What Fig. 3 also shows, in my opinion, is a large range of robustness between different units tested. For example, at D = –80 dBm the worst unit will not function where the total FM power is greater than –31 dBm, while the best unit will work at –14 dBm.
Near the noise-limited desired signal power (–84 dBm according the FCC) these curves turn downwards because of the combination of receiver-generated noise plus the FMI. At –68 dBm, receiver-generated noise is negligible compared to the FMI so these curves have become remarkably straight. Some, but not all, curve upwards at higher desired signal powers. This is probably due to their RF AGC coming into play.
The good news is that FMI can be eliminated by installing a 75 ohm FM band stop filter at the input to the afflicted DTV receiving appliance. A much lower-cost alternative is a 75 ohm high pass filter (HPF) which attenuates FM signals and any low band VHF DTV signals. In many communities, there will be no DTV signals on Channels 2 through 6, so a high pass filter will work fine. However there are some communities with one or more DTV stations on low band channels. A way around this would be an A/B switch to bypass the HPF when a low VHF band channel is desired.
That is so awkward that most viewers would pay the higher price for an FM band stop filter (if available). However some FM band stop filters may affect DTV reception of Channel 6. Clearly, experimentation is in order.
I am wondering now. The same days I could not watch highband were the same days Trip saw a lot of low band skip. I know even on my high band stack when the E's get intense I can see them pretty good. Plus I know that stack receives low band best off the back and side from when Channel 2, 4 and 6 where still on the air. The side of the stack faces west, where most of my E's come from.
Wondering now is it possible the strong E days are overloading my amp? Skip comes in pretty darn strong at times, like you are a couple miles from a station. I know on 6 meters and even 2 meters I have seen E-skip pin the meters or almost. In the order of being 10 go 30 db above 0dbm. If the TV E's are doing that, then I would have to guess I might be overloaded.
I think I will order a HLSJ and try adding it between my high band and the VHF input to the CM7777.