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Old 14-Jun-2019, 12:06 AM   #21
Retired A/V Tech
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,738
There might be complications even with the CM LTE Filter added

The CM 3201 will keep the channel 60 to 69 signals from reaching the antenna.

However, since the CM 3201 LTE filter is a low pass filter, it will not keep spurious signals from the modulator that are below channel 60 from reaching the antenna. Also, if your modulator IS producing spurious signals below channel 60, they will interfere with your OTA signals from the antenna.

You would need a spectrum analyzer or signal level meter to check for spurious signals below channel 60 from YOUR modulator. If they exist, you would need a custom high pass filter at the output of your modulator to block them. Otherwise, your modulator would be considered an illegal unlicensed transmitter.

The advice from ADTech is correct:
Originally Posted by ADTech
(slightly edited)
The problem isn't the splitters, it's most likely that the modulator isn't very "clean" and is transmitting signals that are below the channels that you've selected. That splatter is interfering with the desired UHF OTA signals. FWIW, those two local stations will be leaving those operating channels sometime in the next year or two as a result of the repack.

I don't think that just the LPF (CM3201) will do the needed job. UHF channels 60-64 are far enough removed from UHF channels 43-49 that the modulated analog signals, if clean, shouldn't interfere with the OTA signals.

My recommendation would be to 1) move the analog to 65-69 and then, if necessary, 2) install a HIGH pass filter (~750 MHz) on the modulator's output to suppress the frequencies below that output prior to combining. Unfortunately, that would likely be a custom device. I would still use the suggested attenuation to cool off the whole signal from the modulator.
If that doesn't work, you will have to follow the advice given to you by ExDilbert:
Originally Posted by ExDilbert View Post
It sounds like the RF modulator is generating out of band signals. That's not uncommon with cheap modulators. I see several options to solving the internal interference being caused by the modulator. One is to purchase a better modulator (or 3 modulators) that do not generate out of band signals. An LTE filter will still be required if they are used above channel 39. The other is to purchase a custom band pass filter for the channels being used by the current modulator. (That will likely cost $100+.) Another is to use HDMI, component or composite video inputs (plus audio) on the TV. The last option is the best choice since it will provide a better picture and eliminate any chance of RF interference. If the TV does not have enough inputs, it may be necessary to obtain an external switch.
The output of my modulator is pretty "clean".

The way I prefer to handle the problem of two antennas aimed in different directions is to connect the main antenna to the antenna input of the TV and connect the second antenna to a separate tuner with its output connected to the aux input of the TV; no modulator is required.
If you can not measure it, you can not improve it.
Lord Kelvin, 1883

Last edited by rabbit73; 14-Jun-2019 at 2:53 AM.
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