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Why do cell towers need so much height? Does the height increase antenna gain because it may provide directionality?

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closed as too broad by Chris Stratton, pipe, Harry Svensson, winny, Dmitry Grigoryev Sep 25 '18 at 11:23

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    \$\begingroup\$ It gets the antenna above most of the junk at ground level that could block the signal. Besides, can't you see further from higher up? \$\endgroup\$ – JRE Sep 19 '18 at 13:09
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    \$\begingroup\$ "Why cell towers so high?" If they were only 2 feet tall, you'd trip over them. \$\endgroup\$ – Olin Lathrop Sep 19 '18 at 14:04
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    \$\begingroup\$ @Olin Lathrop that's not a technical aspect . \$\endgroup\$ – ObsessionWithElectricity Sep 19 '18 at 14:05
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    \$\begingroup\$ @ObsessionWithElectricity Technical? maybe not, but it is practical. Practicality drives innovation as much as technical advancements. \$\endgroup\$ – ajsmart Sep 19 '18 at 14:10
  • \$\begingroup\$ But it is not a aspect of tower design. They can easily put a fence even around a tower of height of around 2 ft. \$\endgroup\$ – ObsessionWithElectricity Sep 19 '18 at 14:12
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To avoid the possible number of obstacles between the phones and the cell tower. And height position point is often in sight (or almost) from many ground-level points. Obstacles cause reflections, scattering and absorption effects that compromise the S/N ratio.

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As the other answer states, at a lower position, you start to have more issues with reflection, scattering and absorption. In other words, at a lower position, interference from obstacles are more likely to happen, and negatively affect the range.

You also ask:

Does height increases antenna gain because it may provide directionality?

No, gain is determined by antenna geometry, not the altitude. Also, cell towers are designed to be omnidirectional to get the signal to more people. Increasing altitude just ensures that the transmitted signal travels further with less interference.

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    \$\begingroup\$ Antenna gain isn't determined by "circuitry of the antenna transmission system". It is determined by the geometry of the antenna. \$\endgroup\$ – Olin Lathrop Sep 19 '18 at 14:05
  • \$\begingroup\$ @OlinLathrop noted and edited \$\endgroup\$ – ajsmart Sep 19 '18 at 14:06
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    \$\begingroup\$ Not all are designed to be omni directional. A good percentage are directional depending on the geography, population and anticipated use. - former vzw noc tech. \$\endgroup\$ – Passerby Sep 19 '18 at 14:16
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    \$\begingroup\$ @obsession line of sight isn't required, but it helps. The penalty from increase in distance is tiny compared to the gain from a object free transmission path. An empty field is not typical of most areas where trees and buildings and hills exist. \$\endgroup\$ – Passerby Sep 19 '18 at 14:57
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    \$\begingroup\$ Assuming the pole isn't excessively tall. \$\endgroup\$ – ajsmart Sep 19 '18 at 15:24
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The antenna gain is obviously determined by the antenna. What the altitude affects is path loss caused by "ground clutter". Confusingly the literature does actually refer to this as "antenna gain". See e.g. this IEEE paper which talks of a "base-station antenna height gain factor of 6 dB/octave". Or this google book link which refers to the "Okumura–Hata model" (wikipedia):

$$L_U \; = \; 69.55 \; + \; 26.16 \; \log_{10} f \; - \; 13.82 \; \log_{10} h_B \; - \; C_H \; + \; [44.9 \; - \; 6.55 \; \log_{10} h_B] \; \log_{10} d$$

where $$h_b$$ is the height of the base station in meters.

Interestingly it only seems to matter for urban environments in the Hata model, but I suspect that's because a standard above-tree height is assumed for rural base stations.

There's also a safety aspect as the high-power microwave backhaul links may be powerful enough to be harmful to humans.

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    \$\begingroup\$ Ground is considered like a conductor surface, then can gives interferences due the reflection. Tha's why very low planes flyng over flat ground are difficult to be detect by radars \$\endgroup\$ – Gianluca Conte Sep 19 '18 at 14:34
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In rural areas, where there are relatively few customers, the distance to the horizon becomes important. A single tower might serve customers in an area of many square miles.

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  • \$\begingroup\$ Correct. The same single antenna that only covers 6000 people for 0.4 miles in urban nj would cover 600 people for 10 miles in the Arizona desert. Height is needed to help cover the distance. fudged numbers of course it's been a while since I was handling cell tower trouble tickets. \$\endgroup\$ – Passerby Sep 19 '18 at 16:23
  • \$\begingroup\$ 1) Phones are at ground so there is no benefit from increasing height to prevent attenuation by solid objects. \$\endgroup\$ – ObsessionWithElectricity Sep 19 '18 at 17:41
  • \$\begingroup\$ 2) EVEN 2METER TOWER HAS 5 km of Service range: \$\endgroup\$ – ObsessionWithElectricity Sep 19 '18 at 17:43
  • \$\begingroup\$ Yes, the range is not function of the height. The antenna is ever the same. But if there is a bulding or an hill between you and the antenna the communication channel will be be broken or lost also if you are close less than 2Km from the tower. The concept is the same of the lighthouse. Why the light is on the top of a tower and when possible over some hill? The problem is not the distance from the lighthouse (you will see the lighth at 5mn as well at 10mn), but the obstacles between the light and the ships such as an island or the earth horizon. \$\endgroup\$ – Gianluca Conte Sep 19 '18 at 19:08
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Actually, there is a rule of thumb that doubling the height of the antenna increases the gain by 6 dB. A graph has been plotted to examine this rule of thumb. Three horizontal dipole is taken of length one-quarter wavelength, one-half wavelength and one wavelength respectively and transmitted 14.0 MHz signal using average ground conditions (dielectric constant of 13 and conductivity of .005 Siemens/meter).

enter image description here

It is observed that higher the length of the antenna, higher is the elevation angle and higher is the gain. If we observe the gain difference of the three antennas, we find a difference of 6 dB.

enter image description here

This rule of thumb is also proved by a IEEE based research paper[see ref. 2]. Here is a short note from it:

Using the plane earth model, it has been shown that doubling the height of antenna results in 6 dB gain.[...] By increasing antenna height, one extends the distance at which the 40 dB/decade slope begins. The net effect is that where the 40 dB/decade slope exist(beyond 6 km), lower antenna will have a greater path loss than the higher antenna. Since cell radii is commonly too large, most of the cellular coverage are located in this region and the 6 dB gain for doubling antenna is justified.

enter image description here

Reference

  1. Graphs from here - http://k9la.us/Nov15_A_Rule_of_Thumb_for_Antenna_Gain_vs_Height.pdf
  2. IEEE paper - http://rfeng.net/antenna-height-tilt-pattern.pdf
  3. Relationship of antenna size to gain and directivity
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  • \$\begingroup\$ But someone here has said that gain doesn't increases by height. \$\endgroup\$ – ObsessionWithElectricity Sep 24 '18 at 12:02
  • \$\begingroup\$ It actually does. See the research paper. It proves the statement. \$\endgroup\$ – Nilay Ghosh Sep 25 '18 at 7:45
  • \$\begingroup\$ Downvoter: could you care to explain the downvote? \$\endgroup\$ – Nilay Ghosh Sep 26 '18 at 3:07

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