[time-nuts] AM Broadcast stations as a frequency reference...

Burt I. Weiner biwa at att.net
Thu Oct 13 10:27:02 EDT 2011

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.

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California  U.S.A.
biwa at att.net

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