It is a common phenomenon that as you drive under a bridge your car radio reception often, but not always, fades out for a second of so. What makes this happen?
My car radio reception can be continuously received as I drive up and over hills or between high rise buildings downtown. But drive under an overpass on the interstate and it loses signal for a moment. Is it more steel or concrete in the bridge that does it?
Any radio savvy Dopers know?
When you drive between buildings, the radio signals can bounce off of the buildings. You end up still being able to receive the signal even though you don’t have line of sight to the transmitting antenna.
The geometry of an overpass is such that it’s more difficult to bounce a radio signal under it.
You’re more likely to get an AM radio station to bounce under an underpass than an FM radio station. AM radio signals are vertically polarized and FM signals are horizontally polarized. AM signals bounce more easily off of the ground and off of various atmospheric layers than FM signals do (certain radio frequencies bounce more easily through certain parts of the atmosphere as well, but that’s not relevant to your overpass question). Some FM stations these days broadcast with both a horizontal and vertical polarization in their signals specifically so that the signals can penetrate further into buildings and such for better reception, so the polarization aspect of this might not always be true depending on where you are and what your local transmitters use.
Cell phones often don’t work in buildings with metal roofs for similar reasons, though in that case the metal building also often has a metal frame which further cuts down the signal reception.
Followup, if that’s ok. On a long trip recently, we played a Spotify playlist through our phones that were hooked into the car’s radio. About a tenth of a mile before we got to an overpass, the music would stop momentarily. But it would play fine while beneath it (the exception being one long tunnel).
I don’t generally experience my regular fm radio cutting out as much from an overpass as much as my satellite radio is more sensitive to it, and tall trees.
Really?
It has been ages since I have driven and listened to AM radio but I know FM seems to work fine when passing under an overpass and my recollection is AM stations would completely cut out when doing the same.
To be fair that memory is from the 70’s and tech has changed (I just never listen to AM radio anymore).
On the upside I remember getting an AM station from Idaho in Arizona once. AM can go lots further than FM and if conditions are right it can go really, really far.
OK, wow. Didn’t realize there was a polarization issue. I noticed the fade out today when I was listening to a different radio station than I normally do as I listened to a football game broadcast. Maybe my usual NPR station polarizes their signal in both the horizontal and vertical direction since I don’t recall the signal fade so much when I listen to them?
Around here the transmitters are mostly up on a mountain north of the city which provides for decent line of sight to much of the relatively low lying area.
Weird. That is my recollection, as well. I don’t listen to that much AM radio in the car anymore, but I swear when I do, things get dicey downtown when I’m on Lower Wacker or going under underpasses on the expressway in the way that FM never does. The range of AM is quite farther than FM, but within the city, I don’t think I’ve ever had FM cut out on me, while AM has.
AM radio waves are 100 times larger than FM radio waves (300m vs 3m) and do not fit under bridges.
From here:
“In AM operation, the signal is vertically polarized (Vpol). FM signals were originally horizontally polarized (Hpol), but now, circular polarization (Cpol) is used almost exclusively. Sometimes, the signal separation provided by vertical versus horizontal polarization is used to prevent interference between NCE stations. This is especially useful in the educational FM band when interference is often experienced on TV channel 6, and the interfering NCE station can obtain an extra 6dB of interference reduction by using vertical polarization.”
As noted, wavelength and orientation of the overpass wrt the antenna play a much larger role than polarization.