Signal Delay Question

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lamedog

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Hi, newbie member with newbie question, just wondering:

If I am within a small office building listening to the radio (AM or FM) is there a delay in receiving the signal due to the building and surroundings? Or would I receive the signal at the same time as if there was no building?
I know the signal won't be as strong within the building, but will the signal be delayed?

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Rt169Radio

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No the signal would not be delayed,that would only happen if you listened to am/fm radio from online.The only thing that would happen is the signal might get weak and statically.
 

zz0468

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Since the speed of light is less through objects such as walls, yes, there would be a delay. It would be too short to notice, and would take some pretty accurate test equipment to measure.
 

n5ims

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The building wouldn't cause any delay in the signal as others have said. One thing you may notice if you're watching something live (like a baseball game) and listening to the broadcast of that game on your AM/FM radio is there may be a 6 to 8 second delay in the audio (the runner may be on base when the announcer starts talking about the hit that put him there). This is due to how signals are processed for the broadcast "HD" signal. It takes this amount of time to process the audio for transmission on the HD channel and since the HD radios will switch between the HD signal (if available and strong enough to decode) and the normal audio stream, this delay is added to the standard signal to keep them in sync.
 

UPMan

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Additionally, if it is a local game, you'll often hear the call much earlier on a local radio station than on TV. The remote from the local radio station goes directly to the station, then is broadcast. For TV, the signal goes up to a satellite, down to a network ground station (usually in LA or NYC), back up to the satellite, then back down to your local TV station, then to your TV set. Not counting processing at each node, that TV signal is going much farther...
 

lamedog

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Thanks for the response so far.

Just want to confirm - if I had an oscillator that is accurate to the nanosecond; I would receive (say a beep) at the exact same nanosecond as if there was no building? Or would there be a small delay?
 

kb2vxa

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Pardon me a bit slow on the uptake.
"Since the speed of light is less through objects such as walls, yes, there would be a delay."
The speed of light is the universal constant expressed as C (E=MC2) so please explain.

"...if I had an oscillator that is accurate to the nanosecond..."
That reminds me of a certain computer science teacher who asked this trick question just to confuse her students. Clue: the answer is absolutely true, that's not the trick. We're talking computers here, not physics.
Q: How long is a nanosecond?
A: About eight inches.
Here's your homework assignment class, explain why the eight inches. I won't leave you hanging, if I need to explain I will just to watch you face palm... DOH!
 

zz0468

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The speed of light is the universal constant expressed as C (E=MC2) so please explain.

The speed of light, as expressed in the constant C represents the speed through free space. A wall is not free space, therefore the speed of light is slower than it would be in free space, and would not be represented as a constant.

Stop to think a minute how a lens works. The thicker the glass, the more delay there is for light passing through it.

Think about the velocity factor in coax. That is the percentage of the free space delay that a signal passing through coax travels.

Ever see a lens microwave antenna? I've seen styrofoam used as a lens material because the speed of the wave front is slower through styrofoam than it is in free space.

I could go on...
 

zz0468

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Thanks for the response so far.

Just want to confirm - if I had an oscillator that is accurate to the nanosecond; I would receive (say a beep) at the exact same nanosecond as if there was no building? Or would there be a small delay?

If you had nanosecond resolution in a device that could compare the speed of light in free space, vs through a wall, yes, you would be able to detect the delay.
 

ramal121

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Here's your homework assignment class, explain why the eight inches. I won't leave you hanging, if I need to explain I will just to watch you face palm... DOH!

The distance between your thumb and pinky is a constant, 8 inches. From there you can measure anything accurately.

Actually time and wavelength are reciprocals of each. See Mr. Brown, I WAS paying attention!
 

kb2vxa

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Thanks for the explanation, after all I did ask for one. Oh, speaking of velocity factor it doesn't apply only to coax. Using the formula 300/f in megahertz you have wavelength in free space which comes in handy for figuring out what metre band you're in given the frequency. If you want it in feet it's 492/f and in a bare conductor (V=90%) multiply by .9 or to be quick and simple 468/f. Then there's coax, for solid dielectric V factor is .6 and with foam it's .75 in case you need a delay line or stub filter. Note: to avoid confusion with a measuring meter use the British spelling metre for length.

"The distance between your thumb and pinky is a constant, 8 inches."
No, it's a constant 4 1/2 inches now that I'm fully grown. I'm quite sure that of Andre Roussimoff (you'll recognize him when you look down) was considerably more than 8 inches. Now if you lay three wheat corns taken from the center of the stalk end to end you have a standard inch. Now try laying 24 of them across your hand just for fun.

OK, so WTH does all this have to do with a nanosecond? Cummon, SOMEBODY around here must know SOMETHING about electricity! (Hello, there's your clue for today.)

"See Mr. Brown, I WAS paying attention!"
They call me MISTER Eggers! (My apologies to Sydney Poitier's Mister Tibbs.) OK you were paying attention but you didn't do your homework. You get an F and stay after class for a suitable study period, you WILL be retested.
 
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n5ims

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Thanks for the response so far.

Just want to confirm - if I had an oscillator that is accurate to the nanosecond; I would receive (say a beep) at the exact same nanosecond as if there was no building? Or would there be a small delay?

So we're moving from the practical to the theoretical for your question now. Yes there would be a delay. There may also be differences in when it would be received based on wind direction & speed or even if a butterfly passed gas near the path the signal was traveling. You'd better have a pretty accurate measurement device and correctly factored in the delay from the sensors you use and how they're connected in your calculations.
 

Heyboy

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Perhaps I am missing something in this discussion, but according to my calculations (knowing I am mathematically challenged):

The speed of light is 300,000,000 meters per second, or 300,000,000,000 millimeters per second divided by one billionth of a second (a nanosecond) equals 300 mm per nanosecond or 11.81102362 inches per nanosecond.
 

Token

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So we're moving from the practical to the theoretical for your question now. Yes there would be a delay. There may also be differences in when it would be received based on wind direction & speed or even if a butterfly passed gas near the path the signal was traveling. You'd better have a pretty accurate measurement device and correctly factored in the delay from the sensors you use and how they're connected in your calculations.

Such variations are not in the theoretical at all. Some of us measure and use such delays daily. Of course, for the purposes of the audio conveyed by normal radio communications such variations are vanishingly small. For some other RF applications they are significant. Want to use an X-band passive element phased array antenna? Better be able to predict, control, and measure (measure at least during development, no need to measure in normal use) such delays in the passive phase shifters to less than 90 degrees at the RF involved, or call it 0.025 nanosec, or 25 picoseconds, for a course step.

To extend that premise further, and tie in the OPs original question.

Yes, RF is delayed in passing through various materials, and this delay is dependant on the material and the thickness of the material, such that RF traveling straight to you in space arrives at a different “time” than the same RF having passed through say the brick wall of a building. In general you will not “hear” or be able to detect such changes in arrival time by ear or in normal voice communications. Possibly you could set up such an experiment intentionally, selecting a very low propagation velocity material that was exceptionally thick and actually hear the deltas in arrival time, but I do not think you will find such a situation occurring randomly. On the other hand, the unrelated example of multipath causes such deltas that can be detected by ear fairly regularly.

The delay as RF passes through an object or material, and variations in the delay, is exactly how passive phase shifters work in forming a beam in a phased array antenna. By controlling the propagation delay of the RF through the media, making the path apparently longer or shorter (I say apparent because the physical path might or might not vary, depending on the approach) in the individual phase shift elements you can combine or bend the beam reflecting from (in the case of a front fed system) or passing through (in the case of a back fed system) the array. This delay in propagation time results in a phase shift. Control of this phase shift allows for forming the beam desired.

T!
 
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Token

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Perhaps I am missing something in this discussion, but according to my calculations (knowing I am mathematically challenged):

The speed of light is 300,000,000 meters per second, or 300,000,000,000 millimeters per second divided by one billionth of a second (a nanosecond) equals 300 mm per nanosecond or 11.81102362 inches per nanosecond.

You are correct, if speaking of a vacuum, and RF does essentially travel at the speed of light. However, different materials actually have “different” speeds of light (to waaayyy over simplify). This results in RF traveling at different speeds in different materials or combinations of materials. This applies to every material the RF passes through, brick, water, air, asphalt shingles, etc, not just the cable examples I use below.

If a piece of coax has a velocity of propagation or velocity factor (VF) of 1.00 then RF travels through it as fast as it does in a vacuum, and one nanosecond results in the “distance” you calculated, of about 11.81 inches. But, no coaxial cable has a VF of 1.00, values like 0.65 to 0.85 are common. Coax with a VF of 0.66 will result in the RF traveling about 7.8 inches in one nanosecond, and a VF of 0.68 would result in very close to 8 inches per nanosecond. Ladder line, on the other hand, can have a VF of as high as 0.99, so that the lightnanosecond is pretty close to the vacuum 11.81 inch number.

T!
 
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n5ims

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Such variations are not in the theoretical at all. Some of us measure and use such delays daily. Of course, for the purposes of the audio conveyed by normal radio communications such variations are vanishingly small. For some other RF applications they are significant. Want to use an X-band passive element phased array antenna? Better be able to predict, control, and measure (measure at least during development, no need to measure in normal use) such delays in the passive phase shifters to less than 90 degrees at the RF involved, or call it 0.025 nanosec, or 25 picoseconds, for a course step.

I understand that signal delays are not theoretical, having built phasing harneses for RF as well as needing to keep stereo channels in sync so they still sound correct when listening in mono. My comment was in reference to the OP's wondering if an RF signal in free space vs that same RF signal also traveling through a wall would arrive at the receiver at the same time. In that specific situation, while there would be a difference, it would be nearly impossible to measure. Feeding a signal directly to a 'scope on one channel and through a few thousand feet of cable on another channel it's easy to see the delay on the 'scope display.
 

kb2vxa

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Hold on guys, you're so far off base you're not even close to the ball park. Your 11.8" is correct for radiant energy but there's no way on God's green Earth you can get it down coax or any conductor of electricity. All well and good in fiber optics where V=1 but that's not the subject either.

Why do you keep overlooking the key word "electricity" I keep throwing at you? Toss out light, toss out RF, don't even THINK of AC and here comes the next clue... DC.
 
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Token

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I understand that signal delays are not theoretical, having built phasing harneses for RF as well as needing to keep stereo channels in sync so they still sound correct when listening in mono. My comment was in reference to the OP's wondering if an RF signal in free space vs that same RF signal also traveling through a wall would arrive at the receiver at the same time. In that specific situation, while there would be a difference, it would be nearly impossible to measure.

Not impossible to measure at all, in fact probably very measurable, although specifying a frequency range and wall material would help here. If the brick wall is something like 8” thick and the frequency is in maybe the UHF range I would be willing to bet no special equipment needed. Just two phase matched sample antennas / feedlines (and if they are not phase matched then just understanding the phase relationship at the frequency of interest works also) and a dual channel scope with a high enough sample rate if you wanted to directly sample. You could also feed both samples into separate mixers but with a single LO so everything stays coherent, downconvert to a much more usable frequency, and use an older analogue scope.

The velocity of propagation is related to the dielectric constant of the medium. Cured Portland cement has a dielectric constant of roughly 2.5 ( http://www.asiinstr.com/technical/Dielectric Constants.htm ). Using this we can find that the VF for a cement wall (not counting changes for coverings or rebar) is going to be about 0.63. Assume that the wall is 8 inches thick and the signal in question is on 446.000 MHz, the 70 cm FM simplex calling frequency.

Representing the forward motion of the wave front in degrees of rotation for simplicity from now on. Were the wall not there the RF would travel about 109 degrees of its cycle in 8 inches through the air VF of 1.0 (446.0 MHz yields 360 degrees in 26.482 inches at VF 1.0). The RF will travel 173 degrees of its cycle in the 8” concrete wall assuming it is penetrating the wall perpendicular to the face of the wall. This is a difference of 64 degrees between the open air and through wall. 64 degrees of phase shift should be pretty easy to see on a dual channel oscope. All you need is the two phase matched sampling antennas / feedlines and an oscope with a sample rate high enough to see one full cycle or a fraction of the cycle of the RF, pretty easy to do these days with 500 MHz scopes being almost entry level.

Naturally doing this exercise at HF frequencies yields much smaller phase shifts, say on the order of a degree or two. But the test equipment sees smaller changes easier at lower frequencies.

A hollow cinderblock wall with its air voids would also reduce the phase delta, as would a wooden structure with drywall.

As has been said many times in this thread, you will not hear such a delay by ear, but you most certainly can measure it. In fact, you might get some interesting phase cancellation affects using an FM receiver that could be heard by ear. Not as delays, naturally, but possibly as signal degradation.

Feeding a signal directly to a 'scope on one channel and through a few thousand feet of cable on another channel it's easy to see the delay on the 'scope display.

It will be easy to see only as long as you don’t fall near a 360 degree length on that long run. Why do I say this? Because I have seen a person grab a random long chunk of coax, many wavelengths, and throw it in line, thinking they were going to be shifting phase, only to have the length be near an electrical wavelength, and the actual phase shift change, in degrees of a single cycle and on the scope, being below their ability to discern.

You don’t need thousands of feet, all you need is a portion of a wavelength at the desired frequency. For example, if talking about a 14 MHz 20 meter signal and a coax with .68 VF, the phase shift observed would be the same for a 23 foot, 10.85 inch coax as it would be for a 4804 foot 6.69 inch coax. Or better yet, no coax at all, directly connected to the sample source, on one channel and the other connected via 4780 feet 7.83 inches of .68 VF coax. Phase shift the same, no apparent delay, but in reality 100 wavelengths of delay and you are looking at the zero crossing 7.143 microseconds later but coincident on a dual channel scope, as long as we are talking about a carrier with no frequency or phase modulation present.

T!
 
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