Posted by al germond on October 02, 2009 at 01:09:07:
In Reply to: Re: Re: Re: What About Suburban Repeaters for Weaker AM Stations? posted by Greg Strickland on October 01, 2009 at 20:05:59:
The synchronous pairing of AM stations is almost as old as broadcasting but I haven't read of any recent pairings being developed.
The first technical article describing the WBZ-WBZA pairing appeared in "Radio Engineering" magazine in the late 1920s. At the time, WBZ was located at the Westinghouse Works in Springfield and WBZA functioned as the Boston-area repeater located at Millis, Mass. The article describes the difficulties encountered devising the common crystal drive for the two stations and the vagaries induced by weather and moisture on the leased New England Telco mostly open-line telephone circuit linking the two "plants" as well as establishing a very low fundamental operating frequency and multiplying and precisely maintaining it on the 990 kilocycle frequency both stations occupied beginning on November 11, 1928. The calls were later swapped and Millis was abandoned to the current WBZ directional site at Scituate which is more favorably situated to the greater Boston area.
There has been considerable technical innovation -- both on AM and FM -- in the Boston area. In 1931, the Shepard Stores' WNAC -- then on 1230 kc -- erected this country's first series-fed vertical radiator at Squantum, Mass north of Quincy employing the first Blaw-Knox "diamond" tower erected on this side of the Atlantic at a time when Europe's broadcasters took a greater fancy to this style of mast.
For years, WLLH, Lowell, Mass (1kW on 1400kc) operated a 1 kW on-channel synchronous repeater at Lawrence, Mass NNW of Boston. A check of the FCC database would confirm/deny if this arrangement is still in place.
Articles about synchronous operation on AM appeared rather frequently in both the trade and hobby press during the Thirties.
In 1930, New York's WABC (now WCBS) proposed building a synchronous booster on 860 kc to serve the Washington, D.C. area but this never came to fruition as far as I know.
An article in the "Journal of the Institute of Radio Engineers" describes the synchronization WOC/Davenport, IA with WHO/Des Moines using a monitoring station located in between the two transmitters to keep them precisely locked on frequency. This was tried, written up in a respected technical journal and subsequently abandoned.
Two longer-running synchronous pairings come to mind on clear channels in the East and Midwest that were conducted for a few years prior to WW II.
WTIC/Hartford was synchronized with WBAL/Baltimore at night during NBC Red Network programs.
WBBM/Chicago was synchronized with KFAB/Lincoln (Omaha) during the evening.
Then.... don't know whether the arrangement is still active but a 1 kW station in Kansas City, KS on 1340 kc was synchronized with a similar facility in Pittsburg, KS while both stations were synchronized with a booster facility located in between the two main transmitters at Amoret, Missouri. I recall driving toward the KC area in the middle of the day and noting the hollow slightly out-of-phase effect one typically hears between two AM transmitters that are purportedly synchronized.
No matter how assiduously the engineers try, AM synchronization has never been a foolproof proposition. Maybe synchronism can be coaxed into working during the daytime but at night with varying relationships introduced by the ionespheric reflection plus skywave vs groundwave interaction, synchronization has always come up a cropper in this country.
There's been more activity in Europe due to spectrum scarcity mandated by various conventions -- Prague, Copenhage, Geneva, etc -- but with the same issues at night. The telltale hollow sound has been noticed after sunset on some frequencies during recent trips to the Continent.
There's no synchronous magic here for any of NYC's AM stations.
But there have been several missed opportunities in the USA.
.Failure to seize FM allocation opportunities before digital tuning was adopted more than thirty years ago initially in mobile applications.
.Failure to fully embrace RDS and its unknown application in this country that pairs FM repeaters sprinkled around the area to create seamless coverage beyond the main transmitter.
Please, no more RF on AM!
Amplitude Modulation has withered away in some areas because the FCC has foolishly -- some would say largely due to political pressure -- mismanaged this fragile spectrum. The last thing AM needs is for any more RF to be heaped upon the existing quagmire of noise and interference on the "standard" broadcast band.