Why are more AM stations found above 900 khz?

Question

Why are more AM stations found above 900 khz?

Answer 1

bandwidth. all US AM stations use 10khz, (most european AM stations use 9khz each). There are twice as many 10khz channels above 900 khz to the top of the band as there are 10khz channels below 900 khz. In other words, 900 khz may be the center of your dial, but it is nowhere near the center of the AM band.

Answer 2

Well, the answer to your question has lots of very technical parts.  Here we go:

1.  The antenna required for a lower frequency channel, say 600 KHz is a really major structure.  A 1/4 wave monopole antenna at 600 KHz is about 400 feet tall and requires an 800 ft diameter piece of land to contain all the radials below the monopole.  On the other hand an antenna for say 1500 KHz only needs to be about 160 feet tall and the station only needs to buy a 320 ft diameter plot of land for the monopole and the radials.  So the initial cost for a station above 900 KHz is often much less unless they have to build a switchable directional array to protect a distant station that has channel priority.  But that is another story.    

2)  The 600 KHz station and other station at the bottom of the AM band have a much larger groundwave radius as a result they are in demand by the big, high power and clear channel stations.  Next time you are in your car notice how well you can hear the lower frequency channels at a large distance often more than 100 miles away.  

3)  The higher frequency channels have a much greater skywave that can travel for thousands of miles especially during the winter.  So many of the stations above 900 KHz are daytime community stations only and must shutdown at night so that their skywave does not interfere with a priority channel on that frequency.  There are some 50KW clear channel stations in the high end of the band.  You can often hear them on a winter night as you tune the band.  

4)  The international stations are on 9 KHz centers whereas the US and Canada are on 10 KHz centers, so the higher frequency channels are subject to an undesireable 1 KHz heterodyne from a strong international skywave signal.  The big clear channel stations don't want this because it chases off listeners.  By way of example, I have talked to stations in over 100 countries around the world when operating in the 1800 KHz ham band including places like Mongolia, Japan, Ukraine, Australia etc. so you can see that AM band skywaves can travel a long distance.  

5)  There are licensing issues that make it more likely to be able to get a license for a high band daytime only community channel.  You will not be able to get a low band frequency unless you buy a station.

Well, I hope this gives you some idea why there are more stations above 900 KHz and why some of the channels are a jumble.  

You asked, and now you probably know more than you wanted to know :-)    

Good luck and 73 from the Fisherman

Answer 3

That's a great question, and true in my market too. With my 6 tube string set, below about 750 kilocycles is all but dead. As I go higher up the dial, I hear more and more stations. All the music, save on Brazilian station is clustered at over 1400 kilocycles, some of it going off the dial and into the newer "extended-AM" range. This has to do with reception; stations at the top of the dial recieve a bit better due to wave length concerns involving atmospherics or somesuch. You can see this ghettoizing on FM, too. The bottom of your dial will be college radio and talk. Even if a station has plenty of watts or close proximity (2 miles!) it'll be rather finicky. The stations at the top seem to be the market flagships, and come in crystaline for a pretty long distance considering their wattage as compared to others lower down. Again, probably atmospherics.