Selecting an Antenna Preamplifier for HDTV

Experiencing unreliable HDTV signal reception from local broadcast stations?  You have installed a new antenna and HDTV reception remains erratic.  Perhaps you are planning the installation of a new antenna system for HDTV and are considering the use of a mast-mounted preamplifier. 

Mast-mounted preamplifiers or antenna preamplifiers consist of an amplifier housing and power supply.  The amplifier housing is mounted near the antenna while the power supply is installed indoors.  Most preamplifiers have a single input for connection to a broadband VHF/UHF antenna.  Some preamplifier models are configured for separate VHF and UHF antenna inputs.  Generally, the amplifier is mounted to the antenna mast, within several feet of the antenna output and the power supply is located near an electrical outlet at some location indoors.  Often the power supply may be located within the home/medial distribution panel of a structured wiring system.  The power supply inserts low voltage AC or DC on to the antenna cable to power the amplifier at the antenna.

Is an antenna preamplifier required? 

This is the first question asked when considering the addition of a preamplifier to the antenna system.  A preamplifier can provide considerable improvement where there is excessive distance between the antenna and the home/media distribution panel.  Often people will refer to preamplifiers as “boosters” and while the preamplifier does increase signal levels beyond the antenna, it does not improve the quality of the HDTV signal received from the antenna.  The function of the preamplifier is to “maintain” the signal quality at the output of the antenna to the media/distribution panel.  Therefore the purpose of the preamplifier is to overcome cable system losses by increasing signal levels.

Step #1

At this point, you really need a signal level meter to accurately determine if a preamplifier is required and what type of preamplifier should be purchased.  Select the channel that yields the lowest HDTV signal level and measure the signal level.

Example:

DTV Ch 52:  -8 dBmV

Next, select the channel that yields the highest HDTV signal level.

Example:

DTV Ch 3:  +4 dBmV

Step #2

Estimate the total system loss between the antenna and the central point of distribution or media/distribution panel.  The total system loss (in dB) would be the attenuation from the coaxial cable at the highest HDTV channel in your system.

Example:

DTV Ch 52: -9.10 dB (about 120 ft. of RG-6/U cable)

Step #3

Add the lowest HDTV signal level to the estimated cable attenuation.

Example:

DTV Ch 52:  -9 dB + -8 dBmV = -17 dBmV

This is the expected signal level at the media/distribution panel.  Knowing that the HDTV tuner will fail to operate with a signal level below -12 dBmV, the expected signal level is 5dB below the minimum signal and therefore a preamplifier is required at the antenna. 

Selecting the Preamplifier

There are different antenna preamplifiers for various signal conditions.  The preamplifier’s gain identifies how much the signal will be increased as it passes through the unit.  The practice with preamplifiers is to select a model that provides just the amount of gain required.  Also, media/distribution panels are often configured with system amplifiers and input levels to system amplifiers should be between +0 dBmV and +5 dBmV for HDTV signals. 

Example:

DTV Ch 52: -8 dBmV at the antenna output

Cable Loss: -9 dB from the antenna to the panel

Desired Gain: +18 dB to +20 dB

The gain of the preamplifier minus the system cable loss (between the antenna and the media/distribution panel) will provide a signal level of +1 dBmV at the panel on HDTV Ch 52 in the given example.  Therefore, refer to the manufacturer’s specifications concerning preamplifiers and select a model that has a gain of +18 dB to +20 dB.

Simplifying the Process

If the previous example required too many calculations for your comfort, consider a simplified method that works well for most locations.  Identify the lowest HDTV signal level at the antenna and compare it with the estimated loss anticipated from the cable (between the antenna and the media/distribution panel).  If the signal level measures greater than the loss of the cable, you do not need a preamplifier.  If the cable loss is greater than the measured signal at the output of the antenna, you will need to add a preamplifier at the antenna. 

 

By:
Steve Zahn, Doc. #121105 
© 2005 Used By Permission
szahnengineering@msn.com