AM Broadcast Power Levels

[seminole791]

For AM stations that broadcast at extremely different power levels day and night, how is that done?

An example is WTWB AM 1570 in Auburndale, FL. They are licensed to transmit 5000 watts day and 13 watts at night.

Is the transmitter able to be lowered to that power or does it remain at full power and most of the power sent into a dummy load or some other power wasting device. I just wouldn't think that a transmitter would be stable to run at virtually zero power.

Or is it similar to FM where you have a low powered exciter that is feeding a big transmitter?

Thanks in advance!

 

[RadioMan203]

When I was at WMMM we had four power levels: 1000 watts, 500 watts, 50 watts and 9 watts. The Harris MW-1 was modified by the late Art Silver of Harris to add a third power level so it would do 1000/500/50 watts. For the 9 watt power we used a LPB TX20 transmitter that was switched in for post-sunset power.

 

[BobOnTheJob]

At WIFE 1580 in Connersville,IN, the powers of 250 watts, 45 watts 13 watts and 4.6 watts are all done by the BE AM 500A transmitter with the adjustment of small potentiometer for each level. At WSEZ in Paoli,,IN (1560), using the same transmitter, I had the studio operator listen on a typical portable radio at the studio about 2 miles away while I dialed the power down toward zero. For him to stop hearing the station, the power meter had to be just a tiny bit above zero...well under 1 watt. At 2 watts, it sounded almost as good as at 250 watts. In the early days of WTRE 1330 Greensburg,IN operating their 500 watt transmitter at 36 watts at night, the high low power switch on the Collins 20V3 tube type transmitter was switched to Low (normally 250 watts) and the operator adjusted the Loading control to it's marked 36 watt setting. At the same time, another switch that reduced the audio level was moved to the 36 watt position. At 6AM, the sequence was reversed to return to 500 Watts. Worked great until the BE AM500 was installed around 1994. Needless to say, this couldn't be automated...a live person did the power changes, but in those days, the station was live from 6AM to 10PM 7 days a week. Ah the good old days.

 

[seminole791]

Thanks for the replies folks! Sounds to me that transmitters can be adjusted pretty well from zero to rated power with minor tweaks.

The reason I brought up WTWB specifically, was because I was able to visit the station back in the early '80's in my early teen years. A friend of the family did mornings at the station and I tagged along one morning to see what it was all about. At sign on the station was operating at their low power level of 13 watts. IIRC, it was operated at that power for 45 minutes and then was switched to 5kw. I remember watching the change take place from the studio as there was a very large glass window that separated transmitter from the studio. There were 2 sets of switches and indicator lights. One of the sets of switches were marked "High Voltage" and the other set were marked "Low" and "High". During the power change, the high voltage was turned "off" and then "high" was pressed, and then the high voltage was reapplied. Of course this happened in about 2 seconds, but I will say there was major changes on the meters (most were on the higher end of the scales!) on that old Gates transmitter when she went to full power. I wish I had taken photos of the studio and transmitter because it was quite the sight to see. I don't know the model of the Gates but I'm pretty sure it was the original transmitter when WTWB signed on the mid '50's. The station is still on the air but I would expect that the equipment has been changed by now, considering the stuff was already old in 1983!

Allright, thats enough of the trip down memory lane.

 

[radiorob2.0]

1580 Georgetown, KY (Lexington) had the ability to run 48 watts at night through their three tower array. The daytime authorization was 10kw. They made night time power using a 1kw transmitter's mod monitor tap. The current from that fed into the common point and made power. The main output was fed into a dummy load. They called the 10kw rig "Blowtorch" and the 1kw rig "Pee-Wee". The plant was rebuilt and a new "Blowtorch" went into service that could run the low power levels and "Pee-Wee" was retired.

1110 WCBR Richmond, KY had P/PSA for three watts one hour before/after sunrise/sunset and about one watt two hours before/after sunrise/sunset. Since other 1110's nulled away from Richmond it actually covered the Madison county at night. This changed when WBT lost their array after Hugo and operated temporarily non-directional. During that time, WCBR was a lost cause inside Madison County.

 

[frankberry]

I was the Chief Engineer for WTWB in the late 60's so I am familiar with the old WTWB transmitter.
It was a Gates model BC-5P2. High power was 5kw and low power was 1kw.
In WTWB's case, they switched in a dummy load with an additional resistor between the bottom of the load and ground. They took their 13 watts from the junction of that resistor and the bottom end of the dummy load.

 

[HadYourPhil]

Did that same thing at 1130 WCBX in Eden, NC. The last "dummy load" resistor
was lifted and the 28 watts allowed was coupled into the tower. Gave us a couple of miles.
What I'm curious about are the poor stiffs allowed but one watt...

 

[radiorob2.0]

Did that same thing at 1130 WCBX in Eden, NC. The last "dummy load" resistor
was lifted and the 28 watts allowed was coupled into the tower. Gave us a couple of miles.
What I'm curious about are the poor stiffs allowed but one watt...

You would be amazed if the frequency is fairly clear at night. The Richmond, KY station I mentioned above running a couple of watts of PSA had almost equal coverage of the graveyard at 1340. Of course that was 1988 when radio stations were manned and Richmond was located in a dead zone for the frequency. This may not be the case today.

 

[RadioMan203]

I think there's an AM in Connecticut that had a nighttime power of 3 watts - unless they were automated I can't see a station paying for talent with such a limited power, especially at night.

Like radiorob2.0 mentioned, a clear frequency would do wonders for a low power station. My station with 9 watts could be heard 20 miles away. Granted, it didn't boom in and was far from full quieting but I would listen to it on the way back to my home.

 

[rcs]

WEXT (later WMLB) 1550 in West Hartford, CT has a 2.3 watt pre sunrise authority (PSA) when I was first chief engineer there in 1978. Our day power was 1,000 watts from a RCA transmitter and for the PSA power we ran it at 250 watts into a dummy load and tapped off 2.3 watts to feed the antenna. The trick was finding an RF ammeter with a low enough scale to set the power properly. As a daytime only station they took advantage of the low power PSA to come on the air at 6 am on those months that sunrise was later than that. It depended upon skywave interference from Canada of course but there were mornings I could hear the station at home on a table radio with 2.3 watts and I lived several miles from the transmitter. In 1986 we moved the transmitter site to Bloomfield and went full time so there was no need for the PSA after that.

 

[frankberry]

Back in the early 70's, WGTO, Cypress Gardens, Florida operated with 50,000 watts daytime. They obtained FCC permission to operate with 250 watts during Specified Hours.
The Specified Hours license amounted to pre-sunrise authority.
The two WGTO transmitters at that time were a 50 kilowatt RCA model BTA-50G and a 10 kilowatt RCA model BTA-10H.
To produce the 250 watts, I added the 1 kilowatt power cutback option to the old 10H transmitter and then designed a dummy load to dump 750 watts into a bank of resistors and the remaining 250 watts into the Common Point of the the 50 kilowatt Phasor.
The old 10H modulated like crazy at 1 kilowatt. Even at that low power level, the signal was easily heard from Tampa on the Gulf coast to Melbourne on the Atlantic coast.
I was amazed at the pre-sunrise coverage. Of course, the frequency was 540 kHz. That helped.

 

[seminole791]

frankberry, thanks for sharing your personal insight about WTWB. I appreciate you taking the time to post about this since you have first hand knowledge of the station. What kind of coverage did WTWB have with 13 watts? Was it limited to Auburndale, or did it cover a good portion of Polk County?

 

[Lou_S]

I had a pair of Gates BC-1G's when post-sunset authorization started in the 1980s. One was in Eugene, OR and the other in Lakeport, CA. Both stations got 50 watts PSST, so what I did was rig up a voltage divider to drop the DC voltage to the finals, going from their normal 1500 volts at low power to about 600 volts.

The neat part was it was easy to access the B+ after the modulators imposed the audio on it. So the entire mod consisted of one high voltage relay and a couple of power resistors. There was no need to adjust the modulation when the divider was switched in line.

The procedure was to switch the transmitter from high to low power (which was used pre-sunrise), then increment one step on the remote control and hit "lower" again to drop it to 50 watts. I also had diode logic to kick the transmitter to low power if the PSST position was activated while the transmitter was still at high power. The antenna current was on both remote control positions, with the second one calibrated for the 50 watt PSST. Worked great, and decreasing power in two steps gave the receiver's AGC time to recover. Most listeners never even noticed the cutback, in fact I got a friendly query from one of the other market CEs who lived several miles out of town and had a hard time believing we were actually running 50 watts.

The Eugene station was on a quiet channel - it was midway between the two dominant stations, and both already had deep nulls in line with Eugene. 50 watts covered the metro area just fine.

 

[frankberry]

frankberry, thanks for sharing your personal insight about WTWB. I appreciate you taking the time to post about this since you have first hand knowledge of the station. What kind of coverage did WTWB have with 13 watts? Was it limited to Auburndale, or did it cover a good portion of Polk County?

The WTWB signal isn't all that good to begin with. Short tower and high on the dial.
Their 13 watts covers better than I expected. Good signal for 5+ miles. Listenable signal for another 5 miles or so.
It covers Auburndale, most of Winter Haven and into Lakeland.

 

[DG02816]

At WVCH, we had a small LPB transmitter set for our 6 watt night power, down from 1 kW during the day. The daytime Gates 1 was later adjusted for the 6 watts at night. We get about an 8 mile radius at night, and then CFZM Toronto tramples us. 50 kW into a half-wave high tower at 740 will do that; a fact of life as a Class D on a foreign clear.

 

[DG02816]

Here's a question for those of you who might remember. Years ago , US daytimers on 730,800, 900, 1050, and 1220 were allowed 5 kW for max. power; was this because of the Mexican blowtorches on the frequencies? And if I left any out, just fill 'em in.

 

[frankberry]

Those were probably "regional" frequencies. Limited to 5kw day and 1kw night. 1430 was another regional frequency.

 

[DanStrassberg]

Here's a question for those of you who might remember. Years ago , US daytimers on 730,800, 900, 1050, and 1220 were allowed 5 kW for max. power; was this because of the Mexican blowtorches on the frequencies? And if I left any out, just fill 'em in.

You left out 1570. And I think that 500W-N may have been added while the daytime power limit was still 1 kW for the US Class II-D stations on those six Canadian clear channels. And don't forget that, by US-Mexico treaty, on 1050 and 1220, one each 50 kW-U DA-1 US full-time Class II station was allowed. Those stations (in New York and Cleveland) had to (and still have to) protect the Mexican border. Meanwhile, Canada pretty much had carte blanche to build full-time Class II stations on those six Mexican clear channels. The Canadians also had to protect the Mexican border. The Rio treaty (sometime in the '80s) then allowed Canada to upgrade some of its Class IIs on those six frequencies to Class As. However, they are what I call Class As in name only, because their nighttime skywave coverage is unprotected.

 

[Koredump]

Did that same thing at 1130 WCBX in Eden, NC. The last "dummy load" resistor
was lifted and the 28 watts allowed was coupled into the tower. Gave us a couple of miles.
What I'm curious about are the poor stiffs allowed but one watt...

Oh, God, Phil! That was a punch in the gut! I was the guy who had to kludge that reject load up back in 1973! We got the PSA authorization telegram (remember those) from Ed Wadiak at the Norfolk field office about noon, and Ray Childers was screaming that we had to have our 28 watts the next morning!

I had to drive down from Blacksburg where I was installing the ground system for the WQBX array, rig a tapped-coil power divider out of 1/4" copper water tubing, and rig a relay off the BC-1G's 250 watt nighttime cutback that switched most of the power into the convenient little built-in dummy load, and the remaining 28 watts to the tower....what a kludge! Was so nice back in 1994 when another client with a 2.5 KW. night DA authority bought a Harriss SX-5. Took 30 seconds to tweak the trimpot on one of the power select buttons to get the 2.5 kW. ready.

 

[Bill981]

I was at the 1550 in Lake Geneva, WI when they got the 1W nighttime power, and used it, too! They wanted to carry Brewers night games. I adjusted the power using a FIM, setting the LPB Tx to be exactly 30 dB less than the 1 kW daytime box. Got about 2-3 miles with that watt.