Outside antennas for cellular repeaters

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.

Some general notes about antennas

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.

Antenna types and their properties

Omni-directional

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.

This type of ‘omni-directional’ antenna can have more gain if the radiation pattern is made narrower in the vertical direction than that of a half-wave dipole or quarter-wave monopole. This requires that the antenna be taller. One example is a colinear antenna. Such higher gain antennas may have narrower band-width than a half-wave dipole.

-->

This type of omni-directional antenna is not too distracting visually, being basically just a thin vertical cylinder. Omni-directional antennas can present limitations in use with repeater amplifiers, because of their lack of directionality. While omni-directional antennas do have a deep null directly under them, which can be useful when trying to place the ‘inside’ antenna in a cellular repeater, at higher amplifier gain it can be hard to avoid feedback oscillations since the outside antenna cannot be ‘aimed away’ from the inside antenna. Large vertical separation can help in this case (as can placing the inside antenna directly under the outside antenna).

Panel

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.

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 Reflectors

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

and PCTEL MCR-806 Corner Reflector antenna with gain of 8.5dB in the 806-960 MHz band.

Log periodic

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).

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.

Yagi-Uda

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.

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.

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:

Parabolic reflector

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.

For even higher gain, there are large circular dishes.

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.

Sector 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

where A is the frontal area and λ is the wavelength (the gain in dBi is 10 log10(G)). Sector antennas are available with different horizontal spreads (e.g. 120°, 90°, and 65°) and various vertical spreads (and hence different gains) as well.

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.

Taller sector antennas can provide even more gain by reducing the vertical beam spread further:

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.

Conclusions

When the signal is weak, go for high gain in the antenna. No point in wasting time and money on lower gain antennas. Also, if possible, choose an antenna that has narrow beam width in the vertical direction — while allowing some spread horizontally. This may take some effort, since this excludes many of the antennas offered for sale for use with cellular repeaters (which are, of course, just fine when the signal is not too weak).

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:


(*) Wilson Electronics has become weBoost, which may carry some of the above mentioned products. See also RepeaterStore
Click here to go back to main article on cellular repeaters.
Berthold K.P. Horn, bkph@ai.mit.edu