| View previous topic :: View next topic |
| Author |
Message |
yamaben
EVDO Fledgling
Joined: 19 Jan 2008
Posts: 19
|
 Posted: Tue Jan 22, 2008 1:49 pm Post subject: Height of an antenna |
 |
|
I have read in 3 different posts that best signal strength was obtained when mounted lower. It seems to me the optimum height for a dish or omni would be high enough to obtain line of sight to the tower.
Why does it seem that some members get a better signal at a lower altitude (assuming that cable loss is not a factor)? |
|
| Back to top |
|
 |
Alex
Site Admin
Joined: 19 Sep 2006
Posts: 1945
|
| Posted: Tue Jan 22, 2008 2:01 pm Post subject: |
|
|
for the most part, the reason is because cell tower antennas are setup to provide signal for cell phone users and therefore the antennas are pointed down to the ground, where the people are expected to be.
we've had many reports of users who see great signal/speeds at ground floors of their hotel, and terrible/no signal as they get into their upper floor rooms, even though they are sitting near a window. |
|
| Back to top |
|
|
yamaben
EVDO Fledgling
Joined: 19 Jan 2008
Posts: 19
|
| Posted: Tue Jan 22, 2008 2:15 pm Post subject: |
|
|
| I still say it's black magic. |
|
| Back to top |
|
|
yamaben
EVDO Fledgling
Joined: 19 Jan 2008
Posts: 19
|
| Posted: Tue Jan 22, 2008 2:50 pm Post subject: |
|
|
| Is the LMR400 the appropriate cable for between the antenna and the modem? Seems like that is for 2.4Ghz frequency. That is close enough to the evdo frequency I suppose? Do you have a handy Coax chart or table to refer me to? |
|
| Back to top |
|
|
n6gn
EVDO Junkie
Joined: 22 Aug 2006
Posts: 347
Location: Northern California
|
| Posted: Tue Jan 22, 2008 3:03 pm Post subject: |
|
|
| yamaben wrote: | | Is the LMR400 the appropriate cable for between the antenna and the modem? Seems like that is for 2.4Ghz frequency. That is close enough to the evdo frequency I suppose? Do you have a handy Coax chart or table to refer me to? |
LMR400 has a bit less than 6 dB/100' loss at PCS and is probably a pretty good value.
I think I would question the previous explanation for differences in performance at various elevations being due to the down-tilt of the base antenna though. If one is close enough to the base to actually move significantly, in terms of vertical angle, then signals are likely so strong that you'd you'd still have good connectivity, if only signal strength were the issue.
Perhaps with a VERY tall tower at significant distance one might be in that situation but in that case it is even more likely that the top would be LOS to the base and signals would be strong anyway.
To get into the first null of the base antenna, I suspect requires being a lot further above the target elevation than most people can manage. Even with a (maybe) 20 dB null of the base antenna's first sidelobe, it is likely that there will be enough signal in a LOS situation that things will work well.
I'd look for another explanation as to why things might not work as well when high up, such as self-interference, or other types of co-channel interference.
n6gn |
|
| Back to top |
|
|
mooch
EVDO User
Joined: 02 Dec 2006
Posts: 41
Location: sunbury, ohio
|
| Posted: Tue Jan 22, 2008 3:33 pm Post subject: |
|
|
Can a signal bounce off the slope of the roof in such a way that an Antenna would catch the signal better when it's at the same level as the angled roof?
That's what seems to happen to me. If I have the antenna low enough to be level with the sloped roof my signal will better than when I raise it another 6 feet. However, if I raise the antenna another 15 feet or so higher, then my signal begins to climb again. |
|
| Back to top |
|
|
yamaben
EVDO Fledgling
Joined: 19 Jan 2008
Posts: 19
|
| Posted: Tue Jan 22, 2008 4:01 pm Post subject: |
|
|
| The trouble with some of the posts i've read on antenna height, is that they are not very specific as to the details of height and signal strength. I assume most refrences are to the signal being better when at roof level as opposed to 2 to 3 meters higher. Not many people are willing to raise a 15 meter antenna when the gains will be unknown. |
|
| Back to top |
|
|
n6gn
EVDO Junkie
Joined: 22 Aug 2006
Posts: 347
Location: Northern California
|
| Posted: Tue Jan 22, 2008 4:27 pm Post subject: |
|
|
| mooch wrote: | Can a signal bounce off the slope of the roof in such a way that an Antenna would catch the signal better when it's at the same level as the angled roof?
That's what seems to happen to me. If I have the antenna low enough to be level with the sloped roof my signal will better than when I raise it another 6 feet. However, if I raise the antenna another 15 feet or so higher, then my signal begins to climb again. |
It's not impossible, but not terribly likely. There's the possibility of a "sun over water" increase in signal due to the (properly phased) addition of a 'direct' signal and a reflection. ( this is related to the "you can get sunburned worse when out on the water because your skin 'sees' two suns, one direct and one reflected). In theory it is possible to gain 6 dB that way but it take s very precise alignment and moving the antenna relative to the source of reflection a few inches, or having the characteristics of the 'direct' path change, which is likely if the signal is non-LOS, can cause the gain to become a loss.
It is possible to have a better path or signal/noise ratio at lower elevations by going under obstructions, avoiding sources of interference etc. However, for non-LOS situations, it isn't often that these kinds of situations can be depended upon to stay constant with time.
n6gn |
|
| Back to top |
|
|
|
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
