Are some AM stations allowed a higher field strength than others?

[stormy01]

There has been a discussion that has gotten kind of mixed up within another board, this is an effort to separate that discussion from the original post there, which was about a communications company who most likely is trying to build a 1500' tower in Crystal Lake, IL for MediaFlo, which is a Mobile TV service offered by AT&T and Verizon.

http://boards.radio-info.com/smf/index.php/topic,120024.msg980212.html#new


This is what I have taken out of that subject:

"An AM station would have nothing to gain from such a tall tower. 1,500 feet is almost exactly one wavelength at 670KHz, the lowest frequency Class A station in Chicago. There is some point to use of a half-wavelength tower but a full wavelength buys nothing."


"It'd be super efficient and would require no ground system! 180 over 180 degrees yields, IIRC, 519 mV/m/kW @ 1 km. The typical 200 (or so)-degree series-fed towers used mostly by Class A AMs produce efficiencies right around 400 mV/m/kW @ 1 km. So for an AM already using a 5/9-wavelength tower, moving to a Franklin would be equivalent increasing power to almost 85 kW and would produce a dramatic reduction in high-angle skywave, which limits groundwave coverage during critical hours and at night. HOWEVER, I don't know whether a self-supporting Franklin is technically feasible. All such antennas that I am aware of (and there are only a few remaining) are uniform in cross-section and guy supported. Franklins, though widely respected for their efficiency and the elegance of the concept, have not won many friends in the engineering community because of the maintenance headaches associated with insulating the top section from the bottom section and driving the two sections at the midpoint. Several well-known installations have been decomissioned in the last decade or so and were replaced by much shorter conventional base-insulated, series-fed radiators.

And there is another consideration. With the breakdown of the Class IA channels, there are probably full-time Class B stations on 670 that limit WSCR's nighttime signal strength to its current value of 2680 mV/m @ 1 km. So, were WSCR to upgrade to a super-efficient Franklin antenna, it would almost certainly have to reduce its power to maintain its current 10% skywave contours. However, we have here a catch 22: As a Class A AM, WSCR is REQUIRED to run 50 kW-U. So WSCR would be required to reduce its power (at least at night) and would also be prohibited from doing so;>"


So I recall reading somewhere that WHO in Des Moines has a field strength equivalent to 85kW, and I found where I had read it:

In 2000, WHO, Des Moines, IA, with a 300 degree tall, sectionalized radiator has the highest reported efficiency of 471.54 mV/m/kW at 1 km. With it's 50 kW input, the radiated field is equivalent to 85 kW input to a "conforming" radiator.

http://www.oldradio.com/current/bc_am.htm

Was WHO "grandfathered" somehow, whereas KOA came under a different set of FCC rules and is sending out the equivalent of 50kW in field strength though not actually 50kW to the tower itself? (The FCC database does say "Comments: LIN RESISTOR" under KOA; did the FCC mean to say LINE Resistor?)

Are some AM stations allowed a higher field strength than others for a given power input to the tower? (I am talking mainly about omnidirectional operation here)

 

[LA_Guy]

The tallest practical single fed tower is 5/8 wave (225 electrical degrees). These are mainly used for daytime stations because while they have great gain on the horizon, they also have a lobe that radiates at high angles (50 degrees and above). At night, this lobe would bounce off the ionosphere and come down to the ground between 15-40 miles from the transmitter where it would interfere with the groundwave. Above 225 degrees, the high angle lobe starts getting larger then the lobe on the horizon, and the gain on the horizon begins falling.

Most class A 50 kw fulltime non directional stations use 175 degree towers. This still has decent gain without the high angle lobing (the lobe hasn't quite formed yet-though you can see it as a 'bump' on the elevation plot).

Half wave towers (180 degrees) are popular too-roughly speaking they have twice the gain on the horizon as a 90 degree (1/4 wave) tower. They also have zero high angle lobing.

The final type of tall broadcast antenna is known as the Franklin. It's basically a dipole stood on end and fed from its center (there is an insulator and network up on the tower). Traditional Franklins are 180 degrees over 180, but you can use other mixes too (120 over 180 is used by KDKA I believe). KPFK in Sacramento and WHO use these too. Though in theory they do not need radials, most stations that have them install them anyway.

The problem with the Franklin is the network on the tower is exposed to the temperature extremes of being outdoors and high up-plus tuning and construction can be critical (KDKA's still isn't working correctly-YEARS after it went up).

Due to the high efficiency of the KOA tower, they make their required field (signal) with only about 35 kilowatts into the antenna. Since they want to advertise being a 50 kilowatt station, they are allowed to put a resistor in their system that converts about 15 kW of their transmitter power into heat.

 

[rfry]

Most class A 50 kw fulltime non directional stations use 175 degree towers. This still has decent gain without the high angle lobing (the lobe hasn't quite formed yet-though you can see it as a 'bump' on the elevation plot).

Maybe "175" was a typo? Most Class As use about a 195 degree vertical.

The link below shows the elevation patterns for MW monopoles of various electrical heights. Its the 195-degree vertical that is about the best compromise between groundwave gain and minimal high-angle skywave.

http://i62.photobucket.com/albums/h85/rfry-100/MWElPatComparison.jpg

RF

 

[stormy01]

"Due to the high efficiency of the KOA tower, they make their required field (signal) with only about 35 kilowatts into the antenna. Since they want to advertise being a 50 kilowatt station, they are allowed to put a resistor in their system that converts about 15 kW of their transmitter power into heat."

Ouch. 30% of their power is wasted as heat? Hopefully when KOA replaced their transmitter from tubes to solid state, the reduced power consumption more than made up for the 15kW lost as heat. Wonder why the FCC would not allow any station to go purely by field strength/power/distance from the transmitter instead of input power to the transmitter? After all, waste is waste. Thought that we as a society are becoming more energy efficient...

 

[LA_Guy]

Most class A 50 kw fulltime non directional stations use 175 degree towers. This still has decent gain without the high angle lobing (the lobe hasn't quite formed yet-though you can see it as a 'bump' on the elevation plot).

Maybe "175" was a typo? Most Class As use about a 195 degree vertical.

The link below shows the elevation patterns for MW monopoles of various electrical heights. Its the 195-degree vertical that is about the best compromise between groundwave gain and minimal high-angle skywave.

http://i62.photobucket.com/albums/h85/rfry-100/MWElPatComparison.jpg

RF

Thanks Richard, I meant 195 degrees. It was definitely a typo!

 

[boiseengineer]

I think international treaty allocations could come into play in determining the maximum sky wave field a station could produce.

As far as KOA it's probably a good tradeoff. A little less signal to get rid of the fading in the Denver "suburbs".

 

[DanStrassberg]

I think international treaty allocations could come into play in determining the maximum sky wave field a station could produce.

As far as KOA it's probably a good tradeoff. A little less signal to get rid of the fading in the Denver "suburbs".

AFAIK, international agreements (with Mexico) are indeed what limits KOA's radiation efficiency at night to the Class A minimum of 362.2 mV/m/kW @ 1 km. But there was no tradeoff involved. There WOULD have been a tradeoff if KOA had elected to construct a less efficient tower of ~165 degrees (530'). Such a tower would have produced the Class A minimum efficiency without the need for the radiation-limiting resistor and would produce a vertical radiation pattern without a high-angle lobe. You have to believe that the 665' (207-degree) tower was built because the station ownership (I think it was GE at the time--maybe NBC, though) believed incorrectly that the FCC would eventually approve the higher efficiency. What I have never understood is why KOA did not at least apply to eliminate the resistor during daylight hours. That would have improved the daytime coverage a little bit.

Also, unless my calculations are wrong, KOA burns up only about 9 kW in that resistor, not the higher amounts that some posters have suggested. The efficiency of the 207-degree tower is ~400 mV/m/kW @ 1 km. 362.2 is a little more than 90% of 400. To reduce the inverse-distance field by 9.5%, you have to reduce the antenna-input power by about 18%. 18% of 50,000 is 9000.

 

[nd2023]

Why would a station intentionally waste 15,000 watts?

 

[DanStrassberg]

Why would a station intentionally waste 15,000 watts?

First off, it's not 15,000 watts, as I explained in the post immediately preceding yours; it's closer to 9000 watts.

The reason for wasting the power has been explained in multiple posts in this thread. As a Class A AM, KOA is REQUIRED to run 50 kW. FCC rules say that--in the absence of some temporary emergency condition--the only way a Class A AM that is not grandfathered for lower power (and there is only one such in the US--KNZR) can operate at a power less than 50 kW is to downgrade to Class B. But downgrading to Class B reduces protections to the station's daytime groundwave service and eliminates protections to its nighttime skywave service. The downgrade would thus diminish KOA's stick value. Treaties with Mexico limit KOA's inverse-distance field at 1 km (at least at night) to the Class A minimum of 362.2 mV/m/kW. That would normally require a tower of ~165 degrees or ~530' at 850. KOA's tower is taller and hence more efficient--207 degrees or 665', which produces an inverse-distance field of ~400 mV/m/kW @ 1 km. Shortening the tower would be expensive. Skirting the top 135' and driving the skirt out of phase with the signal introduced at the base could electrically shorten the tower and eliminate the need for the series resistor but that approach would also be expensive to implement (run high-power coax through the lower 530' of tower) and could easily cause maintenance headaches. The resistor is an inexpensive and reliable approach. Also, the resistor could quite possibly be shorted out during the daytime, producing a slight improvement in the daytime coverage, although, if KOA wanted to do that now, the so-called ratchet rule would very likely preclude the change.

 

[rfry]

As a Class A AM, KOA is REQUIRED to run 50 kW. FCC rules say that--in the absence of some temporary emergency condition--the only way a Class A AM that is not grandfathered for lower power (and there is only one such in the US--KNZR) can operate at a power less than 50 kW is to downgrade to Class B.

Somewhat OT in all of this, but still interesting is the differing approach for AM and FM stations that the FCC takes on such matters.

AM licenses require a certain transmitter power output (TPO) depending on their class, regardless of the r.m.s. ERP of the AM station.

FM licenses require a certain r.m.s ERP depending on their class, regardless of TPO.

If the approach that the FCC takes for FM stations was applied to KOA, there would be no issue with them eliminating their ~9 kW series resistor while reducing their TPO to ~41 kW into their existing stick. There would be no difference in the radiated fields for these two configurations, other things equal.

KOA would still be a Class A because they would be radiating the minimum field that the FCC requires of a Class A. And they would be saving the AC power consumption it took to generate (edit: and modulate) that 9 kW of r-f, and to cool the 9 kW resistor.

Seems like a no-brainer, doesn't it?

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[rfry]

The AC power saved by KOA would be the amount it takes to generate a fully modulated 50 kW TPO less that when running at ~41 kW TPO, plus the power it takes to cool the ~14 kW series resistor that such reduced r-f power (fully modulated) would require when reducing the carrier TPO of a 50 kW AM tx to ~41 kW.

The annual savings would be significant, no doubt.

PS: I missed my editing deadline for my previous post.

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