A high gain directional outside antenna is essential. High gain is needed to lift the signal above the unavoidable amplifier noise. At room temperature, and with e.g. 1.25 MHz bandwidth, the thermal noise (kTB) at the amplifier input is equivalent to about -113 dBm (0 ASU) and even a good amplifiers will add perhaps another 3.5 to 8 dB — depending on how many frequency band(s) it covers.
High antenna gain implies directionality (see below). Directionality is also important in reducing the possibility of feedback/oscillation (analogous to ‘microphone squeal’) in the cellular repeater. One should attach a directional outside antenna on the side of the building facing the base station. If the directional antenna is, in addition, aimed at the base station, then it will be is much less likely to pick up stray radiation from the internal antenna — which is ‘behind it’. An important parameter in regard to avoidance of feedback/oscillation is the antenna's front-to-back ratio (FBR), which preferrably should be more than 15 or 20 dB. If the inside antenna is directional (e.g. a panel antenna) then it can often also be aimed away from the outside antenna so as to further reduce the potential for feedback oscillations.
Keep the lead between the outside antenna and the bi-directional amplifier short, since it carries the weakest signal. No point in getting an outside antenna with a bit more gain, only to lose it in attenuation in the cable. Use LMR400 or equivalent cable — at least for runs up to 50' (signal loss is about 3.9 dB per 100' at ‘cellular’ frequencies (824-894 Mhz), 5.8 dB per 100' in the ‘PCS’ band (1850-1990 MHz)). Consider using LMR600 or equivalent cable if a run of 100' or more should be required (signal loss is only about 2.5 dB per 100' at ‘cellular’ frequencies, 3.8 dB per 100' in the PCS band). An alternative for long runs are Cables with air dielectric — 2.33 dB per 100' at ‘cellular’ frequencies, 3.25 dB per 100' in the ‘PCS’ band. The cable from the amplifier to the interior antenna(s) is somewhat less critical since it carries signals of higher power level.
(2) Gain and directionality are interrelated. An antenna is a passive component and does not add power. So the only way the signal in a particular direction can be high in an antenna is that more of the outgoing power is concentrated into a smaller solid angle. So high gain antennas are inherently very directional. To a crude degree of approximation, antenna gain (in dBi) is
where H is the horizontal beam width in degrees (FWHP), and V is the vertical beam width in degrees (FWHP). This is only approximate, of course — actual measurement in a carefully controlled setup, or numerical integration of the three-dimensional antenna pattern are required to get more accurate values for antenna gain.
(3) Antenna gain is measured relative to a reference antenna. Two references are commonly used:
(ii) Half-wave dipole: dBd is the directional power gain (in dB) of an antenna relative to a half-wave dipole antenna (which does not radiate uniformly in all directions).
In some sense, dBd is a measure of more direct practical significance, since one can actually build a half-wave dipole antenna, while one cannot build a truly isotropic antenna. A dipole antenna has about 2.14 dB more power in directions perpendicular to its length than an isotropic antenna would have. As a result, the gain of an antenna relative to an isotropic source (i.e. dBi) is numerically 2.14 dB higher than the gain relative to a dipole (i.e. dBd).
(4) Beam width is typically measured as ‘full width to half power’ (FWHP), that is, out to where the response is 3 dB down from the maximum response. Gain depends on the solid angle of the beam and so can be made high by having the beam be narrow in the vertical direction, or, narrow in the horizontal direction, or, both (see below).
(5) Base station (cell tower) antennas typically are vertically polarized (this depends on the carrier and affects which way the receiving antenna should be mounted). Vertical polarization has somewhat lower attenuation over terrain than horizontal polarization. The orientation of polarization may be altered after refraction or reflection. (Some systems may instead use two polarizations — orthogonal to each other, and each 45 degree from horizontal — in order to implement ‘polarization diversity’ which helps to reduce the occurrence of signal fading).
(6) Some antennas cover multiple bands, although the gains in different bands typically are different. It is difficult to design high gain antennas for simultaneous operation in multiple frequency bands.
(7) The gain of an antenna of a particular design is proportional to the square of the ratio of its size and the wavelength. Hence an antenna for lower frequencies (longer wavelengths) has to be substantially larger for the same gain as a similar antenna made for a higher frequency (shorter wavelengths).
(8) The far field radiation pattern of an antenna is the Fourier transform of its aperture function (Fraunhofer diffraction). (For details see e.g. Computing Antenna Patterns). As a result, the beam width in a particular direction is inversely proportional to the corresponding antenna dimension. A tall antenna, for example, will tend to have narrow beam width vertically. If it is also narrow, then it will have a relatively wide beam horizontally. In this relationship, angular dimensions are measured in radians and spatial dimensions in units of wavelengths.
One consequence is that a high gain antenna for the PCS band (λ ≈ 0.16 m or 6") can be considerably smaller than one of similar gain for the cellular band (λ ≈ 0.35 m or 14"). As another example of this principle, a vertical beam width of say 15° (0.27 radian) in the cellular band would require an antenna at least 1.3 m (i.e. 0.35 m/0.27) tall.
(9) Finally, an antenna has an effective area, which is, the power delivered at the output of the antenna, divided by the power per unit area of the incident radiation. Put another way, the antenna delivers as much power as passes through a patch having the stated effective area. The effective area is related to the antenna power gain.
where λ is the wavelength and G is the power gain (as a ratio, i.e. not in dBi — the gain in dBi is 10 log10(G)). A half-wave dipole, for example, has effective area 0.1303 λ2 (since 2.14 dBi corresponds to a power gain of 1.64)
The effective area can be a useful quantity when doing calculations based on known base station antenna power, base station antenna gain, and distance. For calculation of overall increase in power density to be expected see Gain Calculations.
This refers to omnidirectional in the horizontal direction (azimuth) only, not over the full sphere of directions (unlike the mythical isotropic antenna). A half-wave dipole is an example of a source that has a radiation pattern that is constant in directions in a plane perpendicular to its length. It has a gain of 2.14 dB relative to the isotropic antenna (hence is said to have a gain of 2.14 dBi). A quarter-wave monopole antenna is just one half of a dipole plus a ground plane. A monopole can have higher gain than a dipole because it does not radiate into the space below the ground plane.
An example is the dual band weBoost 50 Ohm Omni Building Mount Antenna (formerly Wilson 301202) antenna with 4.1 dBi gain in the cellular (824-894 MHz) band and 5.1 dBi in the PCS (1850-1990 MHz) band and a radiation pattern width of 360° horizontal and roughly 60° vertical. (18" tall).
An example is the Nanhai Microwave QB800-11V with 11 dBi gain in the 824-894 MHz band and a radiation pattern width of 360° horizontal and 8° vertical. (108" tall).
Panel antennas are somewhat directional, although the gain tends to be moderate. The directionality can help in avoiding feedback/oscillations. Panels are small, flat and unobtrusive, and can be wide-band.
An example is the dual band Wilson 301135(*) antenna with 4.4 dBi gain in the cellular (824-894 MHz) band and 10.6 dBi in the PCS (1850-1990 MHz) band and a radiation pattern of about 70° horizontal and 120° vertical in the cellular band and about 70° horizontal and 50° vertical in the PCS band. (8" x 7" x 2").
Larger panel antennas can be more directional and thus have higher gain. See e.g. TIL-TEK TP-69E-2-40V/H 14 dBi for 689-896 MHz, linearly polarized panel, 40 degree beam width horizontal 36 degree vertical (22.5" x 23.25" x 2.75").
Corner reflector antennas are simple dipoles with a bent reflector, typically with a 90° corner. For example CACR89 corner reflector with 10.2 dBi gain in the 826-960 MHz band
These are wide-band directional antennas and so have the advantage of covering multiple bands (e.g. cellular 824-894 MHz and PCS 1850-1990 MHz). Log periodic antennas provide more gain than short omnidirectional antennas and have good front-to-back ratio (> 20 dB) making placement of outside and inside antennas easier when trying to avoid feedback/oscillation (although, wide-band antennas do tend to have lower gain than comparable single band directional antennas).
An example is the dual band Wilson 304411(*) antenna (12" x 9" x 3") with 8.0 dBi gain in the cellular 824-894 MHz band and 10.5 dBi in the PCS 1850-1990 MHz band. The radiation pattern width is 90° horizontal and 110° vertical in the cellular band and 70° horizontal and 85° vertical in the PCS band.
See also Surecall CM230-W Outdoor Full Band Yagi Directional Antenna with 10 dBi gain in cellular band and 11 dBi gain in PCS band.
The next step up in antenna gain and directionality is the Yagi-Uda design with a driven element, a reflector and numerous directors, all mounted on a long horizontal bar. These mostly are single band antennas and look a lot like miniature versions of antennas used by ham radio operators. The length of the elements and their spacing are optimised to give approximately constant gain throughout the desired band.
An example is the 8 element Wilson 301111(*) with gain 10.8 dBi in the cellular 824-894 MHz band (shown above, 32" long"), and the 9 element weBoost 1900 MHz Yagi Antenna (formerly Wilson 301124) antenna with 12.5 dBi gain in the PCS 1850-1990 MHz band (17" long).
Somewhat higher gain can be obtained using even more elements, but the antennas then get rather long, and it is hard to optimize them for other than a narrow band.
Examples include the 63" long CAY815 YAGI with claimed 15 dBi gain in the 824-896 MHz band.
It is difficult to squeeze this much gain out of the Yagi-Uda design, particularly if it has to cover a relatively wide bandwidth, but apparently a bit more gain may be had by replacing the single reflector rod with a flat plate (or grid) reflector behind the Yagi. Just make sure the antenna is properly oriented for the polarization used by the cell phone service. Since most use vertical polarization, the following, for example, will not work well:
More gain and narrower beams can be obtained using parabolic dishes. First are antennas with rectangular outline (wider than tall) which are a bit easier to handle and install than circular ones.
An example is the ZDAGP800-15-21 antenna which has 15 dBi gain in the 806-896 MHz band (36" wide and 24" high, 15 lb) with a radiation pattern of 21° horizontally and 16° vertically.
For even higher gain, there are large circular dishes.
An example is the ZDAGP800C-23-6 antenna with 23 dBi gain (95" diameter, 55 lb) with a radiation pattern of 6° horizontal and 6° vertical.
With such narrow beamwidth, aiming can be tricky. This can be a particular problem if the best signal comes from an unexpected direction, such as a reflection off a hillside, away from what may appear to be the most direct route to the cell tower. Also, with such narrow beamwidth both vertically and horizontally, there is no way to simultaneously accommodate multiple towers lying in somewhat different directions.
Some more suggestions:
Wilson 301111 Yagi (13 dBi over 700-900 MHz) available from RepeaterStore.
Diamond shaped antenna (15 dBi over 698-960 MHz) from Gamma Nu.
Wide band corner reflector (16 dBi over 700-2700 MHz) from MP Antenna.
The antennas listed above get their gain by limiting both horizontal and vertical beam width. In many cases, it actually makes more sense to use an antenna that narrows the beam strongly in the vertical direction while allowing some beam spread in the horizontal direction, simply because that matches where the sources of electromagnetic radiation of interest are likely to lie. The vertical beam width can be safely narrowed because one would have to be close to a cell tower to have appreciable elevation of the signal from horizontal (and if one is that close one wouldn't need a cellular repeater).
‘Sector’ antennas fit the bill since they are tall and narrow, designed to have a radiation pattern that is narrow in the vertical direction and wide horizontally. These types of antennas are used on cell phone towers as ‘base station’ antennas for just these reasons! Based on the formula for effective area above, an approximate upper bound on the gain of a such an antenna (the actual gain depending on the aperture efficiency) is given by
The narrow vertical beamwidth (and high front-to-back ratio) of the sector antenna makes it easy to position the inside antenna to avoid feedback oscillations. In some cases, the narrow vertical beamwidth can also reduce the negative interference effect of reflections from a body of water — or a more or less flat piece of terrain between the transmitting and the receiving antennas.
One example is the T09140P1000690 antenna from Terrawave which provides a gain of 14 dBi in the 824-960 MHz band and has a radiation pattern that is 90° horizontal and 15° vertical (radome 51" x 11" x 5", weight 28 lb). (Note: four wavelengths tall implies about 1/4 radian vertical beamwidth)
Another example is the ZDADJ800-14-60 antenna from ZDA Communications US LLC which has 14 dBi gain in the 806-896 MHz band with a radiation pattern that is 60° horizontal and 14° vertical (radome 52" x 11" x 5", weight 32 lb). (The ZDADJ900-14-60 antenna has similar properties, but is for the 870-960 MHz band.)
Taller sector antennas can provide even more gain by reducing the vertical beam spread further:
An example is the J0800S18-65 antenna from JACC with 18 dBi gain in the cellular band (824-894 MHz), 65° horizontal beamwidth and 7.5° vertical beamwidth (radome 99.6" x 11.2" x 5.3", 44 lb, including mounting brackets). (Note: seven wavelengths tall implies about 1/7 radian vertical beamwidth)
Also check Nanhai Microwave Communications Equipment Co. for high gain sector antennas.
NOTE: DHL can ship packages up to 3 meters long from China.
Sector antennas are large and somewhat heavier than the other antennas listed above (particularly for the cellular 824-894 MHz band), but do tend to be less visually distracting than some of the alternatives.
In case you are wondering what is inside those long sector antennas, they are typically vertically stacked dipoles spaced roughly a wavelength apart, and about a quarter wavelength in front of a metallic backplane: