Reduced night power directional AM patterens

[Ai4i]

Is the standard practice for all these stations who run extremely low power at night, such as under 250 watts or under 100 watts to use the same directional pattern as they do in the day, but with greatly reduced power? Are there many/any exceptions? Radio-Locator lists them as using the same "number" of towers, just much less power.

 

[w9wi]

Is the standard practice for all these stations who run extremely low power at night, such as under 250 watts or under 100 watts to use the same directional pattern as they do in the day, but with greatly reduced power? Are there many/any exceptions? Radio-Locator lists them as using the same "number" of towers, just much less power.

It used to be if you couldn't use at least 250 watts at night, you couldn't operate at night at all. Some years back (~20) the FCC went through their database of daytime-only stations & assigned each one a night power figure, assuming they would use the same antenna system at night they use during the day. That's where these figures came from.

It's my understanding these stations can *ask* for permission to use a different antenna arrangement at night, and a handful have.[0] I suppose most figure the power is so low as to be not worth the cost of the engineering to request a different antenna. (indeed, many of them figure the power is so low as to not be worth using. Just because a station is listed in the database with just a few watts of night power doesn't necessarily mean it's actually operating at night.)

[0] These former daytimers - "Class D" stations - are not required to cover their city of licensee at night. As a result, some have built their night facilities on a completely different site. For example, AM 900 Lebanon, Tennessee which transmits from the east side of Lebanon during the day -- and from Nashville at night.

 

[DanStrassberg]

Is the standard practice for all these stations who run extremely low power at night, such as under 250 watts or under 100 watts to use the same directional pattern as they do in the day, but with greatly reduced power? Are there many/any exceptions? Radio-Locator lists them as using the same "number" of towers, just much less power.

Using the same DA pattern day and night is common, but nowhere near universal, among Class D AMs that operate DA-D. Cost is obviously a major issue. Assuming that some or all of the daytime towers can be used and no separate night towers are required, owners can rarely justify the cost of a three or four-tower phasor and the engineering to design a night pattern for a night operation that runs less than around 100W. At very low powers (less than 15W or so), it is also common for Class D AMs that are directional by day to run ND at night. A lot depends on the location of the Tx site relative to the location in the market of a major portion of the target-demo population. Remember, when coverage AREA is the prime consideration, everything else being equal, ND almost ALWAYS covers more area than even the simplest directional pattern. The only exceptions to this rule are in markets where there are regions of anomalously high "soil" conductivity (salt water.for example). And in such cases, if you don't include the area of water covered (which the FCC does not), you will probably find that, from the standpoint of coverage area (NOT necessarily population, though) ND always beats DA.

 

[DanStrassberg]

These former daytimers - "Class D" stations - are not required to cover their city of licensee at night. As a result, some have built their night facilities on a completely different site.

Just recently however, Media Bureau Engineering, without issuing a NPRM, dismissed an application for night service using separate day and night sites by a Class D AM because the NIF contour would not cover any part of the CoL. The comment on the dismissal was that the NIF contour MUST cover at least some part of the CoL. This was an about face in FCC policy with none of the regulatory niceties.

The best example of the old policy that I can think of is WRDT, the AM 560 licensed to Monroe MI. Monroe is midway between Toledo and Detroit and, by day, WRDT operates with 500W from a four-tower array that sends a teardrop pattern north-northeast over Detroit. Coverage of Detroit is actually competitive but Detroit is far west in the Eastern time zone and WRDT was not allowed a 500W PSRA because of interference to a co-channel Canadian station to its north. As a result, from at least November through March, WRDT was unable to cover Detroit during AM drive, the most lucrative part of the broadcast day in the highest-revenue months. The solution was a PSRA with 13W ND from a Folded Unipole antenna on a 990' communications tower called the MoTower in (I think) Southfield, which abuts the north side of Detroit. These facilities have, for many years, been licensed for use during all nighttime hours. Given WRDT's low frequency, this antenna is a little over 200 degrees high, making it the tallest antenna of any US AM. It is used only at night for the secondary night service of a Class D AM licensed to a community more than 40 miles away where the night signal can, at best, be heard only by determined AM DXers.

 

[Ai4i]

All useful comments that answer my question well.

I would suggest allowing stations like WRDT to use both their day site to cover the COL @ night and their Detroit site as a synchronous amplifier, but then, I have always loved the synch amp concept.

 

[DanStrassberg]

Before somebody posts a correction, WRDT's antenna is the tallest NON-SECTIONALZED antenna of any US AM. I don't know whether or not any sectionalized antenna is taller. But I am talking about physical height not electrical height, so the added electrical height of a top load should not be counted.

As for synchronous amplifiers, the advent of GPS, which transmits a standard-frequency signal that can be used for synchronizing multiple AM carriers, has enabled significant improvements in the implementation of synchronized AM transmitters. Nevertheless, the idea remains controversial. Hash zones, where signals from multple transmitters combined to produce sub-audible heterodyning (which was not truly SUB-audible--the signal level would rise and fall and would suffer serious distortion when the sidebands persisted but the carrier became suppressed) have, I believe, given way to standing waves, which have the same problems although they either vary only with location and not with time or vary with both location and time. Still, the time-related effects at any given location vary much more slowly. (I believe the beat frequency can now be reduced to one cycle per week or less). However, even if the beat frequency were truly zero, there could be time-varying phase differences among the carriers. Those phase differences might produce effects just as annoying as the frequency differences of old.

The one story I've heard about the GPS-based system being trouble free relates to a Class C (graveyard) AM. I gather that the high background noise on graveyard channels (at least at night) may totally mask the phase variations. At night, the coverage areas of the synchronized transmitters do not overlap as they do during the daytime.

 

[Ai4i]

You might be referring to WBZT, West Palm Beach, which operates a full power synch amp in Pompano Peach, both Florida. The BBC has been synching their domestic AM networks for several decades, I would guess by multiplying and dividing the carrier of something on longwave or VLF. The only apparent heterodyning is caused by moving in and out of phase between the two strongest/closest transmitter sites.

 

[Schroedingers Cat]

If the station applies for Class B facilities above 250 watts or 141 mV/m @ 1 km RMS, or new facilities for PSSA the radiation often must be reduced to meet the 25% RSS rule. The power asssigned for PSSA rules were based on previous skywave prediction and RSS prediction rules. This is why they tried a couple of years ago to reduce licensed power levels on PSSA operations. The result was a massive protest and the stations have been allowed to continue with their licensed PSSA power levels.

Maybe Dan know the answer to this, how they got from 2 hours past sunset to 24 hour operation using the night PSSA power. It seems like Mark Fowler spearheaded the whole thing along with the 1 kW fulltime for Class IVs/Class Cs.

To advertisers and the nontechnical, allowing Class IVs 1000 watts nights was confusing, when many Class II and Class III stations spent huge amounts of money on DAs just to get to 1000 watts night. A Class IV in Cincinnati made claims that they were the same power as WKRC at night, which was misleading at best. 1000 watts on 550 with a former Class III-B facility is far superior to just about any Class IV.

 

[DanStrassberg]

Maybe Dan know the answer to this, how they got from 2 hours past sunset to 24 hour operation using the night PSSA power. It seems like Mark Fowler spearheaded the whole thing along with the 1 kW fulltime for Class IVs/Class Cs.

I believe that PSSA power is almost always less during the second hour past sunset than in the hour immediately after sunset and is usually less in winter months than in other seasons. I've often wondered how many stations with PSSAs just pick a power from the spreadsheet that shows allowed PSSA powers in the various months and time periods that their authorizations cover and use it in both PSSA hours year round. I suspect that this is the practice at the majority of stations, but I don't know that. Anyhow, I think the full nighttime authorization is, as a general rule, for lower power than that prescribed for the month and hour with the the station's most extreme PSSA power reduction. The single night power may then be used for any nighttime hour in any month. Bear in mind, however, that I did not get this understanding by reading any FCC regulation but rather, I got it by studying the PSSAs for a few dozen of the stations for which PSSAs are posted at CDBS. So I could have added 2 + 2 and gotten something radically different from 4.

 

[Schroedingers Cat]

I suspect that in practice, your suspicions are correct, Dan.

Only a handful of people have probably coordinated the whole nighttime signal and interference prediction to DX, but I would say that if the stations are using the licensed power and pattern, "normal" DX is quite predictable from nighttime studies at a particular location. The main exceptions are in the pre and post sunset and sunrise where some stations are on (usually) day power and pattern.

But my observation is that there seems to be a lot more interferince on signals that are predicted to have low NIF contours than there ever used to be, and part of that is the PSSA/PSRA power being used by Class II-S/Class III-S/Class Ds. These are mainly the former III-As and III-Bs. Interference to Class I/Class A stations is usually more obvious, and usually someone notices.

 

[Mike Sheridan]

You might be referring to WBZT, West Palm Beach, which operates a full power synch amp in Pompano Peach, both Florida. The BBC has been synching their domestic AM networks for several decades, I would guess by multiplying and dividing the carrier of something on longwave or VLF. The only apparent heterodyning is caused by moving in and out of phase between the two strongest/closest transmitter sites.

I remember that one when it was WJNO. It gave them a good signal in Coral Springs where I lived for awhile. There was a dead spot in Boynton Beach between West Palm Beach and Pompano that neither site was able to cover at night.

 

[Schroedingers Cat]

The stations that got the most PSSA power were the stations that already had directional antennas that protected some or all of the same directions as would be required for Class B night operation. Some got more than 250 watts, which allowed them to become Class Bs without an additional application. If these are near the center of the population to be served, they can do very well, some having 100,000 or more population within the NIF contour. This is true even if the power is only about 100 watts. With lower power and directional antennas, they were often oriented to serve the area well for daytime already.