FM Signal Analysis FAQ
Written by TV Fool   
Tuesday, 10 April 2007

Plot sample
Plot sample

General

How are these plots generated?

 

Reading the Radar Plot

What is that circular plot?
How do I read the plot?
Can I use this to aim my antenna?

 

Reading the Table

How do I read the table?
What are co-channel and adjacent channel warnings?

 


General

How are these plots generated?

These plots are generated from the FCC's database of all licensed broadcasters.  The signal strength from each of the broadcasts is analyzed for your location using 3D propagation modeling algorithms, and the results are summarized in the plot and table.  Transmitter power, terrain obstructions, Earth curvature, and your antenna height (if specified) are already factored into the report.

 

Reading the Radar Plot

What is that circular plot?

This is called a "radar" plot because of its strong resemblance to a standard radar (radio detection and ranging) display.  Your coordinates are represented by the point at the center of the plot.  The bars show the direction, signal strength, and channel number of the local TV signals in a compact visual way.  If you enter a different location, the directions and lengths of the bars would change to show the conditions at that point.

 

How do I read the plot?

Your location is at the center of the plot.  Each bar on the plot represents a single broadcast channel.  Longer bars represent stronger signals.  The direction of each bar represents the direction of the transmitter relative to your location.  Detailed information about each transmitter is provided in the table to the right.

 

Can I use this to aim my antenna?

Yes.  If you are using an indoor antenna, this report will help identify which side of your home is facing the transmitters.  Larger antennas, especially those installed in the attic or outside, need to be aimed more accurately for best reception.  This report can tell you the precise direction of each transmitter relative to your location.

The top of the plot points due north, exactly as you'd see it on a map.  If a printout of the plot was placed flat and the top of the radar plot was made to point due north, then each of the bars would point directly at each of the transmitters they represent.  The exact direction for each transmitter is also listed under the azimuth column on the table.  Azimuth is measured starting with 0° pointing due north and increasing in a clock-wise direction until reaching 360° at north again.  In other words, north is 0°, east is 90°, south is 180°, west is 270°, and you end up back at north when you reach 360°.

Note: This plot (and maps in general) are referenced to true north, however, most people would use a compass to find their orientation.  Depending on your location, there can be a difference of several degrees between true north and magnetic (compass) north.  A red "N" is drawn on the plot to indicate the direction of magnetic north.  If you are using a compass to orient yourself, the compass needle will be pointed toward this "N" and you can see where the transmitters are located relative to that.  The exact azimuth values relative to magetic north are also provided in the table of transmitter details.  For more information about the Earth's magnetic field, see the http://www.ngdc.noaa.gov/ web site.

 

Reading the Table

How do I read the table?

The table lists several technical details about the local transmitters, ranked from strongest to the weakest for the given location.  The columns of the table are:

    Callsign

These are the call letters that the FCC uses to uniquely identify broadcasters.  Many broadcasters will have multiple entries for their call sign (for translator and booster stations).

Channel

This is the broadcast channel for the station.

Xmit(kW)

This is the amount of power (in kilowatts) that the broadcaster is transmitting in your direction.  Since many broadcasters use directional antennas to focus most of their broadcast energy into populated areas, this power level may be different than the raw power level shown by the FCC.  This value has already been adjusted for your location according to the radiation pattern registered with the FCC.

Rx(dBm)

This is the predicted receiver signal strength of each channel at your location, specified in dBm.

Path

This indicates the path travelled by the signal to get from the transmitter to your location.

LOS: Line-of-sight
1Edge: Single edge diffraction
2Edge: Double edge diffraction
Tropo: Tropospheric scatter

Dist(mi)

Distance from your location to the transmitter, specified in statute miles.

Azimuth

Azimuth direction for the transmitter (0=North), relative to true north.  The numbers have been color coded according to the transmitter direction for easier identification of channel clusters.  Transmitters coming from approximately the same direction will have similar colors, matching the colors in the outer ring of the radar plot.

Magnetic north readings are also provided for easy compass pointing.  When using a compass for orientation, the "North" end of the needle should point to the red colored "N" on the radar plot.  You can use the magnetic north azimuth values (in parentheses) to aim your antenna via compass.

LOS

This is the estimated height necessary (at the given coordinates) in order to reach the line-of-sight path of the transmitter.  This can be used to judge the severity of a terrain obstruction.  This height is specified in feet above ground level.  Note that it is possible to have excellent reception even without line-of-sight access to the transmitter.  This column is provided for informational purposes only.

The most important value to pay attention to is the "Rx(dBm)" value since that is roughly how much signal you have to work with.  Higher values represent stronger signals, but note that the values are negative because of the units being used.  There are a lot of factors that affect each situation differently.  These are only estimates and your particular situation may be significantly better or worse than indicated.  Your mileage may vary.

The "Path" and "LOS" data are mostly for informational purposes only.  Line-of-sight signals paths ARE NOT NECESSARY to have good reception.  FM stations broadcast with very high power and the signals themselves are able to diffract, or "bend", a little over mountains and hilltops.  As long as the diffraction angles between the transmitter and your antenna are very shallow (a pretty common scenario), then there should still be plenty of signal power getting past the obstructions.  These columns are provided to judge the severity of an obstruction for those rare borderline cases where a signal is just slightly out of reach.  For the majority of cases, these values can be ignored.

 

What are co-channel and adjacent channel warnings?

Co-channel interference is when two different transmitters are broadcasting on the same channel and interfere with each other.  In the U.S., thousands of broadcasters are sharing the available channels, so naturally, each channel must be shared by multiple broadcasters.  When the FCC assigns channels to broadcasters, it tries to keep same-channel broadcasters as far apart from each other as possible to minimize interference.  Even so, there are many areas that are "in between" broadcasts on the same channel.  Depending on the severity of the interference, the direction of the transmitters, and the type of antenna being used, the end-user might see both, one, or neither of the channels in places where two channels collide.  The co-channel warning indicator is placed next to channels in the table that are potentially at risk of having a co-channel interference problems.  It does not mean that you will necessarily have problems, but it's just something to watch out for.

Adjacent channel interference has more to do with your receiver.  Some tuners are built with very poor adjacent channel rejection.  That means that if you are trying to watch a weak channel with a very strong signal on an adjacent channel, the tuner can't pick out the weaker channel because the strong one is drowning it out.  This type of scenario might occur if you happen to live right next to a transmitter.  Channels from the nearby transmitter will be much stronger than channels from distant transmitters, and the difference might be enough to prevent you from receiving those channels that happen to have strong channel neighbors.  Most receivers have good adjacent channel rejection and are not very susceptible to this kind of problem, but in case you do have this problem, a warning indicator is placed next to channels at risk of having this kind of interference.

 

Last Updated ( Saturday, 28 June 2008 )
 
 
 
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