Re: Time to update at least one interesting tower site

[Shawn Mamros <mamros@MIT.EDU>]

>What you're saying appears to be that WGBH-FM will be running the equivalent
>of 100 kW at 150m HAAT. The 10.5 kW you mentioned for a B should produce
>coverage equivalent to 50 kW at 150m HAAT. Since 'GBH has been running 100
>kW at a little more than 150m HAAT (roughly 25% more), 21 kW at 323m seems
>to represent a reasonably significant reduction in height-corrected ERP. To
>a first order, to maintain constant coverage as you vary the HAAT, you must
>reduce power in inverse proportion to the square of the HAAT. Thus, 100 kW
>at 187.5m should produce coverage equivalent to 156.25 kW at 150m. I wonder
>what the problem was with getting 30+ kW at 323m.

Maybe there was an issue with having to protect third-adjacent WZBC...

Height-corrected ERP doesn't tell the whole story when it comes to
real-world coverage.  For one thing, there's location.  WGBH-FM's new
location will give them a better signal in the western suburbs, and
I wouldn't be surprised if a fair portion of their donor population
lives out that way.

Also, a lot of WGBH-FM's current high power is actually the result of
using a very high-gain antenna.  Those antennas produce higher ERP for
less actual transmitter power, but they have the drawback of "squashing"
the signal vertically, often to the point of actually reducing signal
strength for those on the ground a short distance from the tower, thus
producing a "hole" in their coverage.  I doubt that they're going with
anything like their current eight-bay antenna at their new location.

IIRC, WBUR(-FM) also "reduced" power when they moved from the BU law
school building to FM-128 a number of years back.  But the move seems
to have actually improved their signal for much of the market.  WGBH-FM's
move seems to work much the same way, trading what on paper is greater
power for greater height at a better location.

-Shawn Mamros
E-mail to: mamros@mit.edu