What is a Donor Antenna?

What is a Donor Antenna?
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A donor antenna is tasked with receiving signals from a carrier radio tower, also known as the donor. Donor antennas are an important component of in-building radio enhancement systems. In fact, they are the MOST important antenna in the system. There are three components to in-building enhancement systems: the donor antenna, bi-directional amplifier, and distributed Antenna.

A donor antenna can be attached externally or internally, but in most cases, it is affixed to the roof of the building. It may also be affixed to the side of a building. For best results, there should be a clear path between the donor antenna and the closest communication tower.

Donor antennas create a two-way interface that brings signals in and out of a building. The downlink is the RF signal direction moving inside the structure, and the uplink is the RF signal as it is sent outside of the building.

BDA (Bi-Directional RF Amplifier)

Highly specialized RF amplifiers are tasked with picking which frequencies to amplify in downlink and uplink pathways. They also enhance RF signal strength moving in and out of the building. Technically known as “signal boosters,” they are regulated under strict federal guidelines that must be adhered by the system designer.

The RF Network

In most cases, non-radiating coaxial cables are used to route RF signals to indoor antennas set up to work with radios.

Radiating coaxial cables are designed to allow low level signals to sneak through the system. Radiating coaxial cables are often used for tunnels and passageways because RF signals lose their strength as they move through a coaxial cable, generally maxing out around 1,000 feet. 

Putting it all Together

A good example of these interconnected parts coming together is a roof top donor antenna that sits directly in sight of the distant radio tower it communicates with. From there, a coaxial cable connects the antenna to the BD RF amplifier, located several stories down inside of the building.

Omni-directional vs. Directional Antennas

Omni-directional antennas are the go-to option when multiple operator base stations require a donor signal. This type of antenna offers signal within a 360-degree radius, but gains are reduced and it cannot solve intercell-interference issues.    

Directional antennas offer greater signal strength and reduce interference. This type of antenna is tasked with drawing a donor signal to just one base station.

Type of donor antenna as well as installation methods will influence the quality of coverage a building receives.

Preventing Interference

A common in-building signal strength system issue is improper isolation or path loss from the roof antenna to the antennas inside of a building.  This issue causes system oscillates, which result in serious interference issues that may be considered illegal. You are not legally permitted to operate a signal booster that oscillates. Gain settings must be reduced to deter oscillations. The Federal Communications Commission (FCC) maintains the right to enforce fines and/or confiscate any equipment that is found to cause interference.

The standard formula for minimum antenna to antenna isolation is: BDA gain + 15 dB 

It’s important to note that additional gain does not equate to greater performance. It is recommended to use the minimum gain setting that proves reliable. Only a qualified radio technician should be tasked with adjusting the gain setting of the BDA.

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