Total posts : 45366
[quote=Ermi Roos]The nit-picking comments that follow do not detract at all from the correctness of the points you have made. … To get 2mV/m field strength at 468 feet requires about 1 mW of radiated power, even without any groundwave losses. The efficiency of the antenna and coupling circuits would have to be slightly over 1%. Such a high efficiency can’t be obtained with a 3 m whip over ground if transmitting the full audio bandwidth is desired.[/quote
In the belief that nit-picking shouldn’t equate to supplying data that differs from another’s post, it should be noted that a ~3-meter x 1/2-inch OD copper radiator on 1610 kHz together with a coil loss of 2 ohms and a ground loss of 8.65 ohms comprise a Part 15 AM radiation system with about 1% efficiency.
The VSWR bandwidth of this system certainly is adequate to provide the r-f/audio frequency response that most modern AM receivers are capable of using. Here is a plot of it: http://i62.photobucket.com/albums/h85/rfry-100/VSWRCompareBasetoCenterLoadedPart15.gif
The ground loss value in my example represents an r-f ground at the high-quality end of those used in most Part 15 AM installations. The goal was to show about the best performance possible, to see if it supported the claims of 1-3 mile coverage for a legal Part 15 AM setup.
[quote]Even if the system were so good that the full 100mW were transformed into radiated power, it would not be possible to produce a field strength that is quite as high as 2mV/ meter at one mile. So, the notion of a 1 to 3 mile range is a dream, especially if the signal quality of licensed broadcast stations is expected.[/quote]
The 2 mV/m contour was chosen because it is a signal that an average, casual radio listener using a cheap indoor radio in a suburban area would notice when tuning across the dial, and could listen to with minimal noise (lacking interference).
But that’s not the coverage limit for those with very good radios and clear paths, a high interest in listening to/promoting the station, and/or Part 15 AM stations using elevated radiating structures (~3-m whip plus the radiating ground lead and radiating ground conductor/flag pole/tower/billboard) that together exceed the 3-m limit for the radiator given in 15.219.