DTV interference on RF channel 14

Concerning my over the air reception of DTV, I have developed an antenna system that works very, very well...all channels available come in strong...but recently, a station to the north of me, WPTZ (RF 14), has become an issue. Although the station is 91 miles away ( I would bet the signal is traveling along the Champlain Valley), after June 12th, it was stable at a 57% Signal Strength. Just recently, I've experienced cyclical drop outs. I'd like to explain what happens and hope one of you has experienced it and knows the cause. Incidently, I have unplugged every electronic device in my small apartment except the TV, and this interference continues, and only occurs on rf 14. WCAX (rf 22) on the same tower remains uneffected in regards to reception here.

Tuning to WPTZ, the signal strength meter shows 57%, suddenly, the SS will spike to 63%, then bounce back and forth from 0% to 50% 5 or 6 times, yesterday I had it bounce 32 times, eventually settling down back at 57%. After 30 to 60 seconds, the whole cycle begins again. There are times and days it doesn't happen. There are days I will watch a clean signal for hours, then the issue will begin, last 45 minutes and stop for hours, and may not occur until the next day.

I initially suspected a cell phone, wireless internet connection, landline wireless, paging system, etc., causing this, but as I continue the investigation, I have noticed that after a while, (and I remain patient), continuously remaining on channel 14, the drop outs stop eventually and will not return, unless I *change channels...or, if I ^turn the rotor to loose the signal...when I *return back to rf 14, or ^swing the antenna back north, the cycle has started again. When the eventual calm finally occurs, the SS locks at 50-52% and barely wavers, indefinately.....unless I change/interupt the data stream to that channel in some fashion.

I have an AOC 26" HDTV, and am now suspecting the tuner is having some issue locking in on channel 14 occasionally, then settles down until I interrupt the signal and it has to deal with the change again.

Maybe someone has an answer...as a ham radio operator, I used to have access to a spectrum analyzer, but I've been inactive for some time and can't borrow one these days, although as I watch day after day, I have doubts there is external interference, simply because when it's not bouncing, it is a very solid signal, as stable as the Albany (53 miles out) stations are.

Any help would be greatly appreciated.

Thanks in advance!!
Bill

Without knowing your exact coordinates of where you live and which direction your signal is coming from, it makes it harder to pin point your problem.

But my guess is that first off, you are too far away from the transmitter for reliable service in your area. A normal service area for any station is usually not more than 65 miles.

The FCC limits power to all stations so that one station does not walk all over another station 100 miles away.

In the days of Analog, one way they ensured that they could limit the amount of interference present was by using a one channel gap between all stations. As per my case KDKA - ch 2 and then up through the bands 4, 6 , 8, 10, 11, 13

In your case, you are at the digital cliff of reception and the reception you enjoy is usually due to tropospheric reception.

The reason your signal fluctuates is due to a phenomena called Flutter.

Back in the analog days, flutter was limited to internal workings of the television, due to the fact that with analog you could have a poor signal - which had static - Gaussian White noise on the audio and Snow - on the picture and still have a stable picture that you could watch and listen to - if you so chose to do.

The Video was transmitted on AM - with a high amount of power, due to the fact that AM was more susceptible to noise. A long time ago it was on 27 Mhz and citizen band radio's interfered with the picture, so things were moved around to protect the video signal.

The audio was transmitted in FM at a much lower power and horizontally polarized - because they found out that television worked better polarized horizontally as opposed to vertically, but in some instances - they switched it when it interfered with something else.

With the digital, it is all or nothing. Either you have a crystal clear picture and sound, or you have pixelation and no sound.

Once the signal reaches the digital cliff, it will travel no further and it travels off into space. If you have experience with Ham Radio's, you would know that the lower the frequency, the further the signal would go with less power, due to the fact that UF was ground following and depended on skip to travel long distances.

With flutter, the radio waves - which are being refracted over the horizon, diffracted and reflected, are not all reaching your location or reaching your location at the same time. Your signal strength meter reading is of no use to us, due to the fact that the television world has never set a standard for the signal strength meter and what is 100% for one receiver might only be 80% for another.

Again, coming on the internet and not furnishing enough information, such as your location and not just a ZIP CODE - does not tell us much.

The loss of the signal is weather dependent, and depending on where you live, you will loose that signal for several months a year when the weather and atmospheric conditions changes and you are left with nothing for the signal to bounce off. The best times for reception is on a clear cloudless day with a high pressure directly over your area of the country and hot humid conditions.

In the fall , winter, and spring, those days are few and far between in the northern half of the United States.

I could go on and on about reception 101, but I guess you get the point.

There are no articles posted in Wikipedia on digital flutter, so all information provided is of my own opinion and not copied from some other author.

WPTZ only has a 69 mile contour
(22.1 kW TPO + 15.45 dB gain = 650 kW ERP)

Because you probably live somewhere near a large body of water, the signal just happens to have a clear field of view for a longer distance then normal transmission / reception - somewhere - where there is trees and buildings and hills and mountains. That is the reason why it travels much further than normal and why reflection plays such a important part of reception.

Your signal bounces off the sky and the water at one point or another.

I have an AOC 26" HDTV, and am now suspecting the tuner is having some issue locking in on channel 14 occasionally, then settles down until I interrupt the signal and it has to deal with the change again.

I hesitate to point out the obvious, but have you tried a different TV/tuner? If not, perhaps you could borrow/buy a digital converter box to test the signal?

this interference continues, and only occurs on rf 14. WCAX (rf 22) on the same tower remains uneffected in regards to reception here.

Have you checked for adjacent channel interference? Do you detect anything on channel 15?

It's hard to tell what might be going on without an exact location, but being 91 miles south-ish from WPTZ means you might be close to a WNYA translator on channel 15. A strong signal on channel 15 might be hurting your sensitivity on channel 14.

Cheers,

At 90 miles you are fortunate to receive a UHF signal, but the symptomology you describe sounds like the tuner might be at fault since after some time on the channel the oscillation subsides, and it can be restarted by losing signal and reaquiring.

JB don't worry about FCC contours, they are as useless as teeth on a chicken since they seem to almost totally ignore topography.

rinardman - your theory misses one point, the signal is not a constant signal. The reason why the signal is not a constant signal is because the signal has to bounce off the Ionosphere and the temperature in the ionosphere fluctuates rapidly with the heating and cooling of the earth and the sun rise and sun set.

In the morning, as the sun comes up the earth is cool, the sun super heats the atmosphere and causes a reverse inversion layer in the atmosphere. Because it is a reverse inversion layer, it does not last long, maybe until 9 AM EST. - this time of year, before the signal has nothing to bounce off, and then is lost again until evening.

In the evening, the earth is warm and the sky is warm, but the heat is lost in the upper atmosphere out into space. That creates a cold and warm zone - due to the fact that hot air rises and cold air falls.

The collision of the two temperatures, combined with a little moisture and you have your inversion layer which causes the signal to bounce off the atmosphere once again.

Because the earth is round and eventually the signal can no longer bounce off the atmosphere, it travels out into space.

At the same time, the tilt of the earth depends on how much direct sunlight it receives.

In the winter, in the norther hemisphere, the earth is actually as close as it gets to the sun in a calendar year. In the summer it is actually the furthest away from the sun, but due to the fact that the earth does not spin true, and wobbles, the earth tilts towards the sun and the direct rays of sunlight heats the earths surface.

Rain scatter blocks signals, but meteor showers actually reflects signals. Sun spots and solar flares affects VHF communications more than it does UHF.

The Perseid meteor shower in late August, the next meteor shower is the Orionids on October 21, 2009.

The easiest way for me to describe Flutter is - if you were to shine a flashlight through a spinning fan. As the light goes through the blades of the fan, you can see parts of the light, but not all the time.

That is similar to Flutter. The signal - (light) wants to travel in a straight line, but the blades of the fan - blocks at times the signal (light) and because the signal is not a constant, the corruption of information in the signal causes the signal to fluctuate and increase and drop =- just like the light trying to go through the fan blades while they are spinning.

The only way to get rid of the Flutter is to move closer to the transmitter, since the curvature of the earth does not allow a direct line of sight of more than 48 miles anywhere on the earth.

Jim - changing the tuner when no signal is present is like changing your mid flight snack on the Hindenburg. It's not going to make all that much difference in the grand scheme of things.

A normal service area for any station is usually not more than 65 miles.


WPTZ (RF 14) and WCAX (RF22) and 3 other stations, broadcast from the top of Mt. Mansfield which is the highest point in Vermont. And as the OP correctly points out, WPTZ and WCAX broadcast from the same tower. Depending on the intervening terrain, line-of-sight OTA reception is possible at 90+ miles for these stations. I live 79.6 miles Northwest of Mt. Mansfield, have LOS, and have no trouble at all receiving those stations. Friends who live in Hudson which is over 90 miles NW from Mt. Mansfield also receive those stations. If the OP is receiving WCAX without problems, he should also be getting WPTZ.

BTW, the analogue shutdown occured on Feb 17 (not June 12) for all of the Plattsburgh-Burlington stations. Prior to Feb 17, WCAX was broadcasting on RF53 because WVNY was broadcasting its analogue signals on RF22.

Thank you all for the interesting theories. I only assumed that my user name might have indicated that radio frequency is not a stranger to me, being an extra class amateur radio op for over 2 decades, but thanks for the refresher....but now, the rest of the story....
I started watching WPTZ yesterday at about 9 a.m.....it was bouncing and pinging as usual at 57% to 0, to 50%. I sat and watched wondering what I could possibly do next. At approximately 10:30 a.m., the signal went to 0%, and the screen dark and remained there for about 5 seconds....when it came back, the marker slowly rose to 82%. It was like a light switch was thrown from bad to good. A few more bounces and the signal stuck at 82%, and remained there the rest of the day, into evening until bed time, when the TV went off. Not one drop out the entire time.
I obviously must continue to monitor this, but I can only assume that there actually WAS a problem at their TV transmitter that only fringe area reception could see, those much closer may not have seen drop out, and therefore did not complain. I did contact WPTZ on 9-13-09 about my issue, but the engineering staff never told me of a problem. An email WILL be sent to THEM regarding this goose chase.
I considered tropospheric ducting, but the weather conditions here didn't seem that was likely, which I confirmed with the forecaster....nil ducting for the entire northeast, and not any predicted any time soon.
On an average, my strongest station is WMHT in Albany showing 97% on the SS meter on my TV....at 82%, WPTZ is now among the strong stations again.
For those interested, here's my exact coordinates:
43.32538060758666
-73.57009649276733
Yes Virginia, good digital TV reception can be had at 91 miles on 470 to 476 megahertz.
Thanks to all that responded.....

For those interested, here's my exact coordinates:
43.32538060758666
-73.57009649276733


Here's a TV Fool report for 50 feet up in the air: http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3dbefb35da102b1f

Did you check for any signal on channel 15? According to these TV Fool estimates, the repeater on channel 15 might be 30 to 35 dB stronger (in the air) than the signal on channel 14. It's almost exactly 180º in the opposite direction, so if your antenna rig has insufficient front-to-back ratio, you might be seeing non-trivial power on the adjacent channel.

The FCC recommended emission mask for DTV can be found here: http://www.atsc.org/standards/a_64b.pdf. It basically says that adjacent channel emissions should be suppressed by at least 47 dB relative to the total channel power (meaning ch 15 in this case). On paper, it looks like you should be able to receive channel 14, but you might be close to the edge of the digital cliff. If there is some variation in the signal (either reduction of channel 14 or extra signal on channel 15) due to tropospheric ducting, tree attenuation, constructive/destructive multipath interference, etc., it might be enough to put you over the edge. The bounces between 0 and 50+% on your receiver's signal meter is a common symptom when hanging out around the edge of the digital cliff.



There are actually two possible problems to consider if you are being affected by adjacent channel interference:

1) There might be too much out-of-band leakage from the stronger channel (ch 15) which ends up as in-band noise on the desired channel (ch 14). If the desired channel cannot maintain at least a 15.2 dB SNR, then the picture will break up or go away.

2) The receiver's adjacent channel rejection might be poor, allowing some energy from the neighboring channel into the RF front-end. This can cause the AGC of the receiver to track the undesired channel's energy and thus cause your desired channel's energy to be attenuated. If the desired channel is being suppressed, it can also reach the point of making the picture break up or go away.

It would help if you had some different receivers to try since the adjacent channel rejection performance for each can be significantly different.



The peculiar bouncing pattern and stable periods may be the result of your receiver using a digital AGC. It's just a theory, but sometimes digital AGCs are programmed to have some hysteresis in the way they change AGC levels. This might explain why channel 14 tends to get "stuck" in either a good or bad state for several seconds at a time.

In theory, one could use a channel 15 JoinTenna (or equivalent notch filter) to take out some of the energy from the undesired channel to make things better.



It's definitely an interesting and peculiar problem to diagnose. Good luck getting to the bottom of it.

Cheers,

From the standpoint of the television station engineers, when a customer living outside the service area of the station contacts them - regarding reception problems you either get a answer you do not want to hear or you get no answer at all.

Television stations have a normal range of about 60 miles for one reason and one reason only. That reason is - because most people will not travel more then a few miles to buy something. No matter how good the deal is.

A used car lot or furniture store or grocery store does not benefit from someone in Punxsutawney Pa being able to watch that channel - when it is broadcast in Cleveland Ohio.

There are times when it is beneficial for the station to transmit longer distances - such as in Alaska or Vermont - where you have a couple of metropolitan areas and the rest is all rural. If the nearest store is 100 miles away and if the next nearest city is 150 miles away, you have to have some way of being able to watch television.

Before the days of Sat TV that was only done with cable television.

Only cable television providers usually are not willing to string thousands of miles of cable to serve a couple hundred customers.

There is no profit in that.

Unfortunately, UHF does not gain anything by putting the antenna up higher. The higher you go, the more power the FCC takes away from you and the less you serve the nearest customers to the transmitter - because the signal passes over their location and lands 100 miles away, where you never intended your signal to be broadcast in the first place.

I once sent a email to the station manager of WDTV - near Clarksburgh WV, telling him that I was able to receive his signal last summer and that I enjoyed their station. I got no reply.

The same was true with WUAB near Cleveland OHIO, they just don't care if someone 150 miles away can watch their station. They are only concerned if someone 20 miles away can or can not get it.

From the standpoint of the television station engineers, when a customer living outside the service area of the station contacts them - regarding reception problems you either get a answer you do not want to hear or you get no answer at all.

Television stations have a normal range of about 60 miles for one reason and one reason only. That reason is - because most people will not travel more then a few miles to buy something. No matter how good the deal is.


Yes, I understand in service area marketing, but if a distant viewer detects a problem, it would have not hurt nor helped them to respond, "yes we have an issue, be patient, we're working on it", as opposed to a short answer revealing no answer, causing that person to go on a wild goose chase, looking for a non-existent problem local to the viewer.
Bottom line, they apparently did have a power problem and have fixed it...in fact, after I got home from work and tuned up WPTZ yesterday, the occational drop to 62% SS lasting 4 or 5 minutes, and occurring several times between 2:30 ad 5:00 p.m. indicated to me there was some tweeking going on up there...the result was a 3% SS gain on my TV...now locking in at 85%

Here's a few pics...
http://i24.photobucket.com/albums/c30/kb2fzq/DSC00085.jpg
http://i24.photobucket.com/albums/c30/kb2fzq/DSC00084.jpg
http://i24.photobucket.com/albums/c30/kb2fzq/DSC00074.jpg

One of the problems wth low-numbered UHF TV statons is that the spectrum space often competes wth LM (Land Mobile) service that is intended to service two-way communications such as taxi-cabs and oil trucks. Since the OP is so far away from Mt. Mansfield, he may be experiencing issues with competing services on channel 14. Within the normal service of area of WPTZ, one would not expect such problems beccause of the strength of the transmitting station, but at a dostance of 85-90 miles, a viewer could have a line-of-sight to Mt. Mansfield and still experience these problems. I do not believe that Canada allows the 470-490 mHz to be used for anything but TV broadcasting, but in the USA, where spectrum space is very tight, I have seen other cities, like Worcester, MA, for example, where a TV station vacated channel 14 for exactly this reason.

...
In theory, one could use a channel 15 JoinTenna (or equivalent notch filter) to take out some of the energy from the undesired channel to make things better.
...
Cheers,

A JoinTenna is too wide a filter for this. A notch would probably work tho. Or better yet a channel 14 Tin Lee filter.

One of the problems wth low-numbered UHF TV statons is that the spectrum space often competes wth LM (Land Mobile) service that is intended to service two-way communications such as taxi-cabs and oil trucks. Since the OP is so far away from Mt. Mansfield, he may be experiencing issues with competing services on channel 14. Within the normal service of area of WPTZ, one would not expect such problems beccause of the strength of the transmitting station, but at a dostance of 85-90 miles, a viewer could have a line-of-sight to Mt. Mansfield and still experience these problems. I do not believe that Canada allows the 470-490 mHz to be used for anything but TV broadcasting, but in the USA, where spectrum space is very tight, I have seen other cities, like Worcester, MA, for example, where a TV station vacated channel 14 for exactly this reason.

My problem didn't involve LM or 2 way comms, or anything, for that matter, here locally....WPTZ, obviously, had an intermittent power amplifier problem, that has been repaired. :banghead: