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Often "overload" is loosely used to mean the maximum signal a preamp can output. This is basically the definition of 1dB gain compression. This might be okay if there is only one strong signal present but it is not useful when two or more strong signals are present which is often the case. In this discussion I’m defining overload to mean the input level to a preamp of two signals that cause the Third Order Intermod products to be strong enough to interfere with the weakest signal that the can be decoded. This occurs at much lower signal levels than does gain compression.

In order to determine overload, IP3 and preamp gain needs to be known. Preamp noise figure is helpful too. Third order intermods increase in amplitude at 3 times the rate of the main signals. This leads to the fictitious number IP3 where the main signals (two equal input signals are needed to run the test) and the intermods are equal in amplitude. It is fictitious because IP3 is always higher than P1dB so the amplifier output can never reach IP3. If you know IP3 and the preamp signal outputs you can calculate the third order intermods quite easily.

Intermods = IP3-3x(IP3-Signal Output)

For example:

IP3 = +30 dBm

Signals = +10 dBm

Intermods = 30 – 3x(30-10) = -30 dBm

It’s time to see how preamp gain and noise figure affect what constitutes overload. I’ll use a 6 MHz bandwidth as a round number for a DTV signal. The noise in a 6 MHz bandwidth at room temperature is -106 dBm (another round number) at the antenna terminals. Let’s say the preamp has 20 dB of gain and a 3 dB noise figure. The noise level at the preamp output will be increased 20 dB + 3 dB or -83 dBm. Any intermod products that are greater than -83 dBm have the potential to interfere with reception of a weak station if they happen to fall in the bandwidth of that station.

Now let’s look at a real preamp and see what the maximum level of two input signals has to be to potentially cause a problem. I’m going to use the new Clearstream Juice preamp and the numbers posted by ADTech:

Gain = 19 dB

Noise Figure = 3 dB on UHF

IP3 = +37 dBm

P1dB = +23 dBm

The output noise level of this preamp is -106 dBm + 19 dB + 3 dB = -84 dBm.

Rearrange the formula above to solve for Signal Output:

Signal Output = IP3 – (IP3-Intermods)/3

Signal Output = 37 – (37 – (-84))/3 = -3.33 dBm

With a preamp gain of 19 dB the maximum input of two signals is -22.33 dBm.

Let’s say we used an antenna with 10 dB of gain. Now look at the NM and Signal Pwr columns in any TV Fool report. The Noise Margin – Signal Power is 90.8. (TV Fool uses -106 dBm as the noise level at the antenna terminals. Since 15.2 dB is minimum Signal-to-Noise to decode a DTV signal, -106 dBm + 15.2 equals the Noise Margin - Signal Pwr or 90.8.)

The maximum Noise Margin without causing preamp overload using a 10 dB gain antenna is:

NM = 90.8 - (10 – (-22.33)) = 58.47 dB.

Any pair of signals with a Noise Margin > 58.5 dB can potentially cause a reception problem on a very weak station using the above preamp and antenna. More strong signals mean even lower Noise Margins can cause a problem.

If you have just one strong signal and the rest are fairly weak (my situation) then you should be able to tolerate a much higher Noise Margin without overload. It all depends on how weak the other stations are.

Let’s take a look at a few more examples.

Tin Lee UHF preamp

Gain = 30 dB

Noise Figure = 2 dB

IP3 = +31 dBm

P1dB = 19 dBm

Output Noise Level =-106 dBm + 30 dB + 2 dB = -74 dBm

Signal Output = 31 – (31 – (-74))/3 = -4 dBm

With a preamp gain of 30 dB the maximum input of two signals is -34.0 dBm.

Using the TV Fool number of 90.8 and antenna gain of 10 dB:

NM = 90.8 - (10 – (-34)) = 46.8 dB (Maximum of any 2 signals)

Next let’s look at the RCA TVPRAMP1R. I measured this preamp but I don’t know IP3. Based on IP3 of the other preamps and P1dB I measured on UHF I’m going to estimate the low 20 dBms. I’ll use +22dBm.

RCA TVPRAMP1R

Gain = 25 dB

Noise Figure = 2 dB

IP3 = +22 dBm

P1dB = +10 dBm

Output Noise Level =-106 dBm + 25 dB + 2 dB = -79 dBm

Signal Output = 22 – (22 – (-79))/3 = -11.67 dBm

With a preamp gain of 25 dB the maximum input of two signals is -36.67 dBm.

Using the TV Fool number of 90.8 and antenna gain of 10 dB:

NM = 90.8 - (10 – (-36.67)) = 44.13 dB (Maximum of any 2 signals)

Finally let’s look at the discontinued Winegard HDP269 that was always at the top of overload resistant preamps. I measured this preamp but again I don’t know IP3. Based on IP3 of the other preamps and P1dB I measured on UHF I’m going to estimate the upper 20 dBms. I’ll use +27dBm.

Winegard HDP269

Gain = 12 dB

Noise Figure = 3 dB

IP3 = +27 dBm

P1dB = +15 dBm

Output Noise Level =-106 dBm + 12 dB + 3 dB = -91 dBm

Signal Output = 27 – (27 – (-91))/3 = -12.33 dBm

With a preamp gain of 12 dB the maximum input of two signals is -24.33 dBm.

Using the TV Fool number of 90.8 and antenna gain of 10 dB:

NM = 90.8 - (10 – (-24.33)) = 56.5 dB (Maximum of any 2 signals)

The bottom line is you want the highest IP3 you can get and no more gain than you need to minimize the possibility of interference from third order intermods to weak stations in a strong signal environment.

I think the advice to not use a preamp with a gain antenna mounted outside when Noise Margins are predicted to be >+50 dB is justified.