OTA TV reception problems

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I tried posting something about this in the past but got no response. I'm h
oling that this time someone might read it who may have a theory as to what
 could be going on.  

I live in southern New Hampshire and watch OTA TV out of Boston, which is a
bout 60 miles from here. Lately I as well as several other people I know ha
ve been experiencing intermittent problems with channel four. While most ot
her channels are presently operating fine, for the past 10 days or so chann
el 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 contras
t 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 n
ever have any problems with that channel. Could propagation be that much di
fferent 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 expl
anation for this. Does anyone have any theories about this? Thanks, Lenny

Re: OTA TV reception problems
On 11/6/2016 4:40 PM, snipped-for-privacy@gmail.com wrote:
Quoted text here. Click to load it
There's signal strength, signal quality/modulation, interference and  
tuner quality.

Having two stations on the same tower reduces the number of variables.

I'm only about 10 miles from the tower, but there are hills all around.
Signal strength is high, but there are a lot of reflections.
I did many experiments varying the antenna direction and changing signal
strength into the tuners.  Best picture stability was not always consistent
with best signal strength.  When trying to use one tuner and one antenna
for multiple stations, I found that I had to attenuate the signal to get
a picture on some stations.

My neighbor has a metal pole building.  I found moving the antenna
to the other end of the house made a dramatic improvement.  Doesn't take
much a reflection that might be from far away to wreak havoc on your
signal due to multipath.  Did Trump put up a new tower in your vicinity?

Currently, I have a win7 computer with 4 tuners.  A single antenna and
distribution amplifier with an output for each tuner.  Over the years,
I've upgraded the tuners so I no longer have to vary the signal
strength to fit a particular tuner.  I find that some tuners
don't like specific channels.  Luckily, media center will let me blacklist
tuners on various channels.

Interference can come from unexpected sources.  Your neighbor's wifi
might be mixing in a corroded gutter junction with some other radio
signal.  The result trashes the signal quality of your TV station.

I once had a towing company radio mix with my radio repeater output
that landed exactly at the IF frequency of my other radio.

If you have access to a spectrum analyzer, you can hook it to your
antenna and look at the relative signal strengths.
Look at the modulation for each channel.  I found that signals were
good when the "bart's head" display was flat on top.  Didn't take much
noise or tilt to render the channel unwatchable, even on very strong
signals.

ATSC is not very forgiving.

Re: OTA TV reception problems
On Sun, 6 Nov 2016 16:40:35 -0800 (PST), snipped-for-privacy@gmail.com
wrote:

Quoted text here. Click to load it

The signal varies in strength and quality, you could be dealing with
knife edge diffraction off of towers, hills, and buildings along the
line of sight.  At 60 miles, you're almost certain to have a potential
problem with Fresnel zone diffraction.
<http://www.proxim.com/products/knowledge-center/calculations/calculations-fresnel-clearance-zone
I can generate a path profile to see if there are any obvious
problems, but I need to know the lat-long of your location and
elevation of your antenna.  Here's a sample output:
<http://802.11junk.com/jeffl/coverage/RST/

Quoted text here. Click to load it

<https://transition.fcc.gov/fcc-bin/tvq?call=WBZ
<https://transition.fcc.gov/fcc-bin/tvq?call=WCVB
The antennas appear to be almost identical.  I just checked the FCC
41dBu service contours for both stations and they are identical.  
<
http://802.11junk.com/jeffl/coverage/KBCZ-WBZ/KVCB-WBZ.jpg

Transmit power difference isn't going to do much.  By all reason and
logic, the signal strengths should be identical assuming the path is
clear of obstructions.  Note that the 41dBu contours do NOT represent
the actual coverage pattern of the stations.  The FCC calculations are
based on the assumption that the earth is flat and lacks mountains and
hills.  These coverage maps should be better:
<http://www.tvfool.com/index.php?option=com_content&task=view&id15%&Itemid=1

You might try checking what might be expected at your location in the
way of TV reception:
<http://www.tvfool.com
Check the calculated signal strengths and see if it's even
theoretically possible to receive either station at your unspecified
location.

Quoted text here. Click to load it

Yes, due to frequency selective fading.  What happens is that you may
have two paths between the TV station and your receiving antenna.  If
the reflected path is some odd numbered multiple of 1/2 wavelength,
the signals will cancel.  At about 500 MHz, 1/2 wavelength is about
1ft.  Move your antenna 1ft in any direction and see if it makes any
difference.

Quoted text here. Click to load it

Oh, lots of theories.  For example, a broken TV tuner that has a
problem on some channels.  Some kind of OTA TV amplifier with an
uneven frequency response.  Badly terminated or connected TV coax
cable causing a notch at one frequency.  TV antenna with a broken
element causing a notch at one frequency.  Local source of
interference on one TV channel, but not the other.  It might be that
one TV station is having transmitter or antenna problems.  I could
probably conjure a few others.  

The easiest way to troubleshoot this is by substitution.  Start with a
different antenna connected directly to the TV with a different
coaxial cable.  It will probably not be as good as a rooftop antenna,
but it should hear something.  Compare the signal levels of the two
stations.  If they're the same, you're problem is somewhere in the
antenna, amp, or coax feed system.  If they're still different, it
might be the TV.  Try a different TV.

Good luck.
--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: OTA TV reception problems
wrote:

Quoted text here. Click to load it

Coverage maps of WBZ and WCVB.  Please note that WBZ has or had a
transmit power upgrade application pending with the FCC to increase
their output to 941kW.  I'm not sure if the coverage map is for 825kW
or 941kW.
<
http://802.11junk.com/jeffl/coverage/KBCZ-WBZ/WBZ-coverage.jpg

<
http://802.11junk.com/jeffl/coverage/KBCZ-WBZ/WCVB-coverage.jpg

Both coverage maps look fairly close.  However, at 60 miles you're
well outside the "normal" coverage area.  I'm wondering why WCVB works
at 60 miles.  What do you have for an antenna and amp at your end?  

I'm otto time for this today.  More tomorrow.

--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: OTA TV reception problems
  snipped-for-privacy@gmail.com wrote:

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Lenny-

If you provide data to Jeff, he can give you a more accurate prediction  
of how well you should receive the station.

As a rough estimate, the range of an signal in miles is the square root  
of twice the sum of transmitting and receiving antenna heights in feet.  
For 60 miles, the sum of heights would be 1800 Feet.

As others mentioned, reflections can make a difference.  Was a large  
building erected about the time you noticed the reduction in signal?

Fred

Re: OTA TV reception problems
On Sunday, November 6, 2016 at 7:40:38 PM UTC-5, snipped-for-privacy@gmail.com wro
te:
Quoted text here. Click to load it
 holing that this time someone might read it who may have a theory as to wh
at could be going on.  
Quoted text here. Click to load it
 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 cha
nnel 4's signal has been in the toilet. This station, WBZ TV operates on UH
F channel 30, runs 825KW, and has an antenna height of 390 meters. By contr
ast Channel 5, WCVB, operates on UHF channel 20, runs 625 KW, and shares th
e 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.  
Quoted text here. Click to load it
planation for this. Does anyone have any theories about this? Thanks, Lenny

Re: OTA TV reception problems
Quoted text here. Click to load it

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.





Re: OTA TV reception problems
snipped-for-privacy@gmail.com wrote:

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Anybody hit or knock down a pole in the area?  I assume your neighborhood is  
wired for cable.  Damage to poles can knock connectors partially loose,  
allowing a LOT of signal to flow onto the outer sheath.  Could be a signal  
right on the same frequency as Chan 4.  I remember a LONG time ago, we had a  
lightning storm, and someone in the area reported TV interference (back in  
the analog days, you could easily tell it was interference) .  Turned out  
the lightning had blown out the output filters on a two-way radio base  
station, so it was radiating at the drive frequency.  Could be something  
similar, a radio transmitter emitting unwanted frequencies.  I've even heard  
of rusty fences and the like developing rectifiying junctions that were  
mixing various broadcast frequencies and blotting out one particular  
channel.

Are the others with the same problem very close to you, or some distance  
away?  If very close, then likely it is a very local interference source,  
might be VERY weak, and darned hard to find.  If they are miles away, then  
it is a larger source, and maybe something the station might be willing to  
send a guy out with a field intensity meter to see if he can find where it  
is coming from.

Hmmm, anybody put up a big building nearby?  Some of these glass towers can  
become excellent near-microwave reflectors, and might be bouncing signals  
from some other area toward your location.

Could also be a wireless security system, baby monitor, wireless cameras, or  
some other gear of that sort that is out of adjustment.


Jon

Re: OTA TV reception problems
Jon Elson wrote:
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    Digital CATV uses QAM modulation, not ATSC.


--  
Never piss off an Engineer!

They don't get mad.

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Re: OTA TV reception problems
On Wed, 9 Nov 2016 15:25:58 -0500, "Michael A. Terrell"

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What is your point?  His point was that a leaky cable system with
signals on channel 4 could interfere with the OP's over the air
channel 4 signal.  That is certainly possible even if different
modulation schemes are used.  (And by channel 4, we mean virtual 4 -
RF 30).

Re: OTA TV reception problems

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Besides using different modulation schemes, cable channels are also
interleaved with OTA channels to prevent mutual interference (via
leakage and ingress).  For example, broadcast channel 30 goes from
566-572MHz OTA.  The closest CATV channel is channel 81, which goes
from 564-570MHz.

Most of the leakage I've seen from CATV systems comes from poor Type-F
coax connector installations by home owners and unterminated coax
cables.  It takes quite a bit of cable leakage to produce interference
with an OTA signal, but it might be possible in weak areas and if the
OTA TV system uses a pre-amplifier.



--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: OTA TV reception problems
Jeff Liebermann wrote:


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The OP is definitly in a weak area (45 miles from stations) and IS using an  
aplifier just under the antenna.  Anybody within a fe miles in the general  
direction of the broadcast antenna could be trashing his signal.  Without  
the proper equipment (portable spectrum analyzer), it could be quite hard to  
find the problem.

Jon

Re: OTA TV reception problems
wrote:

Quoted text here. Click to load it

Yep, and there's another problem:
<
http://802.11junk.com/jeffl/coverage/KBCZ-WBZ/WBZ-Lenny.jpg

That a really rough and crude guestimate of the path, which shows a
150ft pile of dirt blocking the signal.  It could be knife edge
diffraction, but I don't think so.  My guess(tm) is some part of his
system (antenna, amp, coax, splitter, power injector, TV) is broken.

Quoted text here. Click to load it

Yep.  I didn't check for other UHF TV stations in his area.  Overload
(blocking) is possible, but usually affects ALL the channels.  Since I
determined that the theoretical signal strength from both WBZ and KVCB
are almost identical, I don't see a nearby signal trashing one signal,
but not the other.  The lack of complaints about other channels also
points to a specific problem on UHF channel 30.

Quoted text here. Click to load it

Test equipment is always nice to have.  However, I think this one can
be nailed by simply substituting parts of the puzzle and comparing the
effects on both WBZ and KBCZ.  When something is replaced that makes
the signals equal, then the problem has been found.

However, if you really want to use a portable spectrum analyzer, any
of the RTL2832U SDR receiver dongles make a nifty spectrum analyzer.  
<http://www.rtl-sdr.com/?s=spectrum+analyzer
I run several RTL-SDR programs on my Google Nexus 7 tablet that acts
something like a spectrum analyzer.
<https://play.google.com/store/apps/details?id=marto.androsdr2
The basic sensitivity of the typical USB SDR dongle sucks without an
RF preamp, but should be good enough to see what's arriving from the
antenna mounted amplifier.

--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: OTA TV reception problems
Pat wrote:
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   The connectors on the trunklines and feeders are 5/8-27 thread, and  
are quite strong. The housings have a pressed in stainless steel insert  
so the threads don't strip out easily.  Also, the 60VAC @30A, modified  
square wave that powers the trunk and bridging amplifiers is carried on  
the rigid coax. If it is loose enough t radiate, it will have a high  
enough resistance to prevent the amplifier from working. The RayChem  
heatshrink ceramic connectors we used in the mid '80s were so strong  
that the outer aluminum tubing would break, instead of the connector to  
shield connection.


Quoted text here. Click to load it


    Different modulation systems would not mesh, and cause a weak signal  
from phase reversal.  Download the Blonder Tongue Design Guide, and read  
it. CATV systems rarely carried A TV station on the same channel that it  
was transmitted on. For instance, United Video in Cincinnati, had custom  
labels for their converters. Instead of Ch 2 to 37, they were labeled 1  
to 36. So, Channel 12 was actually on Ch 13 to prevent problems with  
direct pickup. The actual Ch. 12 carried one of our in house generated  
channels.


<http://www.blondertongue.com/UserFiles/file/documents/2012%20BRG%20FINAL_lo-res.pdf


--  
Never piss off an Engineer!

They don't get mad.

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Re: OTA TV reception problems
On Sat, 12 Nov 2016 06:32:48 -0500, "Michael A. Terrell"

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Thanks, but the 2014 version is more current:
<http://www.blondertongue.com/UserFiles/file/Marketing%20Literature/2014_BRG_lo-res.pdf
Other documents and catalogs might be of interest:
<http://www.blondertongue.com/about/request_a_catalog.aspx

--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: OTA TV reception problems
Jeff Liebermann wrote:
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    I have several other dates archived, as well. Sometimes you need the  
older information for existing systems.


Quoted text here. Click to load it


    Several other companies had useful publications, but the slow  
conversion to Fiber Aided CATV killed them off.


--  
Never piss off an Engineer!

They don't get mad.

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Re: OTA TV reception problems
On Sat, 12 Nov 2016 23:25:15 -0500, "Michael A. Terrell"

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Good point (after I erased the 2012 version from my machine).
I did some Googling and found the 2008 version of the Blonder Tongue
guide at:
<https://www.sateng.com/downloads/btbroadbandrefguide.pdf
After a quick glance, it seems to have quite a bit on antenna systems,
which are not used very much these days thanks to fiber and satellite
backhauls.

Also, the 2009 version at:
<http://www.blondertongue.com/UserFiles/file/documents/2009%20BBand%20Ref%20Guide.pdf

Quoted text here. Click to load it

The Motorola (now Arris) 2014 guide is what I like to use:
<https://www.arris.com/globalassets/resources/other/cable_technology_pocket_guide.pdf
(6MB) 302 pages.


--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: OTA TV reception problems
Jeff Liebermann wrote:
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    Just like early electrical engineering books that explained the  
things that are just glossed over in current books. P Millet's website  
is a wealth of early electronics books.


--  
Never piss off an Engineer!

They don't get mad.

We've slightly trimmed the long signature. Click to see the full one.
Re: OTA TV reception problems
On Sun, 13 Nov 2016 15:53:54 -0500, "Michael A. Terrell"

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Yep.  
<http://www.tubebooks.org/technical_books_online.htm
I downloaded a mess of those (mostly radio books) a few years ago and
amd sloooowly going through them.  What's interesting to me is that
the origins or reasons behind various modern technical decisions and
standards can be found in the old books.

However, I doubt any of this will help deal with the current OTA TV
reception problem.  I'm not getting any response from CaptainVideo so
I guess I'll drop the project for now.

--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
We've slightly trimmed the long signature. Click to see the full one.
Re: OTA TV reception problems
On Sunday, November 6, 2016 at 4:40:38 PM UTC-8, snipped-for-privacy@gmail.com wro
te:

Quoted text here. Click to load it
 This station, WBZ TV operates on UHF channel 30, runs 825KW, and has an an
tenna height of 390 meters. By contrast Channel 5, WCVB, operates on UHF ch
annel 20, runs 625 KW, and shares the same tower and has it's antenna at th
e same height as channel 4's, and we never have any problems with that chan
nel. Could propagation be that much different 60 MHZ apart? What is really  
weird is that the signal just drops to almost nothing.  

If there's a nearby signal to "channel 30", it could be saturating your inp
ut amplifier.  It's
counterintuitive, but try a passive attenuator and see if it improves your  
reception.  The
attenuator has to be between the antenna and the FIRST amplifier for the RF
 signal,
so if you have a mast-mounted preamp, that's a problem.

Saturation can happen in an RF input amplifier, reducing the signal gain, d
ue to an off-frequency
signal that is completely filtered out in later stages.   I'm not sure if y
our signal indication
would read the spurious signal or not.  

If this is a longterm problem, the best solution is an RF trap tuned for th
e too-strong signal.
That won't hurt your signal strength for the other channel, like an attenua
tor does.

<http://www.mcmelectronics.com/browse/Attenuators/0000000801

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