DTV Reception Signal Quality
Signal Quality is a factor with DTV reception!!
I have spent literally the last 4 months doing field strength tests in our market with some very interesting discoveries about the DTV signal itself. It seems signal quality is a BIG factor that can determine ease of reception of DTV signals, and signal quality is something that the Station Engineers actually control to a certain extent.
Not all reception problems are directly related to an end users antenna, low signal levels, or reception equipment in all cases. A transport stream reader can tell a lot about a signals quality, and that may be the key to a lot of the reception issues that viewers are seeing.
Many stations still do not have the equipment needed to monitor the quality of their signals due to the high costs of the equipment itself. High error vector magnitude, bit error rates, modulation error rates and high SNR ratios are all killers of reliable reception for some DTV signals. Signal quality can vary drastically between different broadcasters in any given market.
If these parameters are near the bottom of the FCC minimum legal specifications, reception will suffer greatly, especially at longer distances from the transmitter ( these parameters along with actual RF power levels help determine where the digital “Cliff” is located ). It also seems that antenna aiming is a critical factor in reliable reception. I went to a cable head end a few weeks ago who was having problems receiving our signal from one of our transmitters.
Our transmitter is located on a 4,000 foot mountain, and the Cable Head End is located on a 4,300 foot mountain with a direct line of sight of only about 23 miles, and they were having occasional unexplained reception issues at random times.
After learning that the RF signal levels were in the great range, at least 20 db above the receivers minimum, the transport stream analyzer was used to aim his antenna, and we improved the critical numbers of his receive antenna by re-aiming it based on the transport stream analyzer numbers and his reception improved drastically.
We are only talking about a few degrees off of optimum aiming, and the analyzer numbers improved greatly. So far, he has had no further issues with reception of our channel. This is an indicator of how critical antenna aiming can be at some more difficult reception locations.
One other issue that contributes to this particular situation is that our transmit antenna has a deep null in the exact direction of the cable head end in question that was intended to protect an analog channel that is now off the air.
Our Broadcast antenna is also side mounted on the tower directly opposite of the receive site, And the combination of the deep null, and the side mounted Broadcast antenna (Which can cause an effect I call tower signal scatter ) can all lead to unreliable reception.
One note to add is that this head end site does have a lot of VHF and UHF 2 way radio and Cell phone antennas, and we may have only nulled out the offending signal by re-aiming the antenna, but I can imagine some viewers who could be experiencing these same issues based on how close they may be to an offending RF source, and a lot of DTV receivers are weak on filtering of RF signals at the front end of the receiver.
If anyone on this forum has any transport stream analyzers available to them, it would be an interesting experiment to show how the signal quality can be effected by antenna aiming itself. I have found that critical signal parameters can be improved by exact aiming of the antenna, and this even applies to signals that are already in an acceptable quality range.
The above scenario with signal nulls, side mounted antennas, and receive sites that have a lot of RF present can all contribute to reception issues for viewers who have no possible way to gauge the quality of any given DTV signal.
The point I am trying to make is that the transition is not complete for many stations. A lot of stations are in the same situation in regards to the transition itself, and a lot of the reception issues viewers are now seeing may be due to circumstances such as this one, and if that is the case then all of the antennas and amps you can buy will never solve these problems. How is the average viewer supposed to know about or deal with all of these possible problems that are yet to be solved in some markets?
If you are having problems with a particular station when most all of the rest of the signals in your area are OK, you may want to call that stations engineering department and inquire about any transmitter site issues or other problems they may be having that can affect reception before you try and solve them yourself by buying amps and antennas that may not solve the problem in question, which could be at the Broadcast end of the DTV signal chain.
There are many broadcasters who are not finished with the DTV transition, and something you do now to solve a problem with a particular station, may not be needed if or when they decide to address their own transmitter site issues, which most of them will do when the cash becomes available. Most Broadcasters are still strapped with loan payments for the new antennas and transmitters, HD equipment for master control etc. so funds to correct these issues may not be immediately available.
See the wave form pictures for examples of good and poor quality DTV signals. When the data levels are not linear across the entire 6mHz channel, the signal quality suffers. If the data levels become as high as the pilot carrier due to a non linear tube or transmitter amplifier, or if it becomes very low and closer to the noise floor, signal quality suffers.
The pilot carrier is the only reference that receivers use to “Find and lock onto” the presence of a DTV signal, and if the data levels are high and not linear, the DTV receiver is sometimes not able to “See” the pilot carrier, or it appears intermittent to the receiver, thus causing drop outs. The DTV transition is not complete for all broadcasters as of now!!
Because someone else already had the highest spot, usually. They had to settle for next-best. You see this on cell towers with multiple carriers’ platforms all the time. The one on top either owns the tower or is the principal tenant.
In our case, and it likely will be the same for many others is the fact that erecting a new tower normally at the low end of the scale, is about a Million dollar undertaking at minimum. Then you have to have the physical room at your site to actually erect a tower, and that takes many additional acres of land that most transmitter sites do not have available. One of our two sites has a 1000 foot tower, and it takes almost 50 acres for the Guy Wire footprint, and taller towers need even more space for guy wire placement.
The plan for most was to side mount at first, then take down the top mounted analog antenna, trash it (Recycle it) and the erect a new top mounted digital antenna, and trash the side mounted digital antenna, or keep it as a backup if your tower is strong enough to support both antennas.
This scenario is taking place as I write this at many transmitter sites even today. Very few stations had the extra land or money to build a new tower, and after all was done, you would either have to tear down the old tower due to FAA regulations and trash it, or possibly lease space on it to someone else such as a cell phone carrier or possibly an FM station.
You are also responsible for the lighting of any tower that exceeds 200 feet, and if you had a second tower and decided to not take it down, you still have to maintain a lighting system in compliance with FCC and FAA regulations. It costs about a minimum of about $2,000 just to get a crew to climb a tower for light maintenance, and the parts are normally around another $2,000 or more to fix most average problems and do maintenance to the tower lights.
It is kind of like you buying a car just to take a vacation, and then not having much use for the car after the vacation is over and not being able to sell it because it was built for a one time special use. You still own the car, but if you don’t need it now, so can you really bear the cost of just throwing it away because it was intended for a single purpose and you are now done with it?
Broadcasters were forced to waste a lot of money on the conversion, and that wasted money has hurt a lot of broadcasters financially. and for some, it may take several years to recover, if they do at all. At my station, we have three antennas to dispose of, three old analog transmitters, and many other items at two transmitter sites that is basically useless junk now, with no useful purpose for it now.
Excellent post Fox TV! I have a story in relation to the above paragraph. I had installed my antenna in the attic and had adequate signal strengths plus 100% signal quality on all my locals. After a month or so, my received CBS broadcast had normal signal strengths but widely fluctuating Signal quality leading to audio breaks, macroblocking and complete dropouts. I called and talked with their engineer late Friday afternoon and he assured me everything was great at their end. “Move the antenna outside” was his recommendation. I tried a temporary roof mount, and yes CBS was fine without dropouts out on the roof. Put the antenna back in the attic (December is not the time to do a roof mount in eastern Idaho). Issues continued on Sunday but were gone on Monday. Did they find something in their signal? Shrug, don’t know but the issue has never occurred again. Calling engineering can be good thing! :thumb:
When the wind picks up around here, the Signal Quality meter fluctuates greatly on the Zinwell 970A….while the Signal Strength remains high. I think the Quality meter is reading Bit Error Rates. This is probably dud to dynamic multipath from the myriad of leaves on the trees in the coastal forest around these parts.
You are not the only one, Escape! It happens on all receivers and your explanation is a correct.
I will attempt to explain the signal meter and what it represents in a new post, after this.
Before I get to the signal meter explanation, I wanted to say “Kudos” to FOXTV for saying signal quality matters and I am pleased that he made this point in an earlier post:
”The fact that broadcast TV signals are now digital, does not change the actual physics of a transmitted radio signal. The actual electrical properties of a radio signal are the same in regards to reception, regardless of the information that radio signal contains, even if that information is in a digital format.”
Bravo Fox TV! Thank you for setting us straight. I have said for some time that the science regarding digital television is neither taught, nor understood.
Signal Quality doesn’t just matter, it is ALL that matters. The ideal communication system is designed to preserve signal integrity (a.k.a. signal quality or signal fidelity) from end-to-end throughout the signal chain.
The truth is that very little has changed. The analog carrier waves being broadcast have just been moved (mostly) up in frequency from VHF (lower frequency, great penetration characteristics) to UHF (higher, weaker frequencies with less penetration and resilience).
What changed was really the type of modulation. Without going into detail about modulation types (AM,FM, PM, etc.) we are now using pulse code modulation (PCM).
By taking advantage of the incredible processing power of computers, we found a way to use those squirrelly, high frequency waves. We now send “packets” or “bits” of numerical information instead of a steady stream of information.
In my way of thinking, the best way to describe this is like a train. The train is the message being sent. The cars are the packets or bits of information. What we’ve forgotten (especially in satellite) is that the tracks (analog carrier waves) should be solid. If the tracks are solid coming in, there will be fewer derailments of the cars.
In the old days, anyone using the airwaves would have been familiar with signal-to-noise ratio (SNR) AND its relation to reception quality. Today, with the digital mantra of “All-or-nothing”, and the “as long as you have lock” mentality, we’ve lost sight of the science altogether.
After seeing this post on the home page, I decided to read it again along with all it’s comments, so why I am commenting so late and again.
I left in Jeff’s post I quotes that to me hit the nail on the head. The transmission of TV carrier wave is still analog. The one point though that FOX makes well is if something isn’t linear in the transmitter, while this didn’t affect NTSC to any great degree it can shorten the distance of the “cliff” from the transmit tower on ATSC.
And to talk to SNR and how so much science seems to get lost in every technological change. Look at your home stereo system, look at DSL. Back 30 to 40 years ago, we all spend as much as we could afford to buy cartridges, and amps that introduce as little noise or distortion as possible, to do what? Increase the signal to noise (and lower distortion which also effects the SNR).
Look at DSL2PLUS compared to ADSL. Recently my ISP upgraded from ADSL ato DSL2PLUS (also called ADSL2). Because of better encoding and decoding the 2PLUS increased my SNR by 6 db. Same wire, same modem and DSLAM, just new protocol. Now this is something DTV can’t do easily but it proves the point about SNR being everything.
Additionally my ISP to receive a substantial discount made me move my DSL from my second line to my first line. I had it on my second line because there were far fewer extensions wired to that line. Well I had to move it to my main or first line that has extensions all over the house and other buildings. The frist line had 6db worse SNR with individual filters than the second had. It also had 4 db higher attenuation (this is key to the point), because it wasn’t as good a pair from the DSLAM to my house. So we checked and it was cut dead ahead to my house (no bridge taps or bad connections), so it was just the nature of the ISP’s pair to my house. I know the local guy (small town) so I suggested we swap pairs between the pins on the DSLAM and my NID. He said that is possible, but he wanted to try something else first. We changed the wire to CAT5 that feed my modem and since it had 4 pairs in it, we used 2 for POTS and one for DSL out of a whole house filter. This is now the point. The attenuation didn’t change (we didn’t expect it to) but the SNR went up to a maximum value of 31 or gained 4 db just by providing my modem a cleaner signal even though it was not as strong a signal.
My point here is SNR was the key to a better signal to my modem. Both from a better protocol, and from better delivery of the signal to my modem. Both DSL and ATSC are packet technologies and getting the “train” there in one piece is critical.
Now I just wish I had the equipment to look at my antennas, but like the average OTA person, we just aim and try.
One other comment is narrow beam width does seem to help most situations with drop outs, either multipath or near the cliff. I proved this to myself when I killed 99% of my wind driven multipath on UHF going from a old CM4221A to a Radio Shack U75-R which had less gain, but a slightly tighter beam width.
Signal Quality comparison of 3 different antennas
Here is an example of the difference in signal quality between 3 different antennas to show how different antenna designs can effect reception of the same signal. This test compares an ancient VHF / UHF combo antenna that is mounted at 25 feet, and it was rescued from the trash truck. I found this antenna on the way to work on a Monday morning a few years ago. It was still attached to the mast pole and in good shape, so I took it home and fixed a few minor problems and still use it today.
The second antenna is a Blonder-tongue UHF 6 element Yagi, but the model is unknown, and it was factory tuned to UHF channel 20 for monitoring of one of our transmitters from my home. these tests were all conducted on channel 20. It is mounted on the same pole as the combo, but at 30 feet, about 5 feet above the Yagi on the same mast pole.
The third antenna is a home brew Clear Stream C2 Knockoff that is TOP SECRET. It has a re-designed feed point, along with a re-designed back plane reflector that purposely narrows the beam width of the antenna. This antenna is being used indoors at only 10 feet above ground level.
The related chart shows data from a signal Analysis of 3 different antenna types aimed at the same transmitter @ 926KW ERP at 30 miles with a 2 edge obstruction as predicted by TVFOOL plot. The data was gathered using a Sencor DTU 236 Bit Stream analyzer.
Please study the attached word document for the results of these tests to see just how much an antennas design and aiming can effect quality and reliable reception of the very same DTV signal. Study the chart carefully for some insight as to how different antennas perform on the same DTV signal
This was originally a single page Microsoft word document, but it was to big to post, so I had to split the chart from the data in order to post it.
In a closing note, the combo antenna WILL NOT decode this signal for reliable reception due to the extremely high echo timing that this antenna sees!!
Definitions below for chart categorical data.
RF Level = Signal Strength of the DTV signal
Pilot Carrier = Also called the pilot signal. The RF or Radio Frequency portion of the DTV signal the tuner uses to “Lock On” to the DTV signal
MER = Modulation Error Rate
Margin = Amount of signal headroom
EVM = Error Vector Magnitude, Could also be considered as a data modulation, or a data reproduction accuracy measurement, and is a very important aspect of the receivability of a DTV signal
EQ Loading = Amount of loading on the receivers adaptive equalizer circuitry, which works to try and cancel out or minimize multi path signals
Echo u Sec (Micro seconds) = Echo timing interval this antenna is seeing in micro-seconds, and is directly related to how far away the reflective object is located
That’s great FoxTV.
Isnt the wider bandwidth of the Combo Yagi the culprit here. The Single Channel BT Yagi is highly optimized for one channel.
Furthermore the loop design of the modified homebrew C2 is also inherently a cleaner signal than the dipole and its multi element derivitives.
From what I understand.
PS – I guess Im not the only one working on a secret Double Loop design antenna…
Excellent observation, and a good question too, as beam width does seem to be an issue with a typical combos inherent design, which allows it to SEE many more reflections from even steeper angles. It would be fun to do some real time data on that, if only I had the antennas to test. In some of my previous posts I mentioned signal quality, and certain antennas I have tested can actually improve “Signal Quality” to a limited extent over another type of design tested in a similar manor such as height, cable used, and accuracy of aiming etc.
Hint: An old VCR cover was used in the construction of my “Secret Double Loop” LOL
Ive been toying with the idea of rolling a double loop over on its side….and wrapping it in a piece of wire fencing in a vertical semicircle around the double loops. More like a Corner Reflector than a Parabolic. This will according to my wild ideas*, narrow the beamwidth horizontally due to orientation and vertically due to the reflector. The reflector will also be acting as parasitic elements reradiating RF energy hopefully reinforcing the gain on the double loop.
Ill let you know how it goes once I try it.
*not so wild, but I know just enough to get into trouble, if you know what I mean
EV – The double loop of C2 generates horizontal polarization when oriented like the number 8. If you flop it on the side with your reflector it will be set to receive vertical polarization which probably won’t work so well with most TV signals. Your ideas on the beams are ok. Hope that helps!
Just to clarify.
The C2 Double Loop also has a narrower bandwidth than the Combo VHF/UHF Yagi, namely its a UHF only antenna (more or less).
And loops have cleaner signal due to the way they “capture” electromagnetic radiation. They “capture” the magnetic side, not the electrical side of the wave as a dipole does.
From what I understand.
For what it’s worth I learned a long time ago from experience that a single bow tie antenna will pick up uhf far better than a single loop antenna will.
Can’t speak on the tapered loop but, yea… There’s a reason why no one put out a two, four, or eight bay loop antenna until recently.
The reason is bandwidth. The bow tie is a wide band structure. The thin full-wave loop is not.
A single thin full-wave loop in front of a reflector was known 5 decades ago to provide about 9 dBi gain. The problem was that its bandwidth was too narrow to cover the UHF bands that existed in those days. Apparently no one ever gave it a second thought until I started looking at it a few years ago.
Since the transition we have a completely new UHF band that is much narrower than before. A thin loop still won’t cover it well, but the tapered loop when properly sized and placed in front of a small reflector does. That was the innovation in the Clearstream loops. We have two issued design patents (covers the looks) and one issued and one pending utility patent (covers function) in the US. Additional patents issued and pending in various other countries.
I hope that helps explain your observations and gives you some insights as to why the new lops are different from your previous experiences.
It never occurred to me there could be a variation of ‘quality’ of transmitted signals: I was aware of some stations altering the bitrate between different shows and commercials.
I had the pleasure of watching the KIRO-7 (39) – Seattle’s CBS affiliate’s tower reconstructed this past summer and I took dozens of photos over three weeks and I posted many on the other forum. I was quite surprised when they added triangular tower sections to the maximum height and then they side-mounted their transmitting antenna. :huh:
I’m guessing they did this with hopes for the future they could use the other two sides to mount antennas for two other channels … or … they did this because they already owned their side mount antenna and the cost of replacing it with a center-mounted ‘stick’ antenna VS adding tower sections was less.
Nevertheless, I (finally) now receive “7” from their main transmitter and also from two distant translators. Regarding MY FOX 13 VHF, it’s line of sight over Elliot Bay, 20 some miles from me and rock-solid on a CM-4228 aimed at one of the above mentioned KIRO/CBS translators, coming in slightly off-side of its ‘beam’ pattern. No reception changes here since the leaves are almost gone from the ‘blocking’ trees. KCPQ (13) used to advertise they have ‘circular polorization’ but I don’t know if that is current info. I’m satisfied.
Well, thanks for steering me clear of that dead end, JER!
I did notice that when I rotate the double loops on my Radio Shack 1880 indoor antenna to the side by side configeration, that it becomes as useless as a teet on a bull. However it sings with them over and under. I have been searching for information on double loops on the internet but havent found many.
I bet you know all about them. 😉
I didnt know that they were polarized like that. Is a single loop also polarized like that?
Im also working on a Corner Reflector Bowtie in a similar configuration ….nothing new here, although they disappeared from the market in the last dozen or so years but were recently revived by Digitenna, who has a nice line of corner reflector bowties.
yes. A loop will take on the polarization that is orthogonal to the feed point.
Feed it at the bottom and it’s horz. Feed it on the side, it’s vertical.
You need to dig out some old antenna books on loop antennas.
Hope I am not to brash in saying this, but trying to come up with new loops and missing that basic info you are shooting in the dark, truly using trial and error on your designs.
One thing I have noticed beaming Jacksonville from Orange Springs. A single or vertically stacked YA-1713 should have a beam width where you can’t see much difference over 20 degrees or more.
But in fact beaming both Jax and WNBW in Gainesville, and WESH out of Orlando, the beam width seems to be about plus or minus 5 to 10 degrees. I am guessing that it’s signal quality I am peaking. Then on other stations such as what I call the Gainesville UHF constellation since their towers are all over town, it doesn’t seem to matter as much. But those signals are so much stronger is probably the clue.
There were some loops made back in the day that were wide sheet metal affairs. GE and Radion are 2 that come to mind. Ive been looking to get a Radion that comes up every so often, but Ive always been outbid.
Here is a picture that inspired me….along with studying some Corner Reflector Bowties in Salvati’s book, TV Antennas & Signal Distribution Systems. I also have a Gavin CR-5 that I picked up on eBay….probably a Radio Shack rebrand. Looks like they just used a section of mast pipe to make large diameter (increased bandwidth) elements for a UHF dipole.