HELP 1900mhz and 850mhz

[chris4434]

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it is generally accepted that 800 mhz frequencies (longer wavelengths) travel farther and penetrate buildings better than 1900 mhz frequencies (shorter wavelengths) I have looked for hours on the Internet trying to find a scientific explanation for this, but i cant find one. does anyone know exactly what makes longer wavelengths penetrate objects better than shorter wavelengths? any help would be greatly appreciated. thanks

[Jay2TheRescue]

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it is generally accepted that 800 mhz frequencies (longer wavelengths) travel farther and penetrate buildings better than 1900 mhz frequencies (shorter wavelengths) I have looked for hours on the Internet trying to find a scientific explanation for this, but i cant find one. does anyone know exactly what makes longer wavelengths penetrate objects better than shorter wavelengths? any help would be greatly appreciated. thanks

Without going into physics too much, that is a property of all electromagnetic waves. The longer the wavelength, the farther the wave will transmit. Take television for example. Channel 2 (The longest wavelength in the TV band) can transmit great distances. I remember as a kid I would watch Channel 2 from Baltimore, MD & I would often be able to get Channel 3 from Philadelphia, PA on our TV at our home 50 miles south of Washington DC. Channel Channel 50, broadcasting from Washington DC was unwatchable.

If I can remember right (Physics class was 15 years ago) it has something to do with more peaks of the wave hitting barriers (building walls, for example) diminishing the strength of the wave.

Maybe someone who's been in physics class a little more recently might chime in and add more.

-Jay

[viewfly]

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it is generally accepted that 800 mhz frequencies (longer wavelengths) travel farther and penetrate buildings better than 1900 mhz frequencies (shorter wavelengths) I have looked for hours on the Internet trying to find a scientific explanation for this, but i cant find one. does anyone know exactly what makes longer wavelengths penetrate objects better than shorter wavelengths? any help would be greatly appreciated. thanks

I'll gladly give an explanation for this (mis)understanding. No math or equations. I'll keep it very simple, at the risk of causing more misunderstanding.

A little background:
First, one, if not the greatest revolution in science was the combination by James C. Maxwell, of the laws of electricity & magnetism with the behavior of light. (around 1860's). That is, wavelengths from audio, radio (am/fm bands), microwave, light and up through gamma and x-rays behave the same and are called electromagnetic waves (E&M).

That said:

1) All wavelengths of E&M waves travel just as far in free space, ie a vacuum. Longer wavelengths do not travel farther,or faster, than shorter ones.

2) But we don't live in a vacuum. The difference is that some wavelengths get attenuated by material, either in the air (like water vapor), or solids (like metals, wood,and so on).

3) Generally, the shorter wavelengths (ie higher frequencies) will be attenuated more, but not always.

4)The basic scientific reason is this: The higher frequencies of E&M waves have shorter spacings between the wavelength peaks of the wave. At these short wavelengths, there is a greater chance that there is a greater match to the electron or electronic bands of an atom or element (like copper), or the vibration modes of a molecule or substances (like water, plastics, whatever). Both will cause energy to be remove from the wave and taken up by the material. In a somewhat similiar fashion, EM waves can be reflected instead of attenuated. At the other end of the scale, really long wavelengths (like those from earthquakes) resonate or couple their energy into bigger things. Have you ever seen bridge or building start oscillating from an earthquake? That structure is absorping the energy from that destructive long wavelength.

5) So by attenuation of materials, higher frequencies sometimes do not travel further. And that depends on the thickness of the material. Think of the two way mirrors one sees in police stations on TV. It is a thin layer of aluminium or silver over glass. Light passing through it is reflected partially, transmittted partially, and absorped partially, so one can see both ways. A thicker layer (as in a normal mirror) and all the light is reflected or absorped (mirrors only reflect about 85-90%): a one way mirror. My 2 way mirrored office window kills my GPS signal, unlike my clear auto window.

6)But it is a general rule, not specific. For example light is at the 100's of terahertz level, 10,000 times higher in frequency that mobile phones. Yet, it can pass through 120miles of solid glass (ie fiber optics). But just move the frequency the smallest amount (but still in the 100Terahertz band) and the OH or oxygen hydrogen chemical bond that is part of a glass structure will kill the signal completely. Likewise, add some hydrogen into glass structure, and even the light frequency that went 120miles, will barely go 100 feet. That is why a lot of work in fiber optics is choosing the right specific wavelength of light and chemically adjusting the glass material.

I hope that wasn't too long, yet simple enough. Bascially, it is the attenuation (and reflections) at specific frequencies that kills the wireless signal. At the right combinations of wavelength/environments, even higher frequencies can travel 'further' than lower ones, but as a general rule, in the world that we live in, lower frequenies are better. Why? Because buildings are complex structures with lot's of materials in them that will reduce the signal from a celluar tower.

Sometimes more power can overcome these, and more power or energy can be placed into lower frequencies (for lots of reasons), but it is not the best way to go. The weak GPS signal I receive (1300-1500Mhz) can be killed by water within tree leaves in the summer, but my 1900Mhz cell will come in fine...the transmitter is closer and more powerful.

Did I confuse you more?

[bobolito]

.... Have you ever seen bridge or building start oscillating from an earthquake?...

No, I don't live in California.

But thanks for the detailed explanation. I learned some new things today.

[Charlyee]

I'll gladly give an explanation for this (mis)understanding. No math or equations. I'll keep it very simple, at the risk of causing more misunderstanding
.................................................. .................................................. .................................................. .................................................. .................................................. ..................................
Did I confuse you more?

Viewfly, very clearly explained thank you. I thought about asking my SO to explain the transmission/attenuation theory (since I only worry about the application of Physics), but decided against it, since everytime I have asked him for something like this I have had to listen to a semester worth of lecture.... .......LOL

[viewfly]

Thanks Bobolito and Charylee. It is a complicated subject, and things are left out like scattering,multipath transmissions and more.

But, for a simple example, I found a plot of attenuation (db/km) from oxygen in the atmosphere verus frequency.

Simply put, the permanent magnetic dielectric moment from oxygen in our atmosphere accounts for a factor of 2 more attentuation at 1900Mhz verus 800 Mhz. The numbers are 0.008dB/km at 1900 Mhz and 0.004dB/km at 800 Mhz. Now, over 20km this still doesn't add up to a lot of dB's, so nothing really to worry about, but this is the general trend of things.

This is something to keep in mine. The relative difference between the two frequencies may seem greater than the actual absolute impact. From what I've seen, cell phones put out twice as much power at 1900 as 850, to compensate for some of these losses.

Water vapor is even less of a problem, heavy rain fall being more important, but also making the ground more conductive which can help propagation. You can see the balancing influences in play that make a simple statement misleading.

[Jay2TheRescue]

Thanks Bobolito and Charylee. It is a complicated subject, and things are left out like scattering,multipath transmissions and more.

But, for a simple example, I found a plot of attenuation (db/km) from oxygen in the atmosphere verus frequency.

Simply put, the permanent magnetic dielectric moment from oxygen in our atmosphere accounts for a factor of 2 more attentuation at 1900Mhz verus 800 Mhz. The numbers are 0.008dB/km at 1900 Mhz and 0.004dB/km at 800 Mhz. Now, over 20km this still doesn't add up to a lot of dB's, so nothing really to worry about, but this is the general trend of things.

This is something to keep in mine. The relative difference between the two frequencies may seem greater than the actual absolute impact. From what I've seen, cell phones put out twice as much power at 1900 as 850, to compensate for some of these losses.

Water vapor is even less of a problem, heavy rain fall being more important, but also making the ground more conductive which can help propagation. You can see the balancing influences in play that make a simple statement misleading.

Water vapor can help too. Thick, high cloud cover can actually reflect the signal back to Earth, increasing range.

[bobolito]

... From what I've seen, cell phones put out twice as much power at 1900 as 850, to compensate for some of these losses...

So why is it that GSM phones output a max. of 2W at 850/900Mhz and only 1W at 1800/1900Mhz? Basically, the other way around....

[viewfly]

So why is it that GSM phones output a max. of 2W at 850/900Mhz and only 1W at 1800/1900Mhz? Basically, the other way around....

Your right! I goofed. I let my fingers do the typing, not my brain.

What I meant to write, is something I've wondered about before from a different posting we exchanged. Is the most important reason that 850 seems better than 1900 because of the higher power at 850? Looking at the propagation in an open space, atmospheric water vapor or oxygen can't account for the difference in terms of dB loss (0.008 x 20km(12miles) is only 0.16dB). But a factor of 2 in power is quite a bit, all things being otherwise equal. And with a power factor of two, plus a relative attenuation reduction of 2x, that is 6db over the same cell to tower distance. That is quite a bit more

Perhaps the mobile phone antenna efficiency is not the same, and it is needed to make up the difference.

Do you know if cell towers also put out different power levels at the two frequencies?

Thanks for catching that.

[Steven2420]

Your right! I goofed. I let my fingers do the typing, not my brain.

What I meant to write, is something I've wondered about before from a different posting we exchanged. Is the most important reason that 850 seems better than 1900 because of the higher power at 850? Looking at the propagation in an open space, atmospheric water vapor or oxygen can't account for the difference in terms of dB loss (0.008 x 20km(12miles) is only 0.16dB). But a factor of 2 in power is quite a bit, all things being otherwise equal. And with a power factor of two, plus a relative attenuation reduction of 2x, that is 6db over the same cell to tower distance. That is quite a bit more

Perhaps the mobile phone antenna efficiency is not the same, and it is needed to make up the difference.

Do you know if cell towers also put out different power levels at the two frequencies?

Thanks for catching that.

Wow....what a lifetime of info. Hey Viewfly, I heard the campus of M.I.T. is beautiful this time of year, is that true! I am also enclosing a Lifetime Achievement Award from MENSA. Great work sir!

LOL--

[Charlyee]

Wow....what a lifetime of info. Hey Viewfly, I heard the campus of M.I.T. is beautiful this time of year, is that true! I am also enclosing a Lifetime Achievement Award from MENSA. Great work sir!

LOL--

So Steven, for us that are not fortunate to have your gift of the gab, would the translation be "viewfly you are brilliant"

viewfly, are the long wavelengths more ground hugging and thus less susceptible to attenuation?
Can sound waves be considered the ultimate example in long wavelenghts, and light waves for short?

[Steven2420]

So Steven, for us that are not fortunate to have your gift of the gab, would the translation be "viewfly you are brilliant"

viewfly, are the long wavelengths more ground hugging and thus less susceptible to attenuation?
Can sound waves be considered the ultimate example in long wavelenghts, and light waves for short?

Charlyee, you are such a flattering young lady. How DO you do it! Listen, you are grouped in there with the Technical Geniuses of WA, so don't be so quick to poke at me there.... --lol. Seriously, listening to people like Viewfly, yourself, Bobolito, Tony, Jay, is worth a degree in Engineering. Thanks for the "Backdoor" compliment!

[walkguru]

Thanks Bobolito and Charylee. It is a complicated subject, and things are left out like scattering,multipath transmissions and more.

But, for a simple example, I found a plot of attenuation (db/km) from oxygen in the atmosphere verus frequency.

Simply put, the permanent magnetic dielectric moment from oxygen in our atmosphere accounts for a factor of 2 more attentuation at 1900Mhz verus 800 Mhz. The numbers are 0.008dB/km at 1900 Mhz and 0.004dB/km at 800 Mhz. Now, over 20km this still doesn't add up to a lot of dB's, so nothing really to worry about, but this is the general trend of things.

This is something to keep in mine. The relative difference between the two frequencies may seem greater than the actual absolute impact. From what I've seen, cell phones put out twice as much power at 1900 as 850, to compensate for some of these losses.

Water vapor is even less of a problem, heavy rain fall being more important, but also making the ground more conductive which can help propagation. You can see the balancing influences in play that make a simple statement misleading.

very interesting reading, thanks for that. so i guess the debate contunius over 850 verses 1900.

[Charlyee]

Charlyee, you are such a flattering young lady. How DO you do it! Listen, you are grouped in there with the Technical Geniuses of WA, so don't be so quick to poke at me there.... --lol. Seriously, listening to people like Viewfly, yourself, Bobolito, Tony, Jay, is worth a degree in Engineering. Thanks for the "Backdoor" compliment!

Steven, I think you gave me a compliment in there somewhere, since I see my name mentioned with names like Bobolito, viewfly, and Tony, who I really respect for their knowledge. So thank youu Steve, I am glad that I was able to help you.

[viewfly]

viewfly, are the long wavelengths more ground hugging and thus less susceptible to attenuation?
Can sound waves be considered the ultimate example in long wavelenghts, and light waves for short?

Hey Charylee!

Very low frequency E & M waves do propagate as ground, or surface waves with very little attenuation...they are also 'ducted' sometimes between the ground and ionosphere, following the curvature of our earth. This makes for really long propagation distances. Our navy experimented with ELF (extremely low freqency) waves, ~45 Hz, in order to contact submerged submarines over extremely long distances. Normally submarines must surface to enable radio communications because of the attenuation of sea water...but not with ELF.

For clarity, 'Sound' waves are mechanical waves of compression of air or material. They are not EM waves. They result when you place a electrical wire to a speaker which converts an electrical signal to a mechanical vibration of the speaker, and then air. Or you can think of seismic waves, due to the mechanical movement of shifting rock beds. Sound waves, say of 60hz, cannot propagate in a vacuum; 60 Hz EM waves can. The ELF EM waves the navy experimented with used 13 mile long high power antennas, not far from your area I think, to propagate 45 Hz EM waves along the ground... but you needed an antenna to 'hear' them. Well, maybe there was a slight buzz if you stood near the source.

In my original post, I placed EM waves in decreasing order of wavelengths. Gamma rays have much shorter wavelengths than light. The order was RF,microwave, light, xrays, gamma. Of course these are just names for areas of a continuous spectrum of EM waves.


BTW, thanks for your compliment. I am a little embarassed by it.
And, steven2420, Cambridge, MA is a little cold this time of year.

[Steven2420]

Hey Charylee!

Very low frequency E & M waves do propagate as ground, or surface waves with very little attenuation...they are also 'ducted' sometimes between the ground and ionosphere, following the curvature of our earth. This makes for really long propagation distances. Our navy experimented with ELF (extremely low freqency) waves, ~45 Hz, in order to contact submerged submarines over extremely long distances. Normally submarines must surface to enable radio communications because of the attenuation of sea water...but not with ELF.

For clarity, 'Sound' waves are mechanical waves of compression of air or material. They are not EM waves. They result when you place a electrical wire to a speaker which converts an electrical signal to a mechanical vibration of the speaker, and then air. Or you can think of seismic waves, due to the mechanical movement of shifting rock beds. Sound waves, say of 60hz, cannot propagate in a vacuum; 60 Hz EM waves can. The ELF EM waves the navy experimented with used 13 mile long high power antennas, not far from your area I think, to propagate 45 Hz EM waves along the ground... but you needed an antenna to 'hear' them. Well, maybe there was a slight buzz if you stood near the source.

In my original post, I placed EM waves in decreasing order of wavelengths. Gamma rays have much shorter wavelengths than light. The order was RF,microwave, light, xrays, gamma. Of course these are just names for areas of a continuous spectrum of EM waves.


BTW, thanks for your compliment. I am a little embarassed by it.
And, steven2420, Cambridge, MA is a little cold this time of year.

I wanna say that Cold/Hot is all relative, but I have lived up north, and it just isn't true. Cold is COLD!!!!

Again, thanks for the info!

[viewfly]

Again, thanks for the info!

Your welcome. Nothing brilliant, just my training and profession.

BTW, I wonder if the original poster has read any of this, and got his/her answer??

Hmmm.

[viewfly]

I did a little bit of research, and found the field flush with data. So for some interesting reading, I found for concrete walls that the average attenuation for cell RF to be in the range of 13dB to 35dB; however I consistently found that attenuation at 850Mhz is about 6dB less than at 1900Mhz: meaning that about 4x more signal passes through a slab of concrete at 850Mhz, but a good 20dB can be lost in total. That, and if more power can be output by the cell (and requested by the tower) at 850, then 1900, it will help quite a bit with coverage inside a building.

But with steel rebar in the concrete, what happens depends on the rebar spacing, and it causes deep periodic dips in the attenuation from across the rf spectrum. And with multiple reflection and just having windows in the concrete can change the average total attenuation a lot. I found that a lot of work is going into designing better windows to help RF coverage.

I also attached a plot of water vaper and oxygen loss vs frequency in the atmosphere. The smooth function is typical of losses like these: note for oxygen the biggest loss is at 60Ghz (10,000db/km) but above and below 60Ghz (30Ghz and 300Ghz, it is less, ~0.02dB/km). So it is not just that increasing freq means more loss; sometimes it is less! But you can see how, for oxygen, 850 is about 2x better than 1900.

viewfly

[viewfly]

note for oxygen the biggest loss is at 60Ghz (10,000db/km) but above and below 60Ghz (30Ghz and 300Ghz, it is less, ~0.02dB/km).
viewfly

obviously, it is 10.000 dB/km (a decimal point , not a comma), not 10,000dB/km!! The mother of all typos.