IDRick

SW, Holl_ands calculations are explained below.


Estimating power input at pre-amp and tv tuner

Holl_ands, a well respected communications engineer, has developed a table that shows max input values for several popular pre-amps. The table is available here: (see: Signal Amplifiers (Amps, Preamps, Distro Amps) - See Chart in Post #1 - Digital Forum).

Example calculations are shown below for power input at the pre-amp and power input at the tv tuner. Holl_ands calculations require a tvfool report for the location. Identify the high power channels (see power column on tvfool report). Strong channels within band are those within 6 dBm of the highest channel. Make a separate list for UHF and VHF channels.

Here are tvfool power projections for an example situation.

Channel 17 -33 dBm
Channel 23 -35 dBm
Channel 36 -41 dBm
Channel 31 -44 dBm
Channel 15 -50 dBm

How many strong UHF channels in this case? Two, ch 17 and 23

What is the average power input from the strong channels? (33+35)/2 = 34 dB

What is the antenna gain? 10 dB

What is the balun/cable loss? 2 dB

What is the pre-amp noise figure? Most quality pre-amps are between 2 and 3 dB, assume 3 dB.

Are there relevant environmental losses?

TV signal will be attenuated with an attic installation, presence of nearby trees, and/or aiming the antenna away from the broadcast towers. In most cases, the user will not have a direct estimate of these losses. Typical attenuation ranges for these factors are: attic (3 to 13 dB), trees (6 to15 dB), and off axis aiming (1 to 20 dB).

What is the calculated input from the strong channels?

Holl_ands calculation procedure is shown below

Average Signal Power from tvfool: -34 dBm
Adjust to peak power: +7 dB
Add in antenna gain: +10 dB
Subtract balun/cable loss: -2 dB
Subtract pre-amp noise figure: -3 dB
Subtract environmental loss: 0 dB
Input Signal at pre-amp = -22 dBm

Which pre-amp should be used in this situation?

Let’s evaluate three pre-amps and compare the max input signal from our two strong channels above versus the max input for two strong UHF channels from Holl_ands table. Please note, Holl_ands has adjustment factors for the max input depending on the number of strong channels (see bottom of table).

HDP 269 max input: -14.6 dBm
AP 8700 max input: -22.3 dBm
CM 7777 max input: -35.8 dBm

Our calculated input exceeds the max input for the CM 7777 and pre-amp overload would be a definite issue. The max input for the AP 8700 is the same as our calculated input. Minor symptoms of overload may occur such as a slight reduction in dynamic range of the pre-amp. The HDP269 is the appropriate pre-amp to use since it’s allowable input power is well above the calculated input signal.

Will we overload the tv tuner?

Holl_ands guideline is to keep the signal power below –10 dBm at the tv tuner. What would be the power level in this case (assume the distribution system includes a 2-way splitter and 100 ft rg-6 cable)?

Average Signal Power from tvfool: -34 dBm
Add in antenna gain: +10 dB
Add in pre-amp gain: +12 dB
Subtract environmental losses: 0 dB
Subtract losses (2 dB balun/cable, 3 dB preamp NF, 3.5 dB splitter, 6 dB cable): -14.5 dB

Signal power at tv tuner = -26.5 dBm, well below the threshold that influences tuner sensitivity.