[BC] Digital coverage and power measurement

[Barry Mishkind]

We have heard from Rich Wood, as he checked out a loaner receiver. It 
might be interesting to learn about other folks experiences, 
especially those with Class A stations.

On another list, there has been an interesting discussion of just how 
much coverage the digital signals are
getting. And a related issue, of how to accurately measure the power 
output of the transmitter. Several have commented that the Bird units 
do not necessarily indicate real world power.

One person wrote:

Don't try to equate FM power with HD power.  Around here, we're
finding that HD coverage is pretty good out to the 60 dBu contour,
which is how it was intended.

One answer, from a "VideoPaul" noted:

That's great, but what do we do here in the midwest where
the stations are expected to perform to the 54dBu?

...

What is everyone's opinion on what level of reliability is acceptable 
for HD radio?
Given that the multicast channels have no analog to blend back to,
I would think that near 100% coverage would be the only part
you'd think is acceptable since they just go silent when the signal 
gets marginal.

This led to John Arndt, in Philly, commenting:

I guess the obvious question I would have to those that don't think 
their HD coverage is as good as their analog...  Are you measuring 
the HD power with a Bird Digital True RMS meter?  The meter on our HD 
transmitters is no where in the ballpark for actual power being fed 
to our HD line and subsequently into our hybrid modified antenna.  If 
I depended on the HD transmitter meters, I'd be 30% under powered!

And then Grady Moates shared this from his driving around:

    WUMB-FM is 660 Watts from atop a water tower in Quincy.  This 
gives us 6.6 Watts of digital.  We get nowhere near our 60 dBu 
contour with this. . . from my driving tests, it appears to be 
limited primarily by first and second adjacent stuff, and by WGBH 
front-end overload from their nearby Great Blue Hill transmitter 
site.  You can only imagine my despair when the FCC declined to grant 
their app to move to Needham.

    I turned my HD carriers up in amplitude about 2 dB one overnight 
during the experimental period last year and did a driving test, and 
discovered:
[1] no perceivable increase to self-interference to      the WUMB 
analog (low-level combined, common      antenna), but [2] not much 
increase, if any, in digital coverage, and [3] elevated interference 
to second adjacent WMLN      on 91.5.

    As you might imagine, It was an unhappy 5 AM as I turned it back 
down.  I was hoping an increase in the amplitude of the HD carriers 
would have helped our HD reception.

    Of course, this was not a rigorous, double-blind, scientific 
test, so the results have no real merit, but it's a nice, informal 
data point.

    Here's another one:  WFPB-FM, Falmouth is on 91.9 FM, with a mild 
cardioid null at about 300 degrees and 6 kW in the vertical polarity 
with a 3-bay half-wave Shively, also low-level combined.
As I drive away from the site, the HD fails about 3 miles away.  The 
problem is that first-adjacent WOMR at 92.1 from Provincetown blasts 
in and wipes out my upper side-band carriers.  Even though the lower 
sideband carriers appear to still be intact, for some reason the 
radio can't successfully decode the HD from the lower sideband 
carriers alone.


Anyone else with a Class A on IBOC, with a report?




_______________________________________________________________________
Barry Mishkind     -       Tucson, AZ    -   520-296-3797