QUESTION ABOUT GRAVEYARD STATIONS

[radiotower40]

The question I have is, are there any 100 watt local channels left? The fcc has them
grandfathered so if they were licsened at 100 watts they be allowed to stay at that
power. The only one I know of is on 1230 in Canada, and I'm not sure if they are still
on.

The second part of the question, and what I'd be very interested to know is how they
would perform coverage wise, both day and night, against all the 1kw stations on those
frequencies?

I know it would be different in places due to ground conditions and tower heights. I would
just like to know how many miles of listenable signal you could expect from a station like this. Thanks

 

[BobOnTheJob]

Depends greatly on how many others stations are on the frequency. There's a station near here than runs about 35 watts at night (not a graveyard--but one channel away from a graveyard freq) & when they switch to that power during the day, you can tell what song is playing at 35 miles. In the Ohio Valley, 100 watts on a graveyard into a 200' tower 'might' get you 3 miles of listenable coverage at night. A 1000 watt graveyard here does good to go 7 miles at night. A little know fact outside of technical circles is that each time you quadruple the power, the signal strength only doubles. So a 1000 watt station is not 8 times stronger than a 62.5 watt station...it's 4 times a strong. So your 100 watter would be approaching 1/3 the strength of a 1000 watter.

 

[rfry]

The second part of the question, and what I'd be very interested to know is how they would perform coverage wise, both day and night, against all the 1kw stations on those frequencies?

Here are some numbers for the conditions shown, assuming a good AM receiving system, and no co-channel or adjacent channel interference:

Frequency = 1240 kHz
Radiator Efficiency = 306 mV/m at 1 km for 1 kW of applied power*
Path Conductivity = 5 mS/m**
Received Groundwave Field Intensity = 150 µV/m***

Applied Power Distance
1,000 W 50 miles
100 W 30 miles

These distances are based on the FCC's propagation charts for the frequency, radiated power, and ground conductivity.

With nighttime interference their useful coverage radius could be less than 1/6th of those distances, as noted by BobonTheJob. Nighttime, secondary coverage from these facilities could extend for hundreds of miles if no interference was present.

* this radiator efficiency is typical of a 1/4-wave monopole using 120 buried radials, each 1/4-wave long (free space)

** Conductivity of 5 mS/m is about an average for a terrestrial path in the US

*** 150 µV/m is about the minimum field needed for useful (if somewhat noisy) reception on a good AM receiver

 

[w9wi]

...
Path Conductivity = 5 mS/m**
...
** Conductivity of 5 mS/m is about an average for a terrestrial path in the US

I don't know if it can be emphasized enough that this figure is WILDLY variable across the country.

The charts in the FCC regulations suggest the groundwave field strength at 50km and 1240KHz nearly quadruples if the conductivity goes from the 5mS/m typical around here (Nashville) to the 15mS/m typical of southern Wisconsin. Conductivity is even higher on the Great Plains.

 

[rfry]

I don't know if it can be emphasized enough that this (conductivity) figure is WILDLY variable across the country.

My post was made to respond to the OP in a way showing the difference in coverage based on radiated power, when other parameters are held constant.

Below is a link to a more comprehensive analysis showing the effects of frequency and conductivity on the field intensity and coverage radius when a 1/2-wave monopole is used to radiate 1 and 50 kW of applied power -- which illustrates the comments of w9wi.

http://i62.photobucket.com/albums/h85/rfry-100/MW_Coverage_Params.gif

 

[Greg Strickland]

I don't know if there are any 100 watt Class C AM stations on local channels. I just took a quick look at AM query and did not see any 100 watt stations on 1230 in the USA.

FCC rules Section 73.37 (c) permits a 100 watt Class C station to increase power to 500 watts.

In addition to ground conductivity, frequency (1230 vs. 1490) and antenna efficiency, night time coverage will be seriously limited by interference. Most graveyard channels in the central part of the USA have so much interference at night that coverage is limited to approximately the 25 mv contour of the desired station.

 

[DanStrassberg]

Most graveyard channels in the central part of the USA have so much interference at night that coverage is limited to approximately the 25 mv contour of the desired station.

25 mV/m also seems to be a common value on the graveyard channels in the northeastern US. Makes you wonder how accurate that value can be. Near the Atlantic and Pacific coasts, the interfering co-channel signals originate in only half the geographic area that you'll find farther from the oceans. OTOH, in areas of high conductivity, such as the Great Plains, there are fewer stations per square mile, so maybe the 25 mV/m number really does apply pretty much everywhere in the 48 contiguous states. I still find it odd, however, that all of the NIFs I've seen for US Class Cs all seem so close to 25 mV/m.

 

[nmoore6676]

Personal listening and tuning experience tells me that 100 watts or less on any AM frequency would be useless, especially at night or during critical hours. When I was living in North Hollywood following the KFI tower disaster and riding on the Metrolink in the early morning I would lose them between Burbank and Downtown. Even at home it was dicey until after sunrise.

Now here in Iowa, one local class D station at 1480 has a 17 watt nighttime authorization and they are useless until they go to full power and I am maybe three or 4 miles from their tower as the crow flies. I am not sure why they even bother especially now that they could get an FM translator.

I scanned some class C channels and didn't find any at 100 watts, there were a few at 250 still and some at 500 but most are at or near 1KW.

 

[satech]

1580 WBCP in Urbana, IL is licensed for 135 watts daytime and 6 watts nighttime (Class D, non-directional). It is said to have the lowest daytime power of any AM station in the USA:

http://www.radio-locator.com/info/WBCP-AM

 

[Old PD]

The last 100 watt fulltime class IV (now Class "C") station to be granted by the FCC was WTHU in Thurmont, Md. in 1966. Originally 100 watts unlimited, it has since increased it's power to 500 w - D / 400 w - N on 1450.

Shortly after the station got it's CP, the FCC changed the minimum power for Class IV stations to 250 watts. The newer Class "D" low power stations have been granted under different rules, most were daytime only stations that were granted very low night time power.

 

[DanStrassberg]

1580 WBCP in Urbana, IL is licensed for 135 watts daytime and 6 watts nighttime (Class D, non-directional). It is said to have the lowest daytime power of any AM station in the USA:
http://www.radio-locator.com/info/WBCP-AM

WBCP's tower is 239 degrees high. Supposedly, the efficiency of a non-segmented radiator with a (standard) ground system consisting of 120 quarter-wave radials tops out at a tower height of 225 degrees. At that height, the inverse-distance field is ~440 mV/m/kW @ 1 km. If my calculation is correct (somebody please check me), WBCP's daytime ID field of 158.23 mV/m @ 1 km works out to 413 mV/m/kW @ 1 km. So the efficiency may well be correct for the tower height. In any event, WBCP more than meets the minimum daytime ID field for a Class D AM: 140.85 mV/m @ 1 km, which is the field that would be produced by a 54-degree tower with a standard ground system and an input power of 250W. All indications are that WBCP is using a tower (most likely an FM tower) that existed before the station moved to its current site.

 

[cd637299]

As far as the original post, he/she may be referring to VOAR in Newfoundland, formerly on 1230....now on 1210 with 10 kW.

cd

 

[rfry]

... If my calculation is correct (somebody please check me), WBCP's daytime ID field of 158.23 mV/m @ 1 km works out to 413 mV/m/kW @ 1 km.

Working backward from the FCC AM Query value showing the WBCP radiator efficiency as 430.64 mV/m at 1 km for 1 kW of applied power, then their IDF at 1 km with 135 watts is:

430.64 * SQRT(135/1000) = 158.23 mV/m (checks)

If the WBCP radiator efficiency was 413 mV/m at 1 km for 1 kW of applied power, that would mean that their IDF at 1 km with 135 watts would be 151.7 mV/m.

Being picky, the phrase "mV/m/kW at 1 km" is very widely used in broadcast circles, but implies that the IDF at 1 km is directly proportional to the change in power. In fact it is not -- it is proportional the square root of the change in power.

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