MATLAB: Longley-Rice propagation values

antennaAntenna Toolboxcontour mapsdiffusionlongley-ricelongleyricepropagation models

I am struggling to understand the results obtained with the Longley-Rice model of the Antenna toolbox.
I try to get the signal strength at a receiver from a TV signal. I keep obtaining really low values (often negative) of signal strength (in dBm) even for recepters really close to the emitter. These values are always lower than the FCC Grade B threshold (-45dBm).
Is the result obtained by sigstrength the difference in signal strength compared to the initial strength at the emitter instead of the absolute value of signal strength at the recepter location?
Thanks!
m = propagationModel('longley-rice','TimeVariabilityTolerance', 0.7)
%%%% 1 Define Transmitter Site %%%
fq = 57000000; % 5.7MHz
tx = txsite('Name','MathWorks', ...
    'Latitude',61.2189, ...
    'Longitude',-149.896, ...
    'Antenna',design(dipole,fq), ...
    'AntennaHeight',39.6, ...        % Units: meters
    'TransmitterFrequency',fq, ... % Units: Hz
    'TransmitterPower',1500);        % Units: Watts, here: 1.5kW
% 2. Define Receiver Sites
% Define receiver sensitivity. Sensitivity is the minimum signal strength in
% power that is necessary for the receiver to accurately detect the signal.
 rxSensitivity = -45; % Units: dBm  --> corresponds to FCC grade B
rx = rxsite('Name', "test", ...
    'Latitude', 61.2188, ... % take really close point as a test
    'Longitude', -149.896, ...
    'Antenna', design(dipole,tx.TransmitterFrequency), ...
    'ReceiverSensitivity', rxSensitivity); 
% get signal strength
   ss = sigstrength(rx,tx,pm)

Best Answer

The output of sigstrength does correspond to the absolute signal strength received. By default the signal strength "Type" is "power", so this corresponds to total received power in units of dBm.
I ran your code in R2020a and saw signal strength that well exceeds the -45 dBm sensitivity:
>> ss = sigstrength(rx,tx,pm)
ss =
11.7868
In addition, the coverage map shows substantial area of coverage over the -45 dBm sensitivity level.
>> coverage(tx,rx,pm,"SignalStrengths",-45:-5)
Related Question