Synchronous Amplifiers

[ai4i]

WBZT(1230), West Palm Beach, Florida, has one in Pompano Beach, Florida, which covers more people than the station's main transmitter. WBT(1110), Charlotte, NC, used to have one in their night time null. Why is WBZT allowed to operate one? Why did WBT's go away? Why do so few, if any, others still exist? Could someone fill me in on these things. Radio-locator does not seem to recognise the existance of WBZT's.

[KI4JQM]

I'd be interested to know this myself. WBT bought a little Class A in Chester, SC to fill in their night time service a few years ago. Synchronous AM sounds like a good idea to me, if you can lock 'em tight.

[davedybas]

While the term "synchronous amplifier" may apply somewhat to the station you are describing, the reality is they are two separate licensed radio stations. The one station that has the null, is directing it's signal away from the other station.

Now the "Synchronous" part....The owner of one station likely bought the second station and has synchronized the RF carrier phase and the audio to the other station to create a larger combined signal. This is done by locking the transmitters oscillators to a GPS signal reference.

Each station is however considered by the FCC as a separate radio station. Each station has separate Call Letters and must ID themselves as such.

[NECRAT]

Each station is however considered by the FCC as a separate radio station. Each station has separate Call Letters and must ID themselves as such.

I know of one instance up in Massachusetts where it isn't run like that. (The legal side that is).

WLLH runs both a transmitter in Lowell (primary) and a syncrhonus in Lawrence. They both have the same call sign, and facility ID, however the Lawrence side is run as an Experimental service, and has done so for a good 30 years now.

[Deep Thought]

Synchronous AM transmitters were never really codified into the mainstream rules and are licensed as experimental stations under Part 74. A quick check on CDBS shows 28 AM records although a bunch of them are for the same couple of stations in Puerto Rico. I know of one excellent example of what you can do with these: WR2XJR in Portsmouth, VA, which is the 700 watt 4-tower nighttime for WPMH in Claremont, VA, a 20 KW class-D running three watts at night from it's daytime Part 73 licensed site. The WLLH example is another good one. KKOB has been running one in Santa Fe forever too, and that one is licensed under the primary callsign. They generally stick "2X" in the middle of the normal calls (KKOB's would be KK2XOB) for the EX operation. The WR2XJR 670 KHz operation is on the WRJR 1010 towers, but when it was licensed WPMH was WRJR while WRJR was WPMH...and the FCC doesn't allow EX callsign changes.

WBZT's in Pompano Beach is also licensed under the main callsign. The file number is BLEX-19940913AB.

With current GPS timing technology a 0.2 Hz tolerance is a lot easier to do than it was 25 years ago, but if you only run it at night the skywave interference from other stations pretty much takes care of any fringe beats or distortions.

[Sam Buca]

I still don't understand how synchronizing the carrier and audio will work. The radiated pattern is circular, so the interference patterns (constructive and destructive) are all over the place. If you synchronize it so that one point is 100% constructive, half a wave length over it won't be.

Wouldn't FM picket fence as you drive? Wouldn't AM give that phasing/flanging sound? How does this work?

[ai4i]

If you synchronize it so that one point is 100% constructive, half a wave length over it won't be.
Wouldn't AM give that phasing/flanging sound?

Actually, it would only be a quarter wave over, since one station would be advanced 90° while the other would be retarded the same amount.

First off, I think WWVB might be a better way to synch the stations than GPS. The BBC has been synching their domestic AM networks for many years, some highway advisory and low power community AM nets are doing the same (one such station in St. Petersburg, FL uses five transmitter sites on 940) and part fifteen networks have also sprung up with questionable legality. The purpose is not to completely rid the interferrence patterns, but areas of equal enough signal strengths between the transmitters to present an issue are shown to be relatively small.

It has always been my oppinion that were all AM stations in this country synched on their respective channels (RF carrier frequencies, not audio) QRM would be much less of an issue. While this would help most/all stations, it would be most adventageous for stations on graveyard channels. There would simply be no heterodyne interferrence, just audio fading in and out. To be objectionable to the listener, this audio would have to be from a much stronger signal than just a carrier going in and out of phase many times per second.

[Shane]

I don't much about this but I'll stick my finger in the socket anyway.

When this is done on FM, I believe there is a "multipath zone" at a point between the transmitters. That zone, I think, would be in areas where the capture ratio is below 2:1 because one signal is not more than twice the strength of the other.

Some stations have been known to "adjust" the location of the multipath zone by raising or reducing the power of a main transmitter or booster depending on which is the more desired signal. If the booster, for example, is closer to a large city and can provide better coverage of it than a main located some distance away, the station might be tempted to reduce the power of the main - or shut it off entirely, city of license be damned - to keep that multipath zone out of the metro area.

Now since the capture ratio on AM is on the order of 100:1, I'm thinking this concept works best for AM null filling or for serving two cities far enough apart that neither signal much reaches the area in between anyway, such as WLLH I suppose, or the overlap area is sparsely populated. I've never been in a receiving situation for this sort of AM operation so I don't know what it really sounds like. The situation in Virginia appears to be a perfect one for the concept judging from the transmitter locations and the night 3W power level of the main way up the river!

I'm not smart enough to figure out what search criteria one can use in CBDS to get this info. What would that be?

[RFWarrior]

Historically, this hasn't been done a lot on AM's because the cost of a referenced, high accuracy oscillator to drive the carrier had a whole bunch of zero's in it - 20 years ago they were going for over 50k - that really makes it cost prohibitive.

To add to the conversations about overlap and multipath, yes, there is some. Phasing of the individual transmitters is done to try to put the interference zones over non-populated areas as much as possible. For FM, in addition to synchronizing the carriers, it's also necessary to synch the pilot signal - and the audio of course needs to be very closely time aligned. These days with GPS receivers in practically every transmitter, it's a whole pile easier than it was 20 years ago

Best,

Jeff

[K9EZ]

WBZT(1230), West Palm Beach, Florida, has one in Pompano Beach, Florida, which covers more people than the station's main transmitter. WBT(1110), Charlotte, NC, used to have one in their night time null. Why is WBZT allowed to operate one? Why did WBT's go away? Why do so few, if any, others still exist? Could someone fill me in on these things. Radio-locator does not seem to recognise the existance of WBZT's.

KKOB in Albuquerque, has a synchronous in Santa Fe. I did talk to the engineers there many years ago, as I was trying to see if I could do the same thing for a station with a very tight pattern in the Midwest.

Transmitter sites are here:

http://www.recnet.com/cdbs/fmq.php?facid=11251&jaws=0

Or here:

http://www.fcc.gov/fcc-bin/amq?list=0&facid=11251

[NECRAT]

I'm thinking this concept works best for AM null filling or for serving two cities far enough apart that neither signal much reaches the area in between anyway, such as WLLH I suppose, or the overlap area is sparsely populated.

That is exactly the case with WLLH. The primary transmitter is in the largest city in Northeastern Massachusetts, with the 2nd located on a rooftop in downtown of the other larger city. The zone between the two is residential and for years very sparesly residential. The last 20 years or so changed that, but it worked back in the day.

Here are the pictures I took of WLLH if you are curious. No transmitters though.
(Not yet at least... )

[Deep Thought]

I'm not smart enough to figure out what search criteria one can use in CBDS to get this info. What would that be?

Go to http://svartifoss2.fcc.gov/prod/cdbs/pu ... p_sear.htm

Select "AM Station" in the Service box.
Type BLEX in the file number prefix (left box of File Number)

Click Submit Application Search.

[NECRAT]

Everything except WLLH shows up with BLEX. For some readon WLLH only shows up as BREX.

[Deep Thought]

Another of the mysteries of CDBS...the file date is 1-1-1978 so it predates even the original VAX database.

[boiseengineer]

If your station gets nailed by a co-channel, try running 4-5 hz higher or lower than the interfering station. Instead of hearing their audio floating underneath you, it will be a low rumble.