In order to suppress nearby cellphone transmitters on old Ch52-69, a so-called
"700 MHz Low Pass Filter" would need to provide BOTH of the following:
1) MINIMAL attenuation on channels below Ch52.
2) LOTS of attenuation starting on 698 MHz and above....
Pico Digital LPF-700 (and other similar, inexpensive CATV Tier filters) meet the
first criteria but it's very iffy on the second. The bandstop specs START
at 800 MHz and says NOTHING for attenuation between 700-800 MHz,
a [ridiculously high] 100 MHz Transition Region
It it's anything like the below low pass filters, there is probably very
little attenuation on old Ch52-54, perhaps a bit more on old Ch55-57
and only reaches the full 50 dB of attenuation on old Ch69.
Although not intended for DTV applications, the fol. Minicircuits 500-700 MHz Band Pass
Filter provides some idea on how much attenuation might be provided in a "typical"
700 MHz rolloff....only -3.6 dB at 740 MHz and -8 dB at 750 MHz and 46+ dB at 800 MHz:
Very typical for multi-pole filters....not much attenuation near the roll-off point, with
most of the attenuation accumulating close to the STOP-BAND spec point.
Minicircuits also provides an Envelope Delay Curve....which can be important for NOT
degrading the Digital TV waveform. Yet another design constraint that inexpensive
CATV Tier filters usually don't have to worry about....
Although 700 MHz isn't one of their "standard" models, Tin Lee can make the
LP7-700 as a special order, with a 30 MHz Transition Region
to 730 MHz....finally
suppressing old Ch57 and above by 40 dB. But LESS EFFECTIVE against Ch52-56,
so it's only a viable candidate if local STRONG cellphone freqs are higher than
Ch56.....and will become LESS EFFECTIVE when Ch52-56 is activated in near future:
If there are STRONG old Ch52 and above signals coming from nearby cellphone
towers (verify with signal level meter), a better approach might be to use one
or more SINGLE CHANNEL STOP-BAND filters, such as Tin-Lee CR-7D (or equivalent)
or a custom BAND-STOP filter. [And add more as other channels are activated]:
Taiwanese supplier Soontai Technologies announced a "LTE Filter" that is
specifically designed to suppress LTE Cellphone frequencies: 698+ MHz in
North America and 790+ MHz in Europe. But it's new, so they haven't yet
posted detailed specs...and you wouldn't want to use a Euro model for N. America:
But to meet both criteria I mentioned above, it's going to have to perform
MUCH BETTER than their standard CATV Low Pass Tier Filters:
A 700 MHz Low Pass Filter would be a special order part, and they only provided
a detailed roll-off curve for their 480 MHz part with a 40 MHz Transition Region
from 480 MHz (-2 dB) to 520 MHz (-56 dB):
It's been well over 2 years since I checked Ch52+ signal levels at my son's location
70-miles SE from Mt. Wilson (L.A.). At that time, Qualcom's Ch55 MediaFlo was 30 dB
stronger than signals from Mt. Wilson and 20 dB weaker than Ch27 (5-mi away...now off-air).
There was another Cellphone signal on Ch58 at about same signal strength.
I should do another check sometime in the near future to see what's new......
I've tried to correlate various on-line cell tower location reports with where I think
the Cell Towers are located....they're probably on the hills in BACKSIDE of the
CM4228's antenna pattern, which was keeping them from being a problem.
But if NEW Cellphone Towers are being co-located with Ch12 (old Ch27 location),
then I might want to retweak CM4228 to make sure they're still in the sidelobe NULL.