gray Line propagation, and mutual twilight, my signal report was a less than strong 4-4. Fumiho was running high power and a Yagi (on 80 meters!), and I was running high power into a vertical with a good ground radial system, and using directional receiving antennas.


Please note that my description of Gray Line propagation is extremely simplified. Day and night do not create a switch which either turns off or on communication. The effect can be subtle, and there are exceptions to the rules, and many variations on the propagation mode. There are also variations from band to band. In other words, 160 meter propagation is not the same as 40 meter propagation. Consult the references for more detailed information.


The background noise level will be an important part of these contacts, since it can easily cover up a desired signal (which is weak at best). It's part of the communication equation, and therefore worth examining. The background level is often in transition during twilight, and the ability to complete a contact is a function of the strength of the desired signal to the noise.


If you spot anything interesting in this data, let me know, and I'll add your information to the page. If there is another type of monitoring that you believe would be informative, please let me know that as well.

I will not be placing the raw data files on the Internet, but I will be saving them. If you are interested in the raw data, or the spreadsheets,  please email me with a request, and I will be happy to send you the files.

References and Links

Here are some references and links that provide background and detailed information on propagation in general, and propagation on the lower bands.

Reference Comment
ON4UN's Low-Band DXing Chapter 1 (third edition) covers propagation in detail. This book is available from the ARRL and other amateur bookstores.
ARRL Propagation Page General information, and a large number of links to reprints and web sites. (some are members only)
The 160-Meter Band: An Enigma Shrouded in Mystery By Cary Oler, and Dr. Theodore J. Cohen, N4XX.  A detailed look at 160 meter propagation.
Topband Propagation Web site with a number of tools designed to aid in evaluating propagation in real time.
hfradio.org Propagation General propagation data and information.
Long-Path and Skewed Path Propagation on the Lower Shortwave Frequencies By Bill Tippett, W4ZV. A look at two of the interesting paths, long path, and skewed path.
Skewed Paths to Europe on the Low Bands

By Carl Luetzelschwab, K9LA. (this article appeared in the August 1999 issue of CQ)

Many more resources exist on the Internet. Search for terms such as gray line propagation. Look for material created by K9LA, or NM7M. Both of these amateurs have authored volumes of propagation information.


We are just about ready for the measurements. In general, I set up my radio to some unoccupied frequency near a given amateur band, and began recording the S Meter. I tried to record data for several days. Unless otherwise modified by a given measurement page, the conditions are as follows:

  1. The radio used is an ICOM 756PRO. This is my main radio, and it supports remote reading of the S Meter.

  2. Preamp set to maximum (#2). The idea here is to discourage the meter from bottoming out at S0 when the band is quiet during the day. In normal operation, the use of preamps on the lower bands is seldom necessary.

  3. The receiver is set to the CW mode, with a 350 Hz filter width.

  4. The receiver AGC is set to the factory mid position, which is 0.5 seconds.

  5. Since my goal was to measure background noise, I need to pick frequencies which do not have signals. This is impossible to guarantee within the amateur bands, so I normally select a frequency right below the bottom of the amateur band. From monitoring I believe that these frequencies are not used.

  6. Data is sampled at an interval of 1 minute. Since S Meter Lite samples the S Meter at a rate of 20 times per second, each sample in the file consists of the average of (60 X 20) or 1200 S Meter readings.

S Meter Reality Check

A quick note on the signal strength values shown in the graphs. An S Meter is really the concatenation of two different scales. S Units range from S0 to S9, then above that, meters usually switch to pure dB. Not only are S Meters not very accurate, but these scales inherently have no relationship to each other. When S Meter Lite is data logging, the two scales must be merged into a single numeric scale. The program supports several different mapping models. In this case, I will be using the synthetic dB scale (see the documentation for more details). I know that my ICOM 756PRO S Meter is far from the ideal meter, especially in the S0 to S9 range. Like most S Meters, the number of true dB between S Units becomes only 1 or 2 as you get near S0. In order for the graphs to have more accurate signal strength values, I have adjusted the signal strength values in the spreadsheet.

On my graphs, a strength of 0 dB is the same as S9 on my meter. The positive values are fairly accurate. Negative values imply that we are moving down through S8 down to S0. It is negative part of the range that I will adjust. The graphs will appear to swing 20 dB below 0.  So, when looking at the data graphs, be careful when looking at the negative dB values. They are not as accurate as the positive values.

In practice, this is a minor issue, since the purpose of the graphs is not to record absolute signal strength values, but to show how the noise drops and peaks between day and night.

Links to Data

Here are the links to each measurement run or session.


Back to my Experimentation Page

Back to my Amateur Radio Page

Last update: Tuesday, September 23, 2003 11:58:46 AM
Back to my Home Page