WCVT (101.7 Stowe, VT)

Garrett Wollman wollman@bimajority.org
Sun Dec 11 13:29:47 EST 2011

<<On Sun, 11 Dec 2011 11:21:47 -0500, Scott Fybush <scott@fybush.com> said:

> On 12/11/2011 9:36 AM, Bill O'Neill wrote:
>> The ERP will be 50 kW according to what the AM host, Brian Harwood, said
>> recently.

> Well no, not exactly.

> WCVT on Mansfield will be (is?) a class C2 station, which is the 
> equivalent of 50 kW at 152 meters above average terrain. But the actual 
> WCVT facilities are just one kilowatt ERP at 811 meters above average 
> terrain. That combination of lower power and much higher antenna height 
> results, *in theory*, in the same distance to the 60 dBu protected contour.

This is a mistake that many non-technical people make -- and in fact,
I made it myself when I was first learning about radio regulation.
The cause is that slippery word "effective", in "effective radiated
power".  In normal English usage, one would expect it to mean
something like "equivalent", but as a matter of engineering jargon,
it's actually much more restricted: take the transmitter power output,
multiply by the transmission line efficiency, then multiply by the
antenna gain.  Nothing else enters into it.  (Normally, engineers, who
are used to working in decibels, a logarithmic scale, would say "TPO
in dB over a kilowatt, minus line loss, plus antenna gain" instead.)

The FCC then plugs the ERP and the height above average terrain into
an empirically-derived formula to determine what the theoretical
coverage radius is, and station classes are actually based on the area
covered -- even though they are specified in terms of prototypical
transmitters rather than the area.  So for a given class, there is a
curve (which you can see in the FCC rules) which represents
permissible combinations of ERP and HAAT; at heights higher than the
"standard", the power must be "derated" in accordance with the curve.
However, the FCC does not allow applicants to compensate for a
lower-than-"standard" height by increasing power above the class
maximum; this is done to encourage stations to put their antennas
higher.  (Also, and for reasons not relevant here, there are many ways
to compute HAAT, and applicants are permitted to use the one most
favorable to them -- possibly more than one in the same application!)

This method of licensing FM stations by coverage area goes back to the
early 1940s, when the FCC started requiring applicants to specify
their service area in addition to the specific transmitter and antenna
they planned to use.  At that time, there were four FM station
classes: A, B, C, and D in order of increasing coverage; the Yankee
Network stations WGTR (44.3) and WMTW (43.9) were both class-D
stations licensed to Boston.


More information about the Boston-Radio-Interest mailing list