[IRCA] IBOC deterioration

["Craig Healy" <bubba@xxxxxxxxxxxxxxxxxxxx>]

One of the advantages of the SDR type radios is seeing the spectrum.  It
really helps when trying to null a pest or see if there are TA/TP carriers.
Great for seeing if LSB or USB will give the best audio.  And, it shows in
good detail the IBOC sidebands.

AM stations have slight seasonal parameter changes in their antenna systems
due to water around the ground system.  IBOC requires a very tightly
controlled set of parameters.  When these drift, things change and not
usually for the better.  Unless a station is frequently monitoring these
changes, it can put things out of tolerance.  This usually shows as added
interference.

One of the crucial tuning tests is a set of spurious sidebands around +/-
27KHz on either side of the main carrier.  It needs to be -65db from that
carrier level.  When all is good, it shows on the SDR as a very slight fuzz
on either side.  When it is pretty obvious, then it's probably out of range.
When setting up WDDZ-550 for day IBOC, I was able to match what the
consultant's spectrum analyzer showed against what the SDR displayed.  I got
a pretty good idea what is good and what is not.

Installation of IBOC is usually done by an out of town consultant who's
there only for that task.  Then the station has no spectrum analyzer and is
blind to changes.  DXers have better eyes and ears than most engineering
staffs.  In Providence, there is another station with day-only IBOC and I've
noticed a significant jump in their +/- 27KHz sidebands.  I'll chat with
their engineer next time I see him, though I don't think there's anything he
can do without the proper equipment.

I have to wonder if the IBOC stations have budgeted for frequent enough
checks to keep their stations in compliance with the FCC Rules and
Regulations.  My guess is no.

So, if a local is trashing stations beyond the first adjacent, they may have
a problem.

I really wish that the SDR-14 Spectravue software had a way to store perhaps
a hundred readings and display the maximums.  That would bring it pretty
close to the multi-thousand dollar consultant's equipment.

Craig Healy
Providence, RI


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