There are some AM broadcast stations in the U.S. that are GPS
referenced. This has primarily come about as a result of Digital
(IBOC) Broadcasting. I do off-air frequency measurements for many
stations, primarily in the Southern California area, so I have a
fairly good idea of who in this area is doing what and their
reference
reliability. In my local area there are three AM stations that are
reliably referenced to GPS most of the time. They are: KFWB 5,000
Watts non-directional on 980 kHz, KNX 50,000 Watts non-directional on
1070 kHz, and KDIS 50,000 Watts directional on 1110 kHz. These
three stations seem committed to the IBOC system, but for how long,
who knows. There are other's, but with them it is sort of an on
again,
off again situation. A couple of caveats are that these stations may
not always be operating on their GPS referenced exciter or
transmitter. These days, individual transmitter maintenance is
mostly
done during the daytime and only antenna repairs are generally done
during the nighttime hours.
Not all stations broadcasting digital IBOC are GPS locked.
Supposedly being GPS locked would allow receivers that are GPS locked
to lock up faster and stay locked better. So far I don't know of a
single IBOC receiver that has the capability of being GPS referenced,
so the GPS referenced transmitter has never been of great importance.
I know of several AM stations that have the capability but have never
bothered to hook up the GPS antenna to their GPS capable exciter and
really have no interest in doing so. In many cases, AM stations have
shut down their IBOC systems due to lack of interest on the part of
the listening radio audience and in some cases due to adjacent
channel
interference caused by the digital sidebands. In some cases they've
actually removed the equipment from their racks and are using it,
appropriately enough, as something to sit on.
So, the bottom line is that while there are some GPS referenced AM
broadcast stations out there, they are not a reliable day to day
reference.
Another aspect of this whole thing is that while it would be simple
enough to GPS lock/reference an AM broadcast transmitter, this may
not
always be desirable. Having two co-channeled stations precisely on
or
very near the same frequency, even though they may be thousands of
miles apart, can have serious fade issues. If they are exactly on
the
same frequency they can cause deep nulls in their coverage depending
on the relative signal strengths. Paging transmitters have in the
past gone to precision offsets of a only a few Hz in order to get
what
they called "rolling fades. While this may be desirable for paging
systems where data can be repeated several times, it can be very
annoying in the broadcast radio world. A listener, depending on
location, could hear consistent deep cyclic fades. As most of you
know, AM broadcast stations are allowed a carrier tolerance of +/- 20
Hz. Co-channel fades are much more tolerable to the listening
audience if they are in the range of 15 to 20 Hz. While this 15 to
20
Hz offset may cause some low frequency intermodulation (tone) in the
audio, it is much less objectionable than a deep, fraction of a Hertz
difference causing a station to slowly come and go. Because of this
I
have several clients that have deliberately opted to go with an
offset
of 10 to 15 Hz, depending on the absolute frequency of an interfering
co-channel station. While broadcast engineers are a scientific
bunch,
they are never the less in "Show Business", and the need to present
their programming (such as it is) in the most listenable manner comes
first.
Burt, K6OQK
Well its a funny thing actually. Looking at an amateur effort its not
all
that hard at 1.6-7.0 MHz to generate reasonable power of say 100-500
watts.
Certainly its not hard to create an exciter at those frequencies that
are
derived from a quality reference. Heck many time nuts have CS
references
etc. the concept is quite flexible if you consider adding some pahse
modulation perhaps. All in all pretty do-able. A few things tend to
get in
the way like rules and regs etc.
But there is an alternate that would be very reasonable. The
broadcast band
AM transmitters run all the time and if their exciters were
controlled by
the reference you would have major portions of large areas covered.
The
Broadcasters are already paying for the power, transmitter, and
antenna upkeep.
Of course nothing like that would happen and am modulation can have
effects
on the carrier.
Regards
Paul
WB8TSL
Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California U.S.A.
[email protected]
www.biwa.cc
K6OQK
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