I live in southern New Hampshire and watch OTA TV out of Boston, which is about 60 miles from here.
Lately I as well as several other people I know have been experiencing intermittent problems with
channel four. While most other channels are presently operating fine, for the past 10 days or so
channel 4's signal has been in the toilet. This station, WBZ TV operates on UHF channel 30, runs
825KW, and has an antenna height of 390 meters. By contrast Channel 5, WCVB, operates on UHF channel
20, runs 625 KW, and shares the same tower and has it's antenna at the same height as channel 4's, and
we never have any problems with that channel. Could propagation be that much different 60 MHZ apart?
What is really weird is that the signal just drops to almost nothing.

I discussed this with the chief engineer at Channel four and he had no explanation for this. Does
anyone have any theories about this? Thanks, Lenny

That sounds as if you may have a multipath situation. The signal from
the transmitters is reaching your receive antenna via two different
paths - e.g. once directly, and once after reflection off of a
building or mountain or ???.

If the two different paths deliver signals that happen to be of nearly
equal strength, and nearly 180 degrees out of phase with one another,
they will largely cancel out when they are combined by your antenna,
and the signal strength will take a huge dive.

Because the effective length of the signal paths (measured in
wavelengths) is a function of the frequency, it's entirely possible
for two different channels transmitted from the same tower to behave
very differently. Even a small frequency difference can shift the
difference-in-path-length by 180 degrees. 60 MHz difference is far
more than enough for this effect to show up.

Other possibilities:

- Somebody may have put up some sort of structure which happens to
resonate at the UHF Channel 30 frequency, thus creating a
frequency-selective reflector.

- It's possible that another station is now operating on Channel 30,
somewhere within antenna range of you, and its signal is now
interfering with WBZ. This might indicate that a new station has
gone on-line (or an existing one has changed frequencies) or might
indicate that there's some tropospheric ducting or other form of
"skip" bringing an out-of-area station's signal to you.

- There might be some form of local interference - a spurious
transmission on or near the Channel 30 frequency.

https://www.fcc.gov/media/television/tv-query can be used to find
stations on specific frequencies. You could plug in your location and
get a list of all stations (or those on channel 30) within a specific
radius of your location.