Re: [IRCA] WDDZ-550 off again, sweeper noted

["Craig Healy" <bubba@xxxxxxxxxxxxxxxxxxxx>]

> What exactly is the purpose of the sweeping/frequency changing oscillator?

It's part of a Hewlett-Packard network analyzer.  There's a lab-grade
directional coupler used, so the load can be measured for both resistance
and reactance.  There is a Smith chart screen display.  It shows the curve
for the frequencies being swept.

Each current transmitter has a requirement for the load as measured at the
final amplifier transistor collectors.  This varies from model to model.
That load will vary according to the power combiner that sums the individual
transistors in that amp.  This optimal load gives the cleanest bandwidth for
the audio, and - sadly in this case - for IBOC signals.  There are
adjustments made to the antenna phasing and matching system to optimize
this.  Sometimes it requires additional components.

There is something called antenna pattern bandwidth for every directional
system.  In some cases, the pattern shape varies significantly from -10KHz
to +10KHz.  If it's too far off, then IBOC doesn't work.  The upper and
lower digital carriers get too far off in power from each other and break
the HD reception.  That's why they do the sweep, to measure the load and
then include the calculated pattern bandwidth to the final analysis of the
system to determine HD compatibility.

There are some stations where the pattern bandwidth is out of the limit
requirements.  WDDZ-550 looks to be in that boat.  The only fix for that is
to increase the tower height to a 447' 90 degree tower from the 300' 60
degrees ones in use now.  Might be tough, as there's an airport not too far
away.  Expensive, too.

Craig Healy
Providence, RI

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