[BC] FM ERP vs TPO

[Richard Fry]

>While 50 kW FM ERP can be derived with any number of
>TPO/antenna gain combinations there is more to consider
> than the electric bill.  The theory works somewhat better
>in the flatlands.  We had a 100 kW in Tulsa and had to
>ramp up the power/lower the antenna gain because of the
>hilly terrain.  High gain antennas have pretty "thin" coverage
>vertically speaking (can't get something for nothing).
>The higher TPO filled in the valley areas between hills
>where coverage was poor.
_______________

Other things equal, ERP in the sectors of the elevation pattern of "high" 
and "low" gain FM antennas won't vary by much when going from the 
horizontal plane down to a few degrees of depression angle -- and that is 
the sector of the elevation pattern that serves the greatest part of the 
coverage area (by far).

Here are two tables showing this:

For 10 Bay Antenna at 1500 ft. HAAT and 100 kW ERP max:

                Distance   Dep. Angle   ERP      dB from    50,50 Field
                  (mi.)       (deg.)     (kW)     Max. ERP    mV/m  dBu
                --------    --------   ------    --------    ----- ----
                   10         1.68      74.85      -1.26     53.5  95
                   20         0.92      91.82      -0.37     15.5  84
                   30         0.71      95.15      -0.22      5.8  75
                   40         0.62      96.19      -0.17      2.5  68
                   50         0.60      96.51      -0.15      1.2  61
                   60*                                        0.57 55

For 4 Bay Antenna at 1500 ft. HAAT and 100 kW ERP max:
                   10         1.68      95.75      -0.19     61.9  96
                   20         0.92      98.71      -0.06     15.8  84
                   30         0.71      99.24      -0.03      5.9  75
                   40         0.62      99.41      -0.03      2.5  68
                   50         0.60      99.46      -0.02      1.2  61
                   60*                                        0.57 55

   * Over radio horizon

The biggest difference in field strength of these two systems occurs close 
to the tower, where the "high" gain antenna has very high field strength in 
the first place.  Probably having more there is not necessary.

For this range of distances and ERPs, these two antennas should provide 
virtually the same fields, regardless of the terrain.

RF