AM Reception At Night

[admin]

Stan: In the days in which I listened to radio 24/7 in the 1960s I noticed that I could only get a few channels during the day but at night I could hear many stations from thousands of miles away. Can you provide a simple explanation for this? Was it competing sighals during the day or perhaps stronger signals emitted by stations at night?

[Scastles]

50,000 watts, generally, and what was called 'night time' skip. Stations like WLS, Chicago, KOMA, Oklahoma City are just a couple examples of these one time AM powerhouses. The reason you were able to pick up these stations at night was due to the 'skip' and because stations across the country which shared the same frequency, or near frequency, were either licensed as daytime only stations, or for low power after sunset. Less congestion at night made the 50,000 watt stations come through easier.

[cjj]

And it's not just the 50,000 watt stations that can be heard further at night. Stan mentioned "night time skip", this is referring to "skywave propagation" which relies on the refraction of radio waves by the ionosphere. The ionosphere is a set of upper atmospheric layers (height 50-1000km) where the air is ionized by solar radiation & cosmic rays. These ionized layers can refract radio waves similar to the way a prism refracts or bends light.

The some layers of the ionosphere are directly affected by the amount of sun falling on the upper atmosphere, so will have a daily cycle as the earth rotates. In particular, the "E" layer, from about 90km-120km mostly disappears at night, making the refractive layer much higher thereby causing radio waves to "skip" further distances. This effect is generally limited to frequencies below about 10MHz, which is why AM (0.52MHz-1.61MHz) radio can be heard from long distances at night, but typically not FM (88MHz-108MHz).

This also the reason that some radio stations are licensed as daytime only, or are required to reduce power at night. Because of the longer distance propagation at night, having stations on the same frequency reduce power minimizes the amount of interference between stations. In the U.S., the FCC has designated a number of frequencies as "clear channel" frequencies. These frequencies generally get assigned to only a few stations allowing these stations to transmit at full power all the time with little chance for interference from other stations. The two "AM powerhouse" stations Stan mentioned, WLS & KOMA are assigned to clear channel frequencies.

[Scastles]

A much better technical explanation for the 'skip', CJ. I must admit engineering was never my strong suit. My technical concerns usually centered around mics, compression and multi-track machines.

[wmthor]

Here a site you might be interested in. It's a listing of Vane Jones' North American Radio-TV Station Guide. Look at the listing by frequency and see which stations were the clear channels.

[cjj]

A much better technical explanation for the 'skip', CJ. I must admit engineering was never my strong suit. My technical concerns usually centered around mics, compression and multi-track machines.

Thanks! Along with being a total nerd, I'm an amateur radio operator and an electrical engineer...

Oh wait, I think those two items fall squarely into the definition of "nerd", huh?

[wmthor]

I remember spending many a night listening to WLS when I was in high school. No wonder I never wanted to get up and go to school. And this was while I was living in Brownsville, TX, which for those that don't know is in South Texas "On The Border By The Sea."

[rictified]

I'm a ham too, KB1OKL and a BCB DXer. I've gotten several continents and many countries on the good old AM radio. Saudi Arabia 1521 is almost a nightly visitor here in MA at night in the winter when the static levels are much lower with a good selective analog receiver. Europe broadcasts 9 KHz apart instead of the 10 KHz apart we do and with good selective receivers and antennas it is possible to pick up many European stations. France, England, Germany and others are fairly common here in MA, also S. America on a good night, the Caribbean and Cuba is in quite a bit on AM. I use old tube communications receivers and the best of the 50's and 60's arguably equal the best of today and are a lot more fun to work on and operate. I regularly listen to AM 740 from Toronto Peter.

[rictified]

I remember spending many a night listening to WLS when I was in high school. No wonder I never wanted to get up and go to school. And this was while I was living in Brownsville, TX, which for those that don't know is in South Texas "On The Border By The Sea."

I get WLS here also in MA, get many of the power houses, although IBOC (HD) which is the scourge of the BCB DXer was starting to kill that with it's 50 KHz wide noise stuck in a 10 KHz wide channel, luckily it's taking off like a lead balloon, especially on the AM band.

[ken_j]

Back in the mid sixties we would spend most summer weekends in nothern Michigan. I would constantly play with the radio to pick up the Detroit and Windsor (CA) stations. A few years back while driving one night I found a New York City station than was playing old standards and big band music. It never came in that good again but I did try on occasion.

[fargobass]

Ken_J's post reminded me of a grad school classmate from 15-20 years ago. This fellow was from southern Michigan, and we were in grad school at LSU in Baton Rouge. He figured out that with the better receivers found in an automobile (compared to what most of us probably have in an inexpensive tabletop radio in our homes), and driving on the interstates in Baton Rouge (which are elevated above street-level throughout the entire city), he could pick up Red Wings broadcasts from Detroit, with minimal static, at night. So, he would drive up and down the interstate for two or three hours at a time, listening to games. I suppose that today, one could listen on the web, saving the gas money.

[rictified]

Ken_J's post reminded me of a grad school classmate from 15-20 years ago. This fellow was from southern Michigan, and we were in grad school at LSU in Baton Rouge. He figured out that with the better receivers found in an automobile (compared to what most of us probably have in an inexpensive tabletop radio in our homes), and driving on the interstates in Baton Rouge (which are elevated above street-level throughout the entire city), he could pick up Red Wings broadcasts from Detroit, with minimal static, at night. So, he would drive up and down the interstate for two or three hours at a time, listening to games. I suppose that today, one could listen on the web, saving the gas money.

He should have gotten the same reception anywhere in the city most likely as AM waves are not line of sight like FM waves are, they bounce off the ionosphere at night. He could have parked in his driveway. The old car radios were really good though, they had to be, they were rugged and more sensitive than most of the cheaper home receivers. Was it WJR 760? If so that's a 50 KW powerhouse that goes across half the country at night.

[fran4001]

Ahh WJR... Indeed still a biggie. I listen here in Scranton PA at night like it was here in town. Same with most of the old biggies West of me. CKLW 800 in Detroit/Windsor, KMOX in St.Louis, WLW, WGN, WHO, WBBM, etc. For whatever reason, maybe directionality/competition issues, the big NY stations just don't cut through like they used to. Still there, but much noisier and less modulation. And they're only 125 miles from me. I still DX alot, and what a thrill it is to hear music of any kind. Back then (from 'late 60s on for me) it was ALL music.

[admin]

Fran: I too remember the excitement of anticipating what new songs I would hear as the night cast its magic over my sleepy town. I often felt as if the ordinary became the extraordinary with the cover of darkness. At last I had arrived. Thanks for your memories as they brought back so very many of my own. There was nothing better than a spoonful of "I believe in magic ...."

[fran4001]

Another fond memory was how many of the AM stations added reverb to fatten the tunes up, sometimes to excess, but still so cool. If you had access to a JC Whitney catalog, you could always order a rear speaker and/or a little under-dash reverb unit too. Or, if you were really cool, like my neighbor Ted Harvey, you got the optional factory reverb and rear speaker in his brand new '67 Pontiac Ventura. Man! How about collecting the local top 40 weekly playlists from the record shop? I had them every week from all the big Scranton PA stations like WARM 590, WSCR 1320, WPTS 1550. Another happy memory was how back then, even in the daytime, we could get WFIL 560, WIP, KYW all in Philly, plus all the big New York stuff. There were 3 times more AM stations available than there were pushbuttons on any radio! And 99% music. That's why now, on the rare moments when I hear an oldie on AM, it stops me in my tracks. To this day, I perform live, I have a few Ipods, a great stereo with thousands upon thousands of CDs, LPs, tapes, 2 hard drives with 3,000 songs in them , yet the thing I listen to at night or on the road? AM radio. Still my all time fave!