Real life radio puzzle
darryl
For those of you in the states that have driven into the city of Chicago
on Interstate 290, you may notice that as you approach about a half mile
to downtown, all of the FM radio stations in the car get turn to static,
but then when traveling under a bridge, the stations all become clear
again. Also while the radio stations are all static, sometimes a
different channel will pop up. For example if I'm tuned to 93.1,
sometimes I may hear glimses of 96.3 or other stations.
I thought maybe this had to do with my goofy car antenna which is built
into the rear windshield, but I noticed this in other vehicles as well.
To make it even more puzzling, I have a portable music player with an FM
transmitter. I thought one day while in this dead zone, I would just
listen to music on my player, so I turned on the transmitter and synced
up the channels, and even that turned to static! It didn't matter what
station I was tuned to.
Does anyone have any ideas what the cause of this is? Also, I didn't
even think to tune to AM radio to see if the same thing happens or not.
-d
Mensanator
wrote:
> This has been puzzling me for years.
> For those of you in the states that have driven into the city of Chicago
> on Interstate 290, you may notice that as you approach about a half mile
> to downtown, all of the FM radio stations in the car get turn to static,
Gee, I've never noticed that, but I rarely go that way.
But is "about a half mile" before the circle? I would
often get on the Ike at the circle from the Kennedy
and then drive to Buckingham Fountain and I don't recall
any chronic radio problems. But maybe that's past your
problem zone.
> but then when traveling under a bridge, the stations all become clear
> again. �Also while the radio stations are all static, sometimes a
> different channel will pop up. �For example if I'm tuned to 93.1,
> sometimes I may hear glimses of 96.3 or other stations.
Hmm...tricky.
>
> I thought maybe this had to do with my goofy car antenna which is built
> into the rear windshield, but I noticed this in other vehicles as well.
>
> To make it even more puzzling, I have a portable music player with an FM
> transmitter. �I thought one day while in this dead zone, I would just
> listen to music on my player, so I turned on the transmitter and synced
> up the channels, and even that turned to static! �It didn't matter what
> station I was tuned to.
>
> Does anyone have any ideas what the cause of this is? �
Not really. You are near some powerful transmitters
atop the Sears Tower, that would be my guess as to
the source. Maybe it's messing up the intermediate
frequency. When your radio is set to 93.1 MHz, your
local oscillator is actually at 455 KHz different
(I forget whether + or -) so the amplifier stages
have a constant bandwith to play with, but I've never
heard that as a cause of problems, it's always been
the over the air signals that had problems (dead zones,
reflections, etc).
Low frequency signals don't penetrate bridges
although high frequencies do, so generally, AM goes
dead under a bridge while FM does not.
And a strong interference signal would also mess up
a small tramitter in your car unless the radio was
fully shielded and the transmitter signal fed directly
into the radio's antenna jack. Having it within a
couple feet of the car antenna wouldn't help.
Dogstar
"darryl" <darryl@unsolicitedemailreducer_tmbg.org> wrote in message
news:Wu-dnc928PR17b_V...@comcast.com...
Intense radioactivity. But more likely electromagnetic interference from
power lines or transformers (not robots in disguise).
-Dogstar
Rich Grise
Is it near any heavy industry, or large medical facility? It sounds
like somebody is transmitting interference - if you can document it,
(i.e., if it's reproducible) you should call your local office of the
FCC and ask about it.
Or, as has been mentioned, it could be a power line arc/corona, in which
case the power company wants to know, because it's costing them money.
Good Luck!
Rich
Barbara Bailey
news:pan.2008.05.08....@example.net:
I take it that your talking about the area around the 90/94/290
clusterfuck^h^h^h^h^h^h^h^h^h^h^h interchange? I've noticed the same
thing occasionally, but have no solid explanation for it.
But, since FM signals can be blocked by structures, my guess would be
that at that particular point, there's something between the towers for
most of the Chicago FM stations (which I believe are mostly on the Sears
Tower) and the road surface.
Antenna Height above Average Terrain on the Sears Tower is 465m. The
interchange is about 800m away on a straight line. Unless I've completely
bollixed my math, that means the signals that are reaching the road
surface at the interchange are coming in at about a 1:1.75 slope, or
roughly a 40-degree angle. A dense stand of sufficiently tall of
buildings could block the FM signal, and the closer to the interchange
they are, the lower they can be and still have that effect.
Mensanator
Although that doesn't explain why his local transmitter doesn't work
or why the stations come back under a bridge. Then again, when I did
field service and had to listen to customers complaints over the
phone,
it soon became apparent that the customer is always wrong. So the
symptoms
reported may be inconsistent. Or not.
E Z Peaces
Most of the filtering in an FM broadcast receiver is in two IF stages.
If powerful RF overdrives a transistor before that, it can produce noise
at the IF frequency. I suppose overdriving could also cause
heterodyning that would convert an unwanted FM station to your IF
frequency.
You are getting close to the Sears Tower, with many megawatts of TV
transmitters. A bridge would help shield you. Closer to the tower,
most o the TV energy would pass over your head.
There are probably FM broadcast receivers that would be able to filter
out that TV energy.
darryl
Well, so far this seems to make the most sense. Thanks!!
Of course I know very little about radio transmissions and stuff, so I
don't know if it's right. I suppose I could look up all about it and
verify, but I'm in general too lazy for that.
-d
James Dow Allen
wrote:
> This has been puzzling me for years.
This thread reminded me of my own real-life FM problem.
Two FM stations broadcast on the same frequency near me;
one is 60 miles to the north, the other may have a stronger
transmitter and is 150 miles to the south. I never
want to hear the former, but the latter played certain
vintage songs from the 1960's. (I even heard Barry McGuire's
Eve of Destruction on that channel, for the first time in
40 years!)
As I drive around, the tuner sometimes gets one station
and sometimes the other; I suppose there are complicated
diffraction patterns. I wonder: is there a circuit,
or perhaps even a simple modification to the antenna,
that would tune in only the station I want?
Solution is no longer urgent: The 60's station changed
its format to Hip-hop. :-(
James Dow Allen
Patrick Hamlyn
I can supply a circuit which will 100% reject the Northern broadcast (disco?) at
that frequency while at the *same time* 100% rejecting the Southern hiphop
broadcast *on the same frequency* 100% as well.
Futhermore, my circuit is immune to the most complex diffraction patterns!
My circuit also allows for a CD player to be added so you never have to hear
anything except pure Rock and Roll ever again, and no ads.
--
Patrick Hamlyn posting from Perth, Western Australia
Windsurfing capital of the Southern Hemisphere
Moderator: polyforms group (polyforms...@egroups.com)
Jim Mack
> On May 8, 10:18 am, darryl <darryl@unsolicitedemailreducer_tmbg.org>
> wrote:
>> This has been puzzling me for years.
>
> This thread reminded me of my own real-life FM problem.
>
> Two FM stations broadcast on the same frequency near me;
> one is 60 miles to the north, the other may have a stronger
> transmitter and is 150 miles to the south. I never
> want to hear the former, but the latter played certain
> vintage songs from the 1960's. (I even heard Barry McGuire's
> Eve of Destruction on that channel, for the first time in
> 40 years!)
>
> As I drive around, the tuner sometimes gets one station
> and sometimes the other; I suppose there are complicated
> diffraction patterns. I wonder: is there a circuit,
> or perhaps even a simple modification to the antenna,
> that would tune in only the station I want?
Not while you're driving around. The only cure is a directional
antenna -- hard to keep aimed while driving, er, aimlessly.
Which station you hear depends on something called the "capture
ratio".
Unlike AM, on which you can get a mixture of the two stations relative
to their strengths, FM has the ability to completely reject the weaker
of two signals, as long as they differ by at least the receiver's
capture ratio.
In a moving vehicle the relative strengths are going to change moment
by moment depending of reflections, etc. No hope.
--
Jim