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norsemantony
EVDO Newbie
Joined: 05 Nov 2008
Posts: 9
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 Posted: Wed Nov 19, 2008 12:08 pm Post subject: How much cable use before signal loss occurs... |
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| How many feet of cable can you run from your outside antenna to inside before you start losing signal strength? thanks |
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n6gn
EVDO Junkie
Joined: 22 Aug 2006
Posts: 555
Location: Northern California
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| Posted: Wed Nov 19, 2008 12:56 pm Post subject: |
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Any length at all adds loss but cable type and length affect the degree. There isn't an easy answer. Good cable like LMR400 has about 6 dB loss per 100' at PCS and about 3.8 at 850 MHz.
There's a nice calculator on the Times Wires site at http://www.timesmicrowave.com/cgi-bin/calculate.pl which will let you figure out many other possibilitites.
In general the question of interest is "when does adding cable length stop being a net help to my situation?" That's even tougher to answer because it depends what benefit the new position provided by the cable length is buying you. The answer to this is very often site dependent, until you reach full LOS to the remote cell site at which point additional cable length is probably going to be of only negative benefit.
A *very* rough rule of thumb is that all else equal (which of course never really applies), doubling antenna height might give 6 dB improvement in signal strength. In fact, if doubling height significantly reduces the amount of foliage, terra firma, etc in between the antenna and cell site, the improvement may be much more than this. As a result, it will probably be relatively uncommon that increasing cable length, in order to increase the antenna height an equal amount, will *not* be a net improvement - at least up to the point that there is so much signal that you'll stop worrying about it.
It's not uncommon to see 20 dB of improvement at PCS due to putting an antenna in a higher/better location compared to lower/poorer. Even at PCS this would indicate that if it requires 300' of LMR400 to achieve this better location, it's still worth it.
n6gn |
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Alex
Site Admin
Joined: 19 Sep 2006
Posts: 2864
Location: Dallas, TX
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| Posted: Wed Nov 19, 2008 1:02 pm Post subject: |
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| n6gn wrote: | | A *very* rough rule of thumb is that all else equal (which of course never really applies), doubling antenna height might give 6 dB improvement in signal strength. In fact, if doubling height significantly reduces the amount of foliage, terra firma, etc in between the antenna and cell site, the improvement may be much more than this. |
you've emphasized the "go higher for better signal" thing before, but i think its best we caution folks to compare their elevation to that of the tower the suspect they are connecting to.
if they are already higher than the tower, i think you'd agree that going higher does not necessarily guarantee any better signal... yes?
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n6gn
EVDO Junkie
Joined: 22 Aug 2006
Posts: 555
Location: Northern California
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| Posted: Wed Nov 19, 2008 1:39 pm Post subject: |
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Alex,
Heights of the cell site and user, per se, make no difference. What matters is the amount of intervening clutter that attenuates signal beyond LOS values. To the degree that height is a predictor of that clutter - most foliage is in the first 100 feet or so above ground, heights do matter.
If this is confusing, just draw a straight line between the actual user antenna (wherever you are considering placing it) and the actual cell antenna (including it's actual location on the tower not the tower itself) and see what is in between. There's no precise way of gauging things but very roughly, for every foot of hardwood foliage (say) on that line, you can expect about a quarter of a dB of extra loss at PCS, somewhat less at 850 MHz.
If going higher doesn't reduce that extra attenuation, going higher won't help. If it does, no matter what height either end is, it almost certainly will.
n6gn |
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Alex
Site Admin
Joined: 19 Sep 2006
Posts: 2864
Location: Dallas, TX
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| Posted: Wed Nov 19, 2008 2:34 pm Post subject: |
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| n6gn wrote: | | Alex, Heights of the cell site and user, per se, make no difference. |
hmm... what i meant was... i have had customers who called us saying they know the tower is at "ground level" and they are at an elevation that is 500, 1000, 2500ft+ higher than that tower.
trees or not, you think going higher than the building they are in would still benefit these folks?
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n6gn
EVDO Junkie
Joined: 22 Aug 2006
Posts: 555
Location: Northern California
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| Posted: Wed Nov 19, 2008 2:48 pm Post subject: |
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If they are having problems - receiving less than a *lot* of signal from their cell, then yes, I think going higher will help since eventually it will get them to LOS which is going to produce plenty of signal. Whether or not that is practical for them is another matter.
n6gn |
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nickjacket
EVDO User
Joined: 28 Apr 2008
Posts: 78
Location: Tucson, AZ
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| Posted: Wed Nov 19, 2008 5:46 pm Post subject: |
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n6gn,
I agree with your antenna height assertion because Line Of Sight can only be just that.
Do carriers limit their coverage by means of tilting the sector panels downward? Maybe Alex meant what he did out of concerns about lining up with the panels? |
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n6gn
EVDO Junkie
Joined: 22 Aug 2006
Posts: 555
Location: Northern California
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| Posted: Wed Nov 19, 2008 7:01 pm Post subject: |
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| nickjacket wrote: | | Do carriers limit their coverage by means of tilting the sector panels downward? Maybe Alex meant what he did out of concerns about lining up with the panels? |
Yes, they sure do. However, even with some considerable tilt, the beams are pretty wide compared to usual tilt angles and the difference due to not being centered in the vertical (elevation) axis is usually not too big a deal compared to the other issues (like lots of foliage, buildings, terra firma etc in between).
I understood Alex to be asking if height in and of itself was enough to void the usefulnes of a user going higher. Thus if there were a user in wooded terrain at 1000' above a valley having a cell site at 100', would it no longer help for the user to raise his/her antenna?
My answer to that was that it would still help if there was intervening clutter causing increased attenuation and drawing a line between the antennas of interest would be the best way to determine what to do.
OTOH If the user were truly overlooking the valley, and thereby already LOS to the cell site, it isn't likely that there would be any problem to discuss at all because the (so-called) free space attenuation alone would not usually be enough to cause a problem within the cell's coverage radius (often 19+ miles for EVDO).
I may not have properly understood his question though.
n6gn |
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Jim_in_VA
EVDO Junkie
Joined: 09 Apr 2007
Posts: 764
Location: On the Chesapeake Bay, Virginia
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| Posted: Sun Nov 23, 2008 7:16 am Post subject: |
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| Quote: | | if they are already higher than the tower, i think you'd agree that going higher does not necessarily guarantee any better signal... yes? |
Different situation in a city vs suburban/rural areas. Moving up in a building exposes your modem to more and more of the sites in the city. The more sites you get exposed to, the higher the pilot pollution level, and thus the greater the damage to the signal you are receiving.
EVDO cannot support an unlimited number of data requests on the same frequency. As the number of requests increases the overall background noise goes up.
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evdoRV
EVDO Fledgling
Joined: 19 Jul 2007
Posts: 22
Location: Colorado
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| Posted: Tue Nov 25, 2008 7:13 am Post subject: |
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Alex wrote: | Quote: | the tower is at "ground level" and they are at an elevation that is 500, 1000, 2500ft+ higher than that tower.
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Alex, this could depend on the tower design needs. For example, if the biggest client base location happens to be at the elevation/s you mention based on the only location the cell provider can put a tower then the engineers might plan for this by installing uptilt base station antenna's. This is typically unlikely but it's certainly possible.
Just to let you know I'm retired as an RF engineer/tech. I designed and installed 800-900 MHz Public Safety Trunked radio systems as well as Cellular sites. Before that I worked for Verizon Wireless back when it was called US West Wireless and later helped design some of the tower sites in Colorado. Many times due to the higher mountains and foothills we would install downtilt antennas because the base station (tower sites) were much higher then the surrounding terrain. Although this is not the norm for typical cell sites. Too high and you cover too much territory. But mostly because the FCC regulates the antenna height above ground and reduces transmitter power out the higher this elevation is. |
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burkem_10
EVDO Fledgling
Joined: 03 Dec 2008
Posts: 13
Location: colon MI
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| Posted: Thu Dec 11, 2008 10:46 pm Post subject: |
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| evdoRV wrote: |
Too high and you cover too much territory. . |
Just a question, but why is covering too much territory bad? I dont really understand, something to do with the FCC? Or would it make the quality of the service worse maybe? |
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n6gn
EVDO Junkie
Joined: 22 Aug 2006
Posts: 555
Location: Northern California
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| Posted: Thu Dec 11, 2008 11:25 pm Post subject: |
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| burkem_10 wrote: | | Just a question, but why is covering too much territory bad? |
Each 1xRTT segment is probably going to have problems above perhaps 20 users or so. Each 1xEVDO segment starts hurting when there are a very few users, sometimes just more than a single user. A higher antenna means more potential users to share the limited capacity.
Also, higher antennas mean other phones/modems on the same RF frequency but a different PN offset (different site/segment somewhere perhaps) will also show up as noise - competing/limiting the users at the same time those users compete/interfere with it.
As rates or # users goes up, the coverage area wants to shrink and the path lengths need to be reduced to provide all users the expected performance.
CDMA manages to trade off capacity vs. coverage to a degree but the aggregate is limited and multiple smaller cells are much better.
n6gn |
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nickjacket
EVDO User
Joined: 28 Apr 2008
Posts: 78
Location: Tucson, AZ
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| Posted: Fri Dec 12, 2008 12:27 am Post subject: |
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That sounds expensive to divide capacity among multiple sites. Wouldn't a carrier want to instead trade site multiplicity for the capacity of a single ideal site that overlooks
an entire area of customers and attenuating obstructions along with comparatively smaller terrain variations?
This afternoon I passed a 10 story building in town that had many 2, 3 and 4 panel groups on most sides of its cement and steel top floor, just below the roof.
There seemed no way that these groups could provide 360 degree coverage, yet coverage maps show saturated areas all over town.
I guess with enough signal overlap between sites that it all works.
But what happens when buildings run out of walls for multiple carriers and other sites can't penetrate the building?
Isn't there an "odd man out" carrier sometimes that has to tolerate a gap? |
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n6gn
EVDO Junkie
Joined: 22 Aug 2006
Posts: 555
Location: Northern California
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| Posted: Fri Dec 12, 2008 9:26 pm Post subject: |
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| nickjacket wrote: | That sounds expensive to divide capacity among multiple sites. Wouldn't a carrier want to instead trade site multiplicity for the capacity of a single ideal site that overlooks
an entire area of customers and attenuating obstructions along with comparatively smaller terrain variations? |
One of the features of CDMA promoted by it's advocates is the ability to combine energy received from multiple sites to reduce errors and improve communications.
Perhaps in the best of all possible worlds [TM] all received energy would be useful for improving communications and none of it would ever be noise, interference or otherwise lost or counter-productive. However, even in this best of cases, the capacity of a cell site (that is, it's entire set of resources both in terms of backhaul capacity and RF communications capacity in the form of transmit power) is finite.
As the level of user demands increases, eventually something has to give. In the real world, one of the things to give is due to the nature of real paths and the high degree of signal loss. Over paths that are on the order of a mile or more and in typical terrain types, the signal level is dropping very rapidly due to loss - absorption and blockage due to the environment. As a result, the amount of signal that reaches the user or base antennas may be ten thousand or even a million times smaller than it would be in a clean, LOS environment.
The user radio is limited to available transmit energy storage by the battery and has a limited antenna size (aperture) limited by "what will fit in your pocket". As a result, as the radio paths (cell size) get longer the amount of waste eventually reaches the point where a given data rate is no longer possible.
Thus the capacity and coverage limitations are fundamental attributes of the physical world. the only fix for them is to improve the radio paths, either by making them more LOS or shortening them.
If network operators chose to build systems out of "monster cells" where there were good or at least better paths to the user base, some of the factors could improve but at the same time the number of users trying to share the resource (the cell) would increase as radius squared. Going from a 1 mile radius to a 10 mile radius could increase the users by 100:1. This kind of increase puts a burden on both the backhaul to support all that communications and also on the protocol to simultaneously share the cell efficiently. Eventually this also requires more spectrum at the same time it risks reducing the amount of spectrum reuse that is possible.
An extreme case of this kind of modification can be seen in satellite internet access where one or a few satellites try to serve a very large geographic area. As you may know, this approach does not scale well and when very many users are involved (as with the satellite services) the degradation due to sharing a common resource can become pretty limiting. Along with the latency problem that is associated with satellite ISPs the slowdown due to many simultaneous users has limited the attractiveness of this kind of service.
In a terrestrial environment, the least wasteful approach when higher data rates are considered is achieved with very high cell density and very short radio paths. This is roughly what happens with WiFi hotspots - they don't work very far from [Starbucks] but they can serve a few users who are close pretty well.
In a mobile comms environment distributed antenna systems and multiple small systems such as nano/pico/femto-cells tend toward this direction. However, these smaller systems have to be funded by fewer and fewer users and a stand-off is eventually reached. This is the crux of the Last Mile Problem.
n6gn |
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SlyFerret
EVDO Heavy User
Joined: 19 Mar 2007
Posts: 116
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| Posted: Sat Dec 13, 2008 1:01 pm Post subject: |
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| burkem_10 wrote: | | evdoRV wrote: |
Too high and you cover too much territory. . |
Just a question, but why is covering too much territory bad? I dont really understand, something to do with the FCC? Or would it make the quality of the service worse maybe? |
Grab a flashlight (like a MagLight) with a beam that you can adjust. Point the light at a point on the wall when the beam is focused on that point, and compare that to when the beam is spread out.
The light is more intense when it's focused on one point than when it is spread out, even though the amount of light from the bulb is the same.
It's the same idea with the cell cites. The carriers try to focus their RF energy in the area that they wish to cover. Sending signal up is wasteful if your subscribers are within a couple hundred feet of ground level.
-SF |
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