AM Radio Reception

May 17, 2005 2:04 PM

Why can I get AM radio stations crystal clear in my car and cannot get them in my house no matter what antenna (that is ones provided by audio equip) I use?
posted by thimk to technology (10 comments total)

One reason may be the reduced fidelity of your car radio versus say a home tuner. There is an inverse correlation between fidelity and reception. Really high end tuners can attend to the signal end of things to overcome this trade-off but the average home tuner has merely adequate reception, but pretty good fidelity. This effect is pronounced in FM; I am not sure about AM. Also, the metal body of your car aids the antenna to give you an overall pretty great antenna in the car.
posted by caddis at 2:43 PM on May 17, 2005

Your car's AM reception is superior because the car's AM antenna is generally well away from any obstructions, while your audio equipment's AM antenna is not only inside your house but also likely near a number of household appliances that are sources of radio interference. I suspect you can reproduce this effect in your car by tuning to an AM radio station and then noting the decrease in signal quality as you pull up to a fast-food drive-through window.

For superior AM reception in your home, try using an exterior AM antenna or locating the AM antenna near a window.
posted by RichardP at 3:02 PM on May 17, 2005

In my experience, small portable radios--particularly the dedicated AM/weatherband type--provide universally superior indoor reception as compared to bookshelf stereos or hi-fis.
posted by kjh at 3:08 PM on May 17, 2005

I was just listening to the AM station in question while driving thru the Burgerworld drive thru. No degradation of signal. Additionally, I get the good reception after driving my car into my garage which, like my house, is made of concrete. Also, it not just one station in question but two.
I have also tried with small weather radios and the like to no avail. I don't want to have to get on of these.
posted by thimk at 5:01 PM on May 17, 2005

thimk,

Does your portable radio fail to work in garage or, like your auto's AM receiver, does it work in the garage?

Are you trying your car radio in the garage with the garage door up or down? Is your garage door metal or wood? The reason I ask is because dry wood is mostly transparent to AM radio waves, so a wooden garage door is sort of like a big window with respect to AM radio.

Carrying a portable AM radio, how far do you have to walk out the door to your home until it starts to work?
posted by RichardP at 5:44 PM on May 17, 2005

Garage Door is metal (Clopay) insulated and the thickest one they make. Also reinforced for hurricanes with i-beams (aluminum).
Portable radio doesn't pick up the station anywhere inn the neighborhood. I have tried a couple of portable radios actually.
posted by thimk at 6:54 PM on May 17, 2005

One possibility is that your indoor stereo's AM receiver is crap. AM reception on a bookshelf stereo is practically a vestigial feature, and if the manufacturer has to make a choice between a receiver that works better and one that's two cents cheaper to make, they'll go with the cheaper circuit, no question.

In a car, the AM radio is more often actually used, so it might have some actual engineering effort behind it.
posted by hattifattener at 9:02 PM on May 17, 2005

Your house is a Faraday cage, especially if it has, say, aluminum siding or steel structural elements; but concrete works pretty well too. It's almost certainly reinforced with rebar.

> The relatively low frequencies (500 to 1600 kHz ...) used for AM transmission correspond to wavelengths of about 200 to 600 metres.... Electromagnetic waves of such wavelength are easily absorbed by large objects. This is why a pocket radio is unsatisfactory when used in a steel frame building. FM transmission, on the other hand, makes use of very high frequencies (VHF), ranging from 88 to 108 MHz ... (these correspond to wavelengths of about 3 metres). Signals in this frequency range, including television signals, are not absorbed by large objects, such as buildings or bridges, but are reflected from them and scattered in all directions.

As RichardP says, you can get an exterior antenna for radio.
posted by dhartung at 10:18 PM on May 17, 2005

TY hattifattener for the best answer so far. House is reinforced with rebar.
posted by thimk at 7:05 AM on May 18, 2005

The experience of kjh mirrors my own. Also, fluorescent lights can play hell with AM reception, turing great swaths of the dial into buzz.
posted by Rash at 9:20 AM on May 18, 2005