Originally Posted by Larry Kenney
Since you're talking about various pre-amps, have you ever found one that would work for me, 3/4 mile from Sutro Tower?
did a comparison chart of preamps. What you want is a preamp with a high max input in dBm and a high SFDR (Spurious-Free Dynamic Range).
The Spurious-Free Dynamic range is the difference between the strongest signal and the weakest desired signal plus the minimum SNR of 16 dB for the weakest signal. IOW, from the top of the strongest signal to the bottom of the weakest desired signal.
As a first approximation you can use the dBm figures on your tvfool signal report. For a more accurate calculation, you would need to measure the signals from your antenna with a signal level meter because the strong signals from a close tower are above your location.
If the calculated SFDR for your signals is equal to or less than the listed SFDR for the preamp, you have a chance. If your calculated SFDR is greater than any listed SFDR, you would need to attenuate your strongest signals.
As you can see from Chuck's chart, the medium gain preamps have the best SFDR. The Tin Lee preamp is an excellent preamp. It has a low NF and a lot of gain which is what Chuck needed for his long coax run, but not the best SFDR. It was the ideal preamp for his location.
But, you're not done yet because you have to match your signals to fit the possible SFDR of your preamp.
If your signals are strong (too hot), the strongest signals will start to cause partial overload of a preamp or tuner, and they will create spurious signals from IMD (Intermodulation Distortion) that will raise the noise floor and reduce the SNR of your weakest signals. The weakest signals will be strong enough, but they will be damaged. IOW, the strongest signals are bumping out the top of the SFDR window.
OTOH, if all your signals are a little too weak, the strong signals will not create spurs from IMD, but the weakest signals will be too weak to decode. IOW, they have fallen out the bottom of the SFRD window.
Your task is to match your signals to the SFDR window. You do this by starting with the signals a little too hot and add attenuation to the signals from the antenna.
It might seem counterintuitive to make the weak signals even weaker, but this will improve the SNR of the weak signals because the spurious signals drop three times as fast as the weak signals when you add attenuation. For every one dB of attenuation, the IMD will go down 3 dB, giving you a potential SNR gain of 2 dB.
My local signals are not strong enough to cause IMD, but I have run attenuator simulations at two locations by adding amplification to my local signals. I'm convinced that the method works.
Ant > 7777HD #2
> variable attenuator > CM3410 > 7777HD #1
> splitter > TV and signal level meter
The low gain (green LED) setting was used for both CM7777HD/Amplify preamps. The 7777HD #2 preamp made the local signals stronger. The CM3410 handles strong signals with low distortion, so it was used to feed the signals to CM7777HD #1, which was the device under test.
A setting of 8 dB on the attenuator gave the strongest signal with a good SNR. Note that the noise floor dropped 20 dB for a 10 dB change in the attenuator. The noise floor was high from electrical interference, and the sum of the NFs from the preamps in series for the simulation. The strength of the signal varied because the antenna was indoors and the signal had to pass through trees and buildings. Oh, and the weather was bad because of a storm coming up the coast from the south. Not ideal, but the test is still valid; it was the best that I could do at my present location.
The point is, the SFDR of the preamp is fixed at certain maximum and minimum dBm values. You must adjust the dynamic range of your signals to match those values to make full use of the preamp SFDR.