The simple station lists you can get off the internet which we have explored in the Mediumwave Station Reference Lists Series will only tell you the first two pieces of information. The third - the directivity of their signals - is still a mystery and the largest piece of the puzzle. What is currently available in print or online?
Bound and printed, the National Radio Club produces the AM Antenna Pattern Book containing reproductions of station pattern plots plotted on a map of the US. They are proportionally correct but only relative and do not reflect the actual coverage areas (sometimes called "contour plots") of stations. The plots are generated by a computer program written by an NRC member. Unfortunately, the current volume is out of print - and the last publication was in 2006 using late 2005 data. Much has changed in the US mediumwave arena since then. Whether a new volume will be produced is anyone's guess.
The FCC's official web site provides us a pattern plot in .PDF form for each mediumwave station using a multi-towered, and thus directional array. Example: daytime WYSL-1040, Avon, NY. A plot for each service is made available, where applicable: Unlimited, Daytime, Nighttime, and Critical Hours (the first two hours of daylight after sunrise and the last two hours of daylight before sunset). The graphic presented shows the array pattern and includes a table listing millivolt per meter levels at 1 kilometer distance for various azimuths from the array.
The FCC chart is not easy to find. You must wade through the site to find the station's facility record, locate the service, and then locate the link for the pattern plot. Once there, in order to make this information useful we have to calculate our return bearing to the station to see where we fall on the pattern. Not an easy task unless you know spherical trigonometry. Even at that, this is only one station out of perhaps many on this frequency, with no correlation to a physical map.
The radio-locator.com site can present a pattern plot on a small localized map giving you a rough idea of coverage for one station. Example: nighttime WYSL-1040. Rough contour lines for Local, Distant, and Fringe levels are shown. Again, this is only one station out of the usually numerous others on the same frequency. Ideal would be a map of the US with the stations and their antenna patterns plotted out so we could see the interaction between them and where our receiving location falls in relation to all.
So, we find no current sources for comprehensive antenna pattern information in map form.
Radio Data MW, a program I have been working on for the last three years, now accomplishes this mapping process. Allow me to introduce another set of new files of interest to mediumwave DXers: Antenna Pattern Maps, produced by Radio Data MW.
Using the actual FCC database files Radio Data MW will auto-generate an interactive HTML pattern map, showing the pattern plots for all stations included at the discretion of the user. A complete set of mediumwave pattern maps can be generated in a matter of minutes. Radio Data MW generates a real pattern plot based on ground conductivity, ground dielectric constant, and can display actual (but approximate of course) signal level boundaries for Local, Distant, Fringe, Extreme mV/m levels, or any custom mV/m level chosen by the user.
The online Google Maps API is used to generate and plot each station on a map of the US. An accurate flag pin is placed at each transmitter location, and in satellite view may be zoomed in to see the actual transmitter site. Map flags are color-coded to indicate Unlimited, Daytime, Nighttime, and Critical Hours services. Each flag has a tooltip-type note, and when hovered over with the mouse will display a note on the station.
A pattern plot for each station is generated and displayed. Each pattern is calculated using standard formulas used by the FCC to compute the base values at one kilometer, and field strength formulas at distance based on the works of many people over the years. See Field Strength Calculations: A History and Field Strength Calculator One, previously posted on RADIO-TIMETRAVELLER.
Finally, an accurate ray path can be drawn from all transmitters to a user-specified receiving location by inputting latitude-longitude coordinates. Super-imposed on the pattern plots, the ray paths show the listener where he or she falls on each station's pattern, a handy guide to knowing where you stand.
Note that these maps are web-based. As stated, they use Google Maps and thus require access to Google. In order to view them you need a connection to the internet. They have been tested in the Internet Explorer, Firefox, Chrome, Opera, and Safari browsers. If using Internet Explorer, best results are had with the latest version, IE 10.
I will post the full pattern map set this weekend.
|Example: 1200 KHz zoomed in on the midwest area showing day and night services.|