[BC] Re: lpfm question

[Mark Humphrey]

For building attenuation, I've seen studies showing as little as 2 dB
(for a wood frame house) up to 25 dB for a concrete and steel office
building.  If the wood frame house is sided with aluminum or has foil
backed insulation in the walls, of course the loss will be higher than
2 dB, so a rough average for all buildings might be 8 to 10 dB.

This factor applies regardless of the transmitter power --  in other
words, if you measure a 60 dBu field strength outside the building and
the attenuation is 10 dB, you should measure 50 dBu inside.  It
shouldn't matter if the station is transmitting with 10 watts from,
say, 2 miles away or with 100 kW from 40 miles away, as long as the
field strength is still 60 dBu outside.  So anything you can do to
improve field strength in the desired coverage area will provide a
proportionate increase indoors.

As an example of the many factors that should be considered when
choosing a site, here's a very detailed EBU document released in 1982
to assist in planning of FM networks.  It goes into much greater depth
than the guidlines given in FCC 73.315:

http://tech.ebu.ch/docs/tech/tech3236.pdf

Regarding the age-old question about power vs. height:

 In 1997, I did a mobile field strength survey of the Philadelphia
market and measured all of the commercial FM stations transmitting
from the Roxborough antenna farm at over 8000 sample points within
roughly 40 miles of the city.  The receive antenna was a vertical
quarter-wave whip cut for 98 MHz and mag-mounted at the center of the
roof of an SUV.   The survey took a week to complete (I drove about
350 miles a day) and was very informative.    At that time, each of
the Class B stations was operating with full facilities and licensed
non-directional, except for WXTU which is required to use a DA.  Only
one station, WFLN 95.7, was operating with 50 kW at 150 meters.   Most
had antenna heights in the 200-270 meter range, and the highest at
that time was WMGK with 8.5 kW at 360 meters.

After completing the drive, I had a map showing all measurement
locations and could draw a box around any group of points and run
averages.  I could also look at the spread between lowest and highest
field strength in any particular area.

The "winner" for market-wide consistency was WMGK, which at that time
was using a single-bay rototiller atop Channel 57's antenna, but WFLN
was one of the worst signals overall.    However, in the Center City
business district (about 8 miles from Roxborough) stations with
antenna heights in the 200 meter range had an apparent 2 to 3 dB
advantage over WMGK.  This is where the "brute force" approach paid
off, but these stations didn't perform as well in many of the suburbs.

Needless to say, test results could vary greatly in other types of
terrain, but the local preference in Philadelphia has been for height
over power.

Mark

On Wed, Feb 18, 2009 at 9:07 AM,  <bmc1956 at aol.com> wrote:
> Thanks to all who offered information on us moving from 100 watts at 100' to 15 watts at 250'.
>
> I have a couple of more questions to ask, if I may.
>
> We are in Lake Charles, LA, flat land, 20 miles from the Gulf.? Tropospheric Ducting is a big problem here.
>
> Would transmitting from a higher HAAT with less wattage help with this problem?
>
> Also, what signal db is required to penetrate houses, apartments, commerical buildings, and mobile homes?