Modulation question

[R. Fry SWL]

...Actual field strength measurements of critical points in the radiation pattern were also required on (as I am trying to recall from so far back) a weekly basis.  These measurement points were specified in the station license. ...

To clarify,  non-directional AM stations are not required by the FCC to routinely measure their radiated fields.  Such measurements were/are required only for directional AM stations -- where the critical points typically are in a pattern minimum on a bearing showing that the station limits their radiation in that direction so as to protect the service area of co-channel and 1st-adjacent stations.

The attached clip is an example of this for several stations on 710 kHz (red = full time, green = daytime, purple = critical hours, black = nighttime).  These patterns show the shapes of the radiation launched in the horizontal plane only, and do not show the effects of ground conductivity or the relative fields of nighttime skywaves.

(Continuing with serious thread drift, I know, but some may be interested in what broadcasters need to do vs. ham operators.)

[K3ZS]

On 60M, the ERP is a factor.   If you have antenna gain you must lower the power so it is 100W PEP ERP.   Since there is no AM allowed, we here don't have to consider it a ham band.

[steve_qix]

To clarify,  non-directional AM stations are not required by the FCC to routinely measure their radiated fields.  Such measurements were/are required only for directional AM stations -- where the critical points typically are in a pattern minimum on a bearing showing that the station limits their radiation in that direction so as to protect the service area of co-channel and 1st-adjacent stations.

Good point!  No regular field strength measurements for non-directional.  Over here, mostly worked at medium market directional stations (when AM).  It just became part of the weekly landscape    During that time, we also built a directional FM operation in Worcester, MA.  That was very interesting!!

If you get a chance, check out the radiation pattern for WRKO (680).  50kW out of Boston.  When they change to their night pattern, their signal literally and completely disappears in this direction.  Turns out, a deep null is "pointing" right at my location.  The array is very sharp, and sometimes it is possible to hear the high frequencies of the modulation (when they're on night pattern), but no carrier.

[R. Fry SWL]

If you get a chance, check out the radiation pattern for WRKO (680).

Sure - WRKO patterns attached (click on thumbnail for a bigger image).

[KA2DZT]

The green pattern is daytime and black is night, correct??

The pattern is altered by shifting the phase to the towers (2 or more).

Is the phase altered by changing feed line length in some way or is some other method or equipment use to do this?

Fred

[R. Fry SWL]

The green pattern is daytime and black is night, correct??

Correct.

The pattern is altered by shifting the phase to the towers (2 or more).   Is the phase altered by changing feed line length in some way or is some other method or equipment use to do this?

Typically, directional AM broadcast stations change patterns by using an alternate configuration of the power dividing and phasing networks at the common point of the array.   These networks usually are installed in a cabinet assembly inside the transmitter building.

The same transmission lines are used from those networks to the matching networks at the base of each tower regardless of the day/night directional pattern that array produces.

Here are the values for the WRKO array:
  
           Twr 1 (Ref)      Twr 2           Twr 3
    DAY
Field         1                   0.9               0.5
Phase       0                   40               104
  NIGHT      
Field         1                 1.904               1
Phase       0                  55.8             104

Also some directional AMs use higher transmitter powers during the day than at night, and/or are non-directional day, and directional night.

An exception in this category is KFMB in San Diego, which is 5 kW non-directional day and 50 kW directional at night.

[KM1H]

I can see WCAP 980 towers from above the tree line about 8 miles away and when they switch to night power/pattern they are in the noise unless there is snow on the ground.

[R. Fry SWL]

I can see WCAP 980 towers from above the tree line about 8 miles away and when they switch to night power/pattern they are in the noise unless there is snow on the ground.

WCAP is directional with different patterns day and night, but they use 5 kW transmitter power in both cases.

Their nighttime pattern has greatly reduced fields toward the west compared to their daytime pattern.

[Steve - K4HX]

Look up any station here.

http://www.radio-locator.com

[W4RFM]

Steve I feel your plain on doing directionals. I had an 8 tower 1% array in Atlanta (680) that was fairly stable. (It had to be, we protected WMPS in Memphis and WPTF in Raleigh.)

[Steve - K4HX]

Seems to be quite a few 6-8 tower arrays on that frequency. WCBM in Baltimore runs a six tower array and has a very sharp pattern at night. I've driven by their tower site in years past when I lived in that area. I wonder who the lucky guy is that has to keep that thing in spec?

[R. Fry SWL]

Steve I feel your pain on doing directionals. I had an 8 tower 1% array in Atlanta (680)...

Imagine the pain of a 12-tower array...

[W4RFM]

My friend Dave Hultsman had a 12 tower for the old KLIF in Dallas. And I think there is one in Florida that has twelve.

[KM1H]

WCAP is directional with different patterns day and night, but they use 5 kW transmitter power in both cases.

Their nighttime pattern has greatly reduced fields toward the west compared to their daytime pattern.

Im to the right of Nashua on the pattern map and right on the red line (just about a perfect due North from the towers) but when they switch the pattern it is in the jumble of noise. On a good S meter it looks like more than 40dB down. With snow on the ground I can hear them fine but weaker.

Considering that going from 15W to 1500W is only 20dB it puts things into perspective.

When driving on I-495 when WRKO switches they are unreadable to the West at I-290 and not useable again until almost at Lowell. Daytime they are gone shortly after I get on I-84 off the Pike. Its a huge difference from the coverage of WBZ and a single tower in a salt water marsh.

[steve_qix]

When driving on I-495 when WRKO switches they are unreadable to the West at I-290 and not useable again until almost at Lowell. Daytime they are gone shortly after I get on I-84 off the Pike. Its a huge difference from the coverage of WBZ and a single tower in a salt water marsh.

I seem to recall that, at least at one time, WBZ had a Westerly cardioid pattern - "beaming" over the city, and the remainder of the US (the pattern looked like something that could easily be created by a 2 tower array).  I haven't looked at this in many years, and they may have switched to a single stick.

[KM1H]

That site and some eyeballs when driving by say one tower. They sure put a good signal down the Med when I was fighting for the USA

[steve_qix]

That site and some eyeballs when driving by say one tower. They sure put a good signal down the Med when I was fighting for the USA

Yeah, they've probably changed the antenna setup since the map I saw of their pattern was made, and that map was not new!!

[R. Fry SWL]

According to current FCC data for WBZ, they are using two towers, DA-1.

Tower 2 is located 90^o east of tower 1, and driven with field = 1 and phase = 86.4^o.

Their pattern, and a Google view of their site are attached.

[KM1H]

My bad, I was thinking of another station down on the South Shore.

[steve_qix]

According to current FCC data for WBZ, they are using two towers, DA-1.

Tower 2 is located 90^o east of tower 1, and driven with field = 1 and phase = 86.4^o.

Their pattern, and a Google view of their site are attached.

That's the pattern I saw on the map more than 40 years ago.  I guess it's still the same!!  They really have a strapping signal most of New England.

[DMOD]

And then there is this three tower dog leg array about a 1.5 miles from my house:


http://www.radio-locator.com/cgi-bin/pat?call=WMT&service=AM&status=L&hours=N

They have some interesting phasing and current partitioning to each tower for their nighttime pattern.

Phil - AC0OB

[R. Fry SWL]

I can see WCAP 980 towers from above the tree line about 8 miles away and when they switch to night power/pattern they are in the noise unless there is snow on the ground.  I'm to the right of Nashua on the pattern map and right on the red line (just about a perfect due North from the towers) but when they switch the pattern it is in the jumble of noise. On a good S meter it looks like more than 40dB down.

At an azimuth bearing of zero degrees and 1 km from the center of the array, FCC data for WCAP shows a field of 805 mV/m day and 48.8 mV/m night.

This is a field reduction of 24.35 dB, which means that WCAP radiates only 0.0037X as much power on that bearing at night as they do during the day.