Repeater units are designed to receive signals from a donor site, amplify and rebroadcast the donor sites signals into poor coverage areas or to extend the coverage range of the donor site.
Repeater :
- These repeaters are bi-directional and do not translate frequency and subsequently are limited in output power and gain.
- Repeaters provide between 50 to 80 dB of gain.
- There are two types of repeater band selective and channel selective.
- Band selective repeater amplifies a band of frequency. Hence it amplifies any frequency that falls within its band.
- Channel selective repeater allows selection of a number of individual channels to amplify and rebroadcast.
- Typically a channel selective repeater allows selection of 2 to 4 channels.
- If the GSM900 or DCS1800 network incorporates frequency hopping, then only band selective repeaters should be used.
- Since repeaters are non-frequency translating, the isolation between the donor-side and mobile-side antenna needs to be much greater than the gain of the repeater.
- If antenna isolation is less than or equal to the repeater gain, the repeater will begin to amplify its own feedback and oscillate. This condition must be avoided.
- The isolation between the antennas needs to be at least 10 dB greater than the gain of the repeater.
- Iso = Grep + 10dB
- Following equation can be used to calculate antenna isolation
- Vertical separation Iso = 28 + 40log( d / l )
- Horizontal separation Iso = 22 + 20log( d / l ) – (GA + GB)
- where d = Separation distance(feet)
- l = Wavelength
- GA = Gain of donor-side antenna in direction of Mobile side antenna.
- GB = Gain of mobile-side antenna in direction of donor- side antenna.
- Separation type is based on site type. If the site is a roof, then horizontal separation should be used while if the site is on a tower vertical separation should be used.
- If directional antennas are used then the required horizontal spacing is given by
- Iso = 22 + 20log( d / l ) – (Gamax – Gaback) – (Gbmax – Gbback )
- and
- Iso = Grep + 10dB
- If gain of the directional antenna is 17dBi and front-to-back >25dB and the repeater( gain of 80dB) is used on GSM900 then
- 80 + 10 = 22 + 20log(d/1) – (17-25) – (17-25)
- 90 = 22 +20log(d) + 8 + 8
- 22+20log(d) = 74
- 20log(d) = 52
- log(d) = 2.6
- d = 398 feet
- Hence a horizontal separation of 398 feet is required to provide a isolation of 90dB between the donor-side and the mobile-side antenna which is quite unreasonable.
- Large antenna separation produces large cable losses and reduces repeater output power.
- Some obstruction should be in between both the antennas to reduce this separation.
- If vertical spacing is given by
- Iso = 28 + 40log( d / l )
- and
- Iso = Grep + 10dB
- if the repeater is used on GSM900 then
- 80 + 10 = 28 + 40log(d/1)
- 90 = 28 + 40log(d)
- 40log(d) = 62
- d = 35.48 feet