Why do we often see that 3 microwave antennas are mounted on the BTS tower? Why 3 not 2 or 1?

إن توفير شدة إشارة مناسبة في منطقة الاستقبال تتحقق عندما تكون زاوية اشعاع الهوائي لاتزيد عن120 درجة كحد أقصى تبعا لمستوى التشويش وبالاتلي لتوفير تغطية بمقدار360 درجة يجب استخدام  ثلاثة هوائيات وربما أكثر لتفادي حدوث فجوات أو مناطق ميتة , هذا حتى يحين استخدام الهوائيات الذكية وتفضلوا بقبول الاحترام

Actually, to save implementation cost, telecom companies avoid doing wired connectivity betweent BTSs,  so they prefer to connect BTSs by Microwave links, so the links will be partially Meshed, in otherword a single site may connected to multi sites, for a single site you may use singe antenna TX/RX, or2 antenna TX/RX in case you need to do1+1 link backup.
but if the site is leaf it can have a single or two links only, that happened alot, but the data coverege (2G/3G/4G) they always use3 sectors, since it is the recommended cell shape.

If  1 microwave antenna on tower Its called singel site and If3 microwave antennas on tower Its called hub site.

Hi,

Sorry for your misunderstanding my friend, that may not a MW antennas, that may be RF antennas which you are asking about. Micro wave antennas are round in shape while RF long rectangular.

The3 RF antennas are connected to3 feeders of Base transreceiver stations (BTS), named alpha,beta,gama. they are the3 sectors of BTS. They are used to provide coverage to mobile or wireless users. By using3 antennas we can cover or transmit signal all around (360 Degree) a mobile tower.

But Microwave antennas are for different use, it is used to connect a BTS Tower (site) with another.

And there is no limitations in number, that is number of MW anttenna on a tower depends on Number of Another towers it connected with.

With my limited Knowledge it's for two reasons mainly first one is like everyone said it helps to cover  the whole surrounding area  and second one is for connect other region BTS'es other than connecting through Fiber or anyother wired means

You can also put1 or2 antennas but usually they cover120 degree angle. To cover the whole area you need360 angle which is the area to complete the circle.

120x3 =360 Degree

well3 directional antennas are helpful to cover the surrouning area in maximum of360 degrees. As you see thy are placed in such a way that they have around120 degree difference to total of360 means circular coverage per BTS 

للتغطيه الشامله فى محيط الدائره وهوه360 درجه على هيئه مثلث

most of the sites have three antennas to cover 360 degrees /120 degrees per sector 

but actually it's based on business need so, we find some sites have 4 antennas due to high traffic in this area 90 degree per sector and another sites located in highway Rd or Railway have 2 antennas 180 degree per sector

Conclusion: number of antennas totally based on area traffic and business need of the site

 Microwave antenna number doesn't related to the coverage(parabolic antenna for transmission between sites).

sectorized antenna almost are3 because sector antenna horizontal coverage isdegree, but in road sites we need2 sector antenna and in rushed area we install4 antenna .

coverage depends on how much sector antenna we install.

to give full recovery area 360 degree