Thank you for the TVFool report for your location.
Adding a splitter in between the UHF Antenna and the AD combiner resolves the issue but with an extra 3.5 dB of losses...And defeats the purpose of the low insertion loss of a diplexer...
I think I understand why the splitter before the AD UVSJ helped. Your indoor test with the UVSJ shows that the UVSJ is OK, but you have an overload problem from your strong local signals. The overload causes IMD (Intermodulation Distortion) which creates spurious signals in a preamp or tuner. These spurious signals (spurs) interfere with the reception of your weaker signals because they raise the noise floor that reduces the SNR of the weak signals.
When you added the splitter, it acted as an attenuator. For every 1 dB of attenuation, the IMD is reduced 3 dB. The weaker signals are also attenuated 1 dB, so there is a net gain of 2 dB in the SNR. The splitter made all signals 3.5 dB weaker, the IMD was reduced 10.5 dB, and the net gain in SNR was 7 dB.
Overload can be estimated using the NM numbers or the signal power numbers in dBm.
CBFT has a NM of 75.9 dB, even before adding the antenna gain. If you add a conservative antenna gain of 12 dBd, that brings the NM up to 87.9 dB; clearly overload territory.
Interpreting Noise Margin in the TV Fool Report
Chuck's Digital TV Page
I didn't use CIVM because it is in a different direction.
Using signal power converted to dBmV:
CBFT is -14.9 dBm = +33.9 dBmV
Max input of CM7778 is +34 dBmV; just barely OK
but if you add the antenna gain:
CBFT 33.9 dBmV + 12 dBd antenna = 45.9 dBmV; preamp overload
CBFT -14.9 dBm + 12 dBd antenna + 16 dB preamp = +13.1 dBm; tuner overload
ATSC Recommended Practice:
Receiver Performance Guidelines
Document A/74:2010, 7 April 2010
RECEIVER PERFORMANCE GUIDELINES
A DTV receiver should achieve a bit error rate in the transport stream of no worse than 3x10E-6 (i.e., the FCC Advisory Committee on Advanced Television Service, ACATS, Threshold of Visibility, TOV) for input RF signal levels directly to the tuner from –83 dBm to –5 dBm
for both the VHF and UHF bands.
5.2 Multi-Signal Overload
The DTV receiver should accommodate more than one undesired, high-level, NSTC or DTV
signal at its input, received from transmission facilities that are in close proximity to one another. For purposes of this guideline, it should be assumed that multiple signals, each approaching –8 dBm
, will exist at the input of the receiver.
THREE TYPES OF OVERLOAD
There are three types of preamp or tuner overload, in order of increasing signal strength:
1. The strong signals almost cause enough intermodulation distortion (IMD) to interfere with the reception of weak desired signals, but the spurious signals are at or below the noise floor of the weak signals. This is the point that holl_ands uses in his preamp charts to obtain max SFDR (Spurious Free Dynamic Range). No damage will happen.
As the strongest signals continue to increase in strength, more of the weaker signals are damaged until you reach:
2. The strong signals cause overload to the preamp or tuner that makes it impossible to receive any signals. No damage will happen. The strongest signals are still there, but they can't be decoded because the IMD products have damaged them so that they contain more errors (high BER....bit error ratio/rate) than can be corrected by the FEC (forward error correction).
3. The signals are so strong that the input transistor is toast. You are not likely to encounter OTA signals that strong, unless you live next door to a high power transmitter and you have your high gain antenna aimed at the transmitter's antenna.
As a general rule, tuners can tolerate stronger signals than preamps before overload. The difference in strength is approx. equal to the preamp gain.