FM radio fade at traffic lights

I think everyone has got to have experienced this. I'm sitting at a red light, and the FM radio station I'm listening to sometimes fades and becomes static-y. I inch up a bit, and it comes back full. Then I inch up again, and it starts to fade.

Yet driving down the highway, it sounds solid.

Why does this happen? Is the antenna somehow out of phase when you're in a certain fixed position? Or am I feeling the full effect of reflected signals when I'm stopped?

potentially a light with some kind of FM emitter nearby ? EMI? using a directional setup that is in phase with certain positions?

geography of location? downtown? rural ? large buildings?

Many areas I drive in So Cal have street lights that have lots of RFI that cover HF through VHF. Some city buses are also huge noise sources that wipe out my radios when I get within about 30ft. Someday I'll take some spectrum analyzer measurements and submit a complaint.
prcguy

CapStar362 said:

potentially a light with some kind of FM emitter nearby ? EMI? using a directional setup that is in phase with certain positions?

geography of location? downtown? rural ? large buildings?

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its called picket fencing

it is from the arrival of out of phase signals, reflections. also called multipath.

LED traffic lights are a major source of RFI in the VHF range.

Radio interference from LED lighting | EMC and Regulatory Compliance

wtp said:

it is from the arrival of out of phase signals, reflections. also called multipath.

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Which can be exacerbated with some of the crappy antenna systems on some cars---

yes

Thayne said:

Which can be exacerbated with some of the crappy antenna systems on some cars---

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if the light was generationg interference it would get worse as you got closer.

Much of the time, it's because a big rig

or large vehicle is between you and the distance station. Thus causing out of phase signals. And as others have posted also.

Rayleigh fading and local noise sources.

And, some large intersections with electric power lines, telephone lines, cable TV lines, and the power/controls for the traffic signals, make a wonderful Faraday cage for AM radio, too. Just two or three cars from the intersection the signal is great, but while passing through the intersection the radio goes blank and then reawakens after exiting the intersection. This happens more in the rural rather than in the urban areas.

I have not noticed it much lately, but I have had trouble in the past when listening to a station that was not local. I usually listened to a 12kw station that is no more than a mile from me when in the city, so the RF is strong, but a similar station 20 miles away...yep, same experience.

On my scanner, or if I kick my VHF off PL when in my work truck....yeah, I get tons of noise when at any of the newer upgraded-signal intersections in our city. There have been tons of upgrades in the past year, so now on VHF I hear controller noise, and the VHF SCADA doesn't help either.

There is an automotive plant I go by everyday, their intersection is getting a huge upgrade, but I probably won't notice any worse issues there, because the plant has 5 VHF repeaters, a paging TX and they are busy...so there is enough intermod to scramble your thoughts! Not sure why this plant continues to use 50 watt VHF repeaters for all functions of a 100 acre campus....(10 watts would work...the portables are only 5!) and why they are not on a UHF trunk, which would work much better. I was told that when they opened in 1974 with like 25 employees, that they had about 10 VHF radios on 154.570...and so as they expanded, and so the did the communications need, they added radios and frequencies and just stayed on VHF because MARFAC (Coordinator) had freqs to issue. So for the love of a Ritron on 154.570...there is a mess today. LOL. I would love to see a TRBO system go in there, it would work well.

zz0468 said:

Rayleigh fading and local noise sources.

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I had not heard of Rayleigh fading; just read about it.

Seems like there would be a couple factors here, like the lights, all the lines (power and otherwise overhead), and maybe the control boxes themselves.

It's not an all-out obliteration of the FM signal, nor is it picket-fencing. From what I read...it does sound like a combo of Rayleigh fading (and noise.)

speed

if you look through a picket fence while driving, you can see through it
if you listen to the radio as you drive you can hear it

if you stop at a fence it is hard to see through the pickets
if you stop at the light you will be hearing through the pickets

it is speed related so the slower you go the more you will notice it.
and yes i can agree on the big truck thing blocking signals.

i can not say with absolute certainty it is this, but it fits the best especially with the creep up and it works.
and will guess if you creep up it will go in and out.

So I think this is a really tiny fence

It's a 25 mph road, so I'm going pretty slow. FM radio is 100% full quieting. As I approach the intersection, and slow to a stop...if I stop in the right place, I experience the fade. Creep up a foot or so, then it comes back. Another foot, it fades.

Light turns green, I'm on my way, all is well again. The zone I'm talking about here is about 20 feet. So I suppose it could be a limited picket-fencing effect too.

you got it

a small area with a bunch of metal poles to block and reflect signals, add a few more for a faraday cage...

In-ground loop detector for traffic detection?

Jacob

wtp said:

if you look through a picket fence while driving, you can see through it
if you listen to the radio as you drive you can hear it

if you stop at a fence it is hard to see through the pickets
if you stop at the light you will be hearing through the pickets

it is speed related so the slower you go the more you will notice it.
and yes i can agree on the big truck thing blocking signals.

i can not say with absolute certainty it is this, but it fits the best especially with the creep up and it works.
and will guess if you creep up it will go in and out.

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GREAT example. And yes, this "issue" predates computer driven traffic lights, so you can blame them. It also happens if you stop at a stop sign, and AFAIK those are not yet electronic at most intersections.

It is definitely multipath fading caused by the Differential Time of Arrival (DTOA) of the reflected FM broadcast signal (also known as picket fencing, but only when you are moving fast enough to hear the swish, swish, swish coming from "between the fence posts" /areas of better then worse signal). The signal is arriving from multiple directions due to reflection off large objects like buildings. It is more pronounced at some frequencies because the wavelength of a given frequency may be better suited to reflecting in an urban environment.
When two signals arrive at the same point (your antenna) after traveling different distances from the transmitter (due to reflection along the path) they are going to arrive at a fractionally different point in time. This will cause a phase shift as the two signals are applied to your receiver. If the shift is at or near 180 degrees out of phase a null (dead / weak spot) will appear. If they are 90 degrees out of phase the signal may actually be enhanced. This happens with all things in physics that have sinusoidal waveforms.... whether sound or radio.... and even light. And even a rock thrown into a still pond of water that forms waves. Throw another rock and watch where the two sets of waves smack into each other... some will be cancelled, some will be made bigger/stronger.
So when you move your vehicle ahead (or back) you are moving the antenna to a different distance from each of the two or more reflected signals.... and therefore changing the time of arrival (TOA) of the signals... and also the phase relationship. The waves are hitting your antenna after having traveled different distances. Mathematically, Velocity is equal to Frequency times Wavelength. Time is implicit in each of the components of that formula.... the time it takes for the signal to travel from transmitter to multiple reflected paths to reach your antenna.... to the time it takes the modulated signal to complete one full cycle (frequency).
You can hear this in audible sound in music. Ask any guitarist what a phase shifter box does. Or listen to the Doobie Brothers 70's hit "Listen to the Music".... there is a long section of audio phase shifting in that song about midway through.
This effect is also present with cellphones. Ever noticed how when you move your cellphone the signal improves or gets worse? In the case of cellphones it is usually only inches.... because the wavelength at cellular frequencies is much shorter than at FM broadcast, where you might have to move a couple feet.
This used to be a major problem with 802.11a, b, and g WiFi. It was also known as multipath reflection and used to cause severe dead spots in a WiFi coverage area. With the advent of 802.11n and 802.11ac and Digital Signal Processors (DSP) chips, the WiFi receiver is able to use multiple antennas (antenna diversity) to combine ALL of the various reflected signals that are being received and then the DSP performs phase-shifting to ENHANCE the signal, ensuring that there are no 180-degree out of phase signals.
So why are there not multiple FM antennas on your car that use antenna diversity to eliminate the "traffic light effect"? Cost.... and aesthetics. As an Amateur Radio operator (ham) I don't mind being told my car looks like a porcupine with all the antennas. But the average car buyer is not going to want multiple radio antennas.... or have to pay for them.

scanningisfun said:

In-ground loop detector for traffic detection?

Jacob

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i doubt this. those loops are nothing more than a very low power EMF that the loop can sense the change of the magnetic field from the cast iron of your engine block disturbing the field.