WITS/Red Sox

[Dan Strassberg]

Since I can't think of a good tutorial, I'll do my best to give you a 5-cent
tour of the most important terms.

I'm embarassed to say that I don't know what the C in CDBS stands for. I'm
quite sure that Scott Fybush knows. Garrett probably knows too. The DBS part
stands for database system. The most obvious guess for the C is computer,
but I believe that would be wrong.

Another term that may have mystified you is azimuth, which is not really a
radio term at all; it's a term from map-making and cartography. Any good
dictionary should contain a definition.

mV/m stands for millivolts/meter, the unit of measurement of electric-field
intensity, which is electrical-engineer speak for signal strength.

mV/m/kW @ 1 km = mV/m per kilowatt at 1 kilometer, the standardized unit of
efficiency of medium-wave radiators (AM transmitting antennas). It
normalizes the signal to an antenna-input power of 1 kW by dividing the
actual signal strength by the square root of the antenna-input power in
kilowatts. For example to get WUNR's pattern RMS you would multiply 410
mV/m/kW @ 1 km by the square root of 5 (2.23) because WUNR uses an
antenna-input power of 5 kW.

RMS is root-mean-square, a mathematical term meaning the square root of the
sum of the squares. In obtaining a pattern for an AM station, the quantities
that are squared and summed are the 72 inverse-distance field-strength
values at 5-degree intervals around the compass. After you calculate the sum
of these squares, you divide by 72 (to obtain the average of the summed
squares) and then take the square root of the average value.

A non-directional station has a circular pattern (plotted on
polar-coordinate graph paper) So you can (in theory, at least) measure the
signal strength along any radial and obtain the pattern RMS. Of course, the
real world is more complicated. Variations in the soil conductivity will
result in different values along different radials. "But wait," you say,
these are inverse-distance fields. Aren't they supposed to be independent of
soil conductivity?" Yes, but in practice the measured values can't be; so
you have to correct for soil conductivity by measuring the signal strength
along each radial at many points and using a graphical procedure to match
the attenuation vs distance curve to the curve for a particular soil
conductivity.

Directional stations' patterns have inverse-distance signal strength that
varies widely along different radials. A good local example is WBIX whose
night-pattern values range from around 2 mV/m @ 1 km west-southwest of the
transmitter site to about 1500 mV/m @ 1 km east of the transmitter.

NIF stands for nighttime interference-free, the FCC's (rather conservative)
estimate of the signal-strength contour within which an AM station has a
more-or-less interference-free signal at night. The RSS (root sum square) of
the interfering 10% skywave signals of the co-channel and
first-adjacent-channel stations enter into the NIF calculation. RSS is
calulated in a manner similar to RMS except that you don't divide the sum of
the squares by the number of quantities that you squared. 10%-skywave refers
to the signal-strength value that a skywave signal equals or exceeds 10% of
the time. If you get the sense that skywave signal strengths are
unpredictable, except on a statistical basis, you are correct. The concept
of xx% skywave is a way of quantifying values that vary in a more-or-less
random manner.

You may see the terms 50%-exclusion NIF and 25%-exclusion NIF (although I
did not use those terms in my most recent posting). The 50%-exclusion NIF
excludes interfering signals that are less than or equial to 50% of the
largest one. The more conservative (and hence usually slightly higher) 25%
exclusion NIF excludes signals that are less than or equal to 25% of the
largest one. The rationale for the exclusion is that when you square a value
that is 50% of the largest number in a group or series of numbers, the
square is only 25% of the largest square. When you square a value that is
25% of the largest number in a group or series, the square is only 6.25% of
the largest square.

I hope that gets you started. Typical electrical engineering curricula
prepare EEs to understand these concepts, but even the best Master's program
in EE doesn't cover the specifics of this arcane subject area. The
practitioners, most of whom are EEs, have had to pick up the details on
their own. Scott has them down and I believe he majored in History and
Journalism at Brandeis, so you don't HAVE to be an EE--but it certainly
helps.

--
Dan Strassberg, dan.strassberg@att.net
eFax 707-215-6367

----- Original Message -----
From: "Doug Drown" <revdoug1@verizon.net>
To: "Dan Strassberg" <dan.strassberg@att.net>; "Laurence Glavin"
<lglavin@mail.com>; "Garrett Wollman" <wollman@csail.mit.edu>;
<bri@bostonradio.org>
Sent: Saturday, October 07, 2006 7:08 PM
Subject: Re: WITS/Red Sox


> <<WKOX, which, at night, currently also uses a two-tower array to produce
a
> modified cardioid pattern, has even taller towers. (They are top-loaded to
> an electrical length of 214 degrees.) These produce even higher
>  efficiency--426 mV/m @ 1 km using a power of 1 kW.>>
>
> Question from a broadcasting non-professsional (I've done a lot of
> broadcasting but have little understanding of radio engineering other than
> running a board): I've read a lot of posts like this in this forum, and
> would like to know if any of you could recommend any good textbooks that
> would explain, in comparatively simple terms, the sort of thing you're
> talking about here.  I find it fascinating, but I don't know what all the
> terms mean, and aside from knowing (for example) what kHz, kW, and "null"
> refer to, the technical lingo is almost entirely lost on me.   Any
> suggestions?
>
> -Doug
>
> ----- Original Message -----
> From: "Dan Strassberg" <dan.strassberg@att.net>
> To: "Laurence Glavin" <lglavin@mail.com>; "Garrett Wollman"
> <wollman@csail.mit.edu>; <bri@bostonradio.org>
> Sent: Saturday, October 07, 2006 5:26 PM
> Subject: Re: WITS/Red Sox
>
>
> > WUNR uses the same two-tower directional pattern day and night. The
towers
> > are spaced 105 degrees apart along an axis at 70 degrees. As you
observed,
> > the towers are quite tall--greater than half wave--205 degrees to be
> > precise. Consequently, the efficiency shown in CDBS is quite high--410
> > mV/m/kW @ 1 km, although it has been speculated that maybe the ground
> > system, which is almost 60 years old if it has never been replaced, is
in
> > serious disrepair. A deteriorated ground system could explain an
> efficiency
> > significantly lower than the value shown in CDBS. The pattern is a
> modified
> > cardioid with radiation minima 31 or 32 degrees off axis at 219 degrees
to
> > protect WWRL and at 282 degrees to protect E Longmeadow. (A true
cardioid
> > consists of towers exactly 90 degrees apart and has only one radiation
> > minimum, along the axis of the array.)
> >
> > WKOX, which, at night, currently also uses a two-tower array to produce
a
> > modified cardioid pattern, has even taller towers. (They are top-loaded
to
> > an electrical length of 214 degrees.) These produce even higher
> > efficiency--426 mV/m @ 1 km using a power of 1 kW. The axis of WKOX's
> array
> > is 35 degrees; in other words, WKOX's current pattern is rotated
> > counterclockwise from WUNR's pattern by 35 degrees. I believe that WKOX
> was
> > built that way because, when the move to 1200 was granted, secondary
> > stations on what had been Class IA channels were restricted to 1 kW at
> > night. The 35-degree orientation allowed a 1 kW signal to cover
> Marlborough
> > pretty well at night. Had WKOX been allowed higher night power when it
> first
> > moved to 1200, the orientation of its towers would probably have been
more
> > clockwise because the azimuth of WOAI is 33 degrees clockwise with
respect
> > to the azimuth of WKOX's towers. (And then later on when WBIX moved in
> with
> > WKOX, it would not have needed to use lower power during critical
hours.)
> >
> > When it looked as if Newton would never approve tower construction at
750
> > Sawmill Brook Parkway, I urged that WKOX move there anyhow and use
WUNR's
> > two existing towers to operate DA-2. It turns out that the existing WUNR
> > towers have, within inches, the same physical spacing as the WKOX towers
> > (105 degrees at 1600 is 78.75 degrees at 1200). Running 50 kW-D from the
> > existing WUNR towers should have presented no problem for WKOX. The
night
> > power would have had to be much lower than 50 kW, however. Because the
> WUNR
> > towers' more clockwise orientation would have reduced radiation to the
> > northwest, it might have been possible for WKOX to run 5 kW at night and
> > still adequately protect CFGO--I'm not sure. But even with 5 kW from a
> site
> > within the City of Newton, it did not appear that WKOX could deliver an
> NIF
> > signal to the requisite 80% of the CoL's population. Hence, the FCC
would
> > have had to grant a waiver.
> >
> > At 1200, WUNR's existing towers are 153.75 degrees high. The resulting
> > efficiency would have been about 7.5% higher than what WKOX will achieve
> > from its new 195' towers--the equivalent of more than 15% higher
> power--not
> > a huge difference, but not trivial either. Since there will be five
towers
> > at the site (of which WKOX will use only three), and two of the three
> > stations that share the site are on frequencies lower than 1600, the
area
> > covered by the new ground system will be greater than that covered by
the
> > existing WUNR ground system. Part of the reason that WKOX is projecting
an
> > efficiency as high as 339 mV/m/kW @ 1 km, is that the ground system of
the
> > two towers that WKOX will not use is still considered to be part of
WKOX's
> > ground system. If WKOX were going to use the existing WUNR towers, a
> larger
> > than normal ground system could also have been employed, somewhat
> improving
> > on the antenna efficiency that would have been achieved from the
> > 153.75-degree towers.
> >
> > --
> > Dan Strassberg, dan.strassberg@att.net
> > eFax 707-215-6367
> >
> > ----- Original Message -----
> > From: "Laurence Glavin" <lglavin@mail.com>
> > To: "Dan Strassberg" <dan.strassberg@att.net>; "Garrett Wollman"
> > <wollman@csail.mit.edu>; <bri@bostonradio.org>
> > Sent: Saturday, October 07, 2006 12:15 PM
> > Subject: Re: WITS/Red Sox
> >
> >
> > >----- Original Message -----
> > >From: "Dan Strassberg"
> > >To: "Laurence Glavin" , "Garrett Wollman" , bri@bostonradio.org
> > >Subject: Re: WITS/Red Sox
> > >Date: Fri, 6 Oct 2006 14:49:15 -0400 In any event, if you find WKOX's
> > current signal listenable for
> > >only a short distance past Worcester (I find it pretty good all the way
> to
> > >the Connecticut border on I-84), the new, "stronger" signal will become
> > >unsatisfactory to you near the west side of Framingham.
> >
> >
> > In this particulat case, I wasn't doing any radio-geekery, that is
trying
> to
> > hold a station to see how far it would go;  I was actively listening to
> > Stephanie Miller and wanted to continue to do so.  I knew AM 1600 in
> > suburban Springfield was carrying it and when I flipped to 1600 I
> > expected a battle between it and WUNR, but was surprised to get the
> > station I wanted quite clearly.  In the past I never had any reason to
> > sample AM 1600 out of Longmeadow, so I had no idea what I'd find.
> > It does indicate that even during the day, WUNR's signal WNW is no great
> > shakes,
> > which backs up your previous assertion that the Samill Brook location
> > isn't exceptional, even though those towers APPEAR efficient for a
> > 1600 khz operation.
> >
> > --
> > ___________________________________________________
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> >
> >
> >
> >
> >
> >
>