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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