This is a question about AM radio reception in my car.

September 4, 2007 8:14 AM

This is a question about AM radio reception in my car. I currently live two states away from where I grew up, and every now and then I drive back for a visit. When I start getting close, I try to tune in to my hometown public radio station, 910 AM. It seems like I can never get good reception until I'm just 20 or 30 miles out. When I leave my hometown again, though, having tuned into the station in town, I get good clear reception for nearly 200 miles. Why does this happen?

It really does seem to be a regular thing. My antenna is imprinted in the rear right window of my car. Is it possible that once my radio has been tuned into the station, it can maintain it longer? Or am I just imagining things?

posted by thirteenkiller to Technology (15 answers total)

Antenna orientation is very important in receiving radio signals, especially AM. My guess is the antenna in your window is 'shielded' by the car when you're driving towards town (and the station's transmitting antenna) and facing it when you leave. Even if there are structures that might reflect the signal to your car, it'll be so weak you can't pick it up.

Next time you're driving into town, turn the car around about 60 miles away and see if you get the signal.
posted by jdfan at 8:26 AM on September 4, 2007

AM radio does not 'lock on' to a carrier signal, so it's really all about tuning. I assume you are using a digital radio tuner. You might try slightly de-tuning to 900 MHz or 920 MHz and see what happens.

At night, AM radio stations use much less power to transmit, so that may account for some of the discrepancy depending on your routine travel times. The AM radio signal travels much farther during the day.

Your car antenna is most likely tuned for FM radio, not AM radio frequencies, so it's not a major part of the equation.
posted by Argyle at 8:26 AM on September 4, 2007

Are you always driving there in the morning and coming back in the evening, or vice versa? I want to say that Argyle has it reversed and AM radio travels much further at night, but I may be wrong. In any case, travel time might be the main point.
posted by mikeh at 8:33 AM on September 4, 2007

I must disagree with JDFan. Antenna orientation is not very important. And AM wavelengths are so long (on the order of 300 meters) that reflections and shielding are negligible effects.

I must also disagree with Argyle. You're listening to an AM station at 910 KHz, and their transmit power stays the same at night. For complicated reasons, the range increases at night. (It's related to the solar wind.)

Here in Portland OR we used to be able to pick up KSL (Salt Lake City) at night. (Might still be able to, but I don't own a radio to check it.)

I hypothesize that you're arriving in daylight and leaving at night. That would be the reason why.
posted by Steven C. Den Beste at 8:39 AM on September 4, 2007

I'm going to throw out another theory akin to the antena orientation issue: your engine's electrical system is interfering with the signal on the way in, but not on the way out.
posted by Pollomacho at 9:02 AM on September 4, 2007

Re: the day/night distinction. From WikiPedia:

Medium wave and short wave radio signals act differently during daytime and nighttime. During the day, AM signals travel by groundwave, diffracting around the curve of the earth over a distance up to a few hundred miles (or kilometers) from the signal transmitter. However, after sunset, changes in the ionosphere cause AM signals to travel by skywave, enabling AM radio stations to be heard much farther from their point of origin than is normal during the day. This phenomenon can be easily observed by scanning an AM radio dial at night.
posted by pardonyou? at 9:12 AM on September 4, 2007

FWIW, this happens to me too. I can get 710AM out of Amarillo almost three hours away when I'm headed southeast. But coming back, I can't get it until about 30 miles out.

My antenna is the traditional kind, mounted on the right side of my hood.
posted by Bud Dickman at 9:17 AM on September 4, 2007

And it may well be a day/night thing, but I have taken this trip many times, leaving and arriving both during the night and the day.

Or maybe I'm misremembering. It's possible. :)
posted by Bud Dickman at 9:18 AM on September 4, 2007

My antenna is the traditional kind,

IIRC, that isn't the AM antenna. Only for FM. AM antennas are loopy things. Although I'm not sure where the AM antenna is in a car?
posted by smackfu at 10:01 AM on September 4, 2007

I'm usually arriving after dark and leaving in the morning or mid-day.
posted by thirteenkiller at 10:09 AM on September 4, 2007

To reduce night time interference (due to the ionospheric diurnal effect), most AM stations, except powerful "clear channel" stations like WSM (Nashville), WSB (Atlanta), WGN (Chicago), etc., switch to night time directional antenna arrays, which change the omni-directional propogation of their ground waves to multi-lobed patterns favoring certain directions, and cutting interfering signal in the direction of other stations sharing their same frequency assignment. And, Steven C. Den Beste is incorrect in saying all AM stations maintain constant power for both daytime and night operation; to reduce night time common channel interference beyond what directional antenna arrays can do, many AM stations are required to drop their night time power, as a license requirement, as well. Some stations, such as KMTI AM, are required to use directional antenna arrays even during daytime operation, because of the relationship of their transmitter sites to their service area, and to adjacent same channel stations, but their night time power may be less than a 1/10 of their night time power, due to license restrictions.
posted by paulsc at 10:12 AM on September 4, 2007

Traditional AM radio antennae are loopy things, yes. In a compact radio, it's usually coil of wire around a core, inside the case. The thing that extends is the FM antenna. In old skool radios (big wooden cabinet furniture things), there's usually a coil of wire around back somewhere. I haven't seen the inside of a car radio in such a long time I forget what they use. I think it might be the exterior antenna.

Improving reception on AM radios can involve getting a better coil and/or hanging a long wire off the antenna.

I'm gonna second paulsc, though. Stations have highly directional signals these days (especially so on the cluttered FM band). And power is dependent on so many variables (some keep full power, others don't).

It is a bit odd that reception would be better in the day, though. Most AM stations have far better signal propagation at night.
posted by jdfan at 10:49 AM on September 4, 2007

I think we're on to something. From the station website (on the staff bios page, where I hadn't thought to look): WSUI operates with a daytime power of 5,000 watts and a nighttime directional power of 5,000 watts from a transmitter located south of Iowa City.

I drive in from the east. According to this FCC query, there's a station in southeastern Illinois operating at 910 AM, and a couple in Wisconsin, so I suppose my Iowa City station would point their directional array in a generally non-eastward direction? Those seem to be the three closest stations on the same frequency.
posted by thirteenkiller at 10:50 AM on September 4, 2007

Do these day and night coverage maps help at all?
posted by smackfu at 11:07 AM on September 4, 2007

Wow, yeah. That pretty much exactly describes the coverage limits I have been experiencing. Awesome.
posted by thirteenkiller at 11:24 AM on September 4, 2007