[BC] IBOC Tentatively Identified as Interfering Signal

[Phil Alexander]

On 14 Aug 2006 at 16:36, Kent Winrich wrote:

> In regards to the interference issuse, again I will refer to 680/700/720 in
> Chicago/Cincy .  70 miles NORTH of Chicago I am able to get WLW during the
> day while WSCR and WGN run IBOC (WGN has IBOC turned off as of this
> writing).  WLW is over 400 miles away.  I haev not had any difference in my
> ability to receive WLW.  In Milwaukee WTMJ is running IBOC.  In the main
> lobe of 620 WTMJ, (250kW ERP) I am able to get tiny 640 WMFN, 1.2 kW in
> Zeeland, MI (about 80 miles away).  What am I missing?

Those are all second adjacent situations that should not be a problem
provided transmission systems are linear and maintained that way with
an automatic pre-distortion system.

The issue of lost "bonus coverage" is a first adjacent situation where
the IBOC subcarriers are essentially causing co-channel IX  to a "first
adjacent" analog signal where the D/u ratio of analog to subcarrier IX
is less than about 25:1, or 50:1 if you are a purist, intolerant of any
noise floor increase.

In the present context of daytime operations it does come up here and
there where there are pattern b/w issues at + or - 10 kHz in some of
the newer patterns, however, I expect this is only the tip of the
iceberg compared with nighttime operations where patterns are generally
sharper, with considerably greater relative null-fill in the +/- 10 kHz
area, and these emissions are propagated via skywave. Many of the older
in-line arrays can be expected to be especially problematic. This topic
was virtually ignored in the RM.


---------------------------------------------
Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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