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Mary Robinson is all about real-world information on the switch to digital television signals. The success of the DTV switch will depend on regular folks understanding new technology and getting it running in their homes. Ms. Robinson is weighing in on those nitty-gritty details, sharing her enthusiasm for TV-signal technology with those who are less technically inclined. She’s developed an expertise through years of hands-on experimentation, pulling in signals from the rooftop of her Texas home. Now she’s a resource for consumers struggling with the digital switch. We discovered Mary right here on TVWeek.com, where she reliably dispensed information in the comments section of this story, First Digital TV Converter Box Wins Government Approval about the digital switch. Let’s keep the conversation rolling!

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Digital Transition Answers



Keeping Your Signal Strength Up

August 21, 2008 5:50 PM

I would like to review some of the things that we have talked about on the different forums these last few months.

First of all, I want to encourage anyone who may need to depend on over-the-air television for any reason to go ahead and get set up for the digital transition now. We are in the waning days of summer, and a lot of you who live up in the northern reaches of the U.S. soon will be facing winter. I can’t think of anything more miserable than trying to put up an antenna, or add a signal boosting pre-amp, during February in Michigan, Maine, North Dakota (or South Dakota, for that matter), Montana or a lot of other places. And believe me, I know that you who are down here in the South are waiting for cooler weather to start your projects, but please don’t put them off any longer than you have to.

Signal strength is measured in units called decibels, abbreviated dBs. They usually are shown as a negative number. TVFool.com suggests a signal strength of at least -110 dBs at the antenna is necessary to be able to drive the tuner. But this is dependent on the sensitivity of the tuner. Unless your tuner is really sensitive in your digital equipment, that will not be nearly enough signal strength to get a picture.

I have found that a signal strength somewhere in the -90 dB range is more real-world signal strength. This is the signal strength at the antenna. For the tuner to be able to use it, it will need amplifying.

At the transmitting antenna, the signal strength would be designated as 0. The farther away you are from the transmitter, the more signal loss you have (-dBs). Signal strength also can be affected by terrain, buildings, trees and other obstacles between the transmitter and your antenna. Even the wall between the transmitter and that attic antenna will cause signal loss (5 dBs in some cases). That is why outside is better than inside, and the higher the antenna is over those obstacles, generally, the better.

If you need a pre-amplifier, be sure to pick one that is very low-noise. Pre-amps are mounted close to antenna. In an analog signal, noise looks like snow. For a digital tuner to lock on to the signal, it would have to have the equivalent of a clear, no-snow analog signal. Most Channel Master pre-amps are very low-noise. A noise rating of 0.5 dB is excellent and 2 dBs is very good.

Amplifiers should be linear. They should boost what is put into them and nothing more. If there is too much amplification, then they can become nonlinear and overload the tuner. The good news is that overload almost never causes damage to the tuner.

If you add an amp, and the reception gets worse, you may have a condition known as overload. This has happened to me. I know from experience.

The amplification side of the pre-amp also is measured in dBs. This is called gain. Antennas, too, always have a gain factor built into them. How much gain depends on their type and construction. Finding the gain rating of an antenna is not always an easy task. A lot of the time, an antenna will be merely listed as a medium-gain or high-gain antenna. The Channel Master model 3671 high-gain, deep fringe antenna has a gain rating of 5.6 dBs on channels 2-6 (VHF—low), 10.9 dBs on channels 7-13 (VHF—high) and 10 dBs on channels 14-69 (UHF).

When figuring the total gain of a system, this number for the antenna does not change. When figuring total gain, you would take the gain rating of the antenna and add the gain rating for the pre-amp, which for the CM 7777 pre-amp is 23 dBs on VHF (low and high) and 26 dBs on UHF. Then you would start deducting the loss of gain.

RG-6 is the preferred standard for coaxial cable. RG-6 has a gain loss of about 1dB for every 18 feet of length. Keep your coax runs as short as possible. RG-59 has a lot more loss of gain per foot, so stay away from it.

A one-to-two splitter will cost you 3 dBs. A balun, also called a transformer, or matching transformer will cost you about 1 to 2 dBs. Every 3 dB of gain loss equals half your signal strength.

For example, a station that puts a -80 dB signal at your Channel Master 3671 antenna will be increased by 10 dBs on UHF channel 20 (this is an approximation only). So you now have a signal strength of -70 dBs. Adding the CM 7777 pre-amp adds another 26 dBs. So we are now at -44 dBs. If you have a 54 foot RG-6 cable run down to the TV, then you would deduct 3 dBs loss for a total of -47 dBs. If you split that somewhere in the line, then you lose another 3 dBs, for an end signal strength of -50 dBs, and so on.

You can eliminate the loss from a splitter by using what is called a distribution amp, which typically comes in a one-into-two or a one-into-four version.

So there you have our review at the six-month mark before the transition. And just a reminder, there may be a pop quiz next week!

Support your local LPTV broadcasters!


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Comments (6)

It's been a long odyssey since I started upgrading for digital in the spring.

Here in the Quiet Corner of CT / Last Green Valley we're more or less in the outskirts of three TV markets --- Hartford (large hills block reception), Providence and Boston. Digital transition upon us, it was going to take some heroic efforts to maintain whatever channel selection we had.

Picked up the Insignia box at Best Buy. Had a big Radio Shack on a chimney mount for years of spotty analog, which I took down. Replaced it with a tripod mount toward the middle of the peaked roof with a reclaimed 8-foot section of mast and the Antennas Direct (AD) DB-8, pointed at Boston, mostly for their PBS selection, tho their other carriers were spotty or , and then towards Providence which mostly came in reliably except for their PBS, WSBE (virtual 36) and a spotty WPRI.

Bought an AD XG-91 to try for two markets w/o fussing with a rotor (which I still flatly refuse b/c of logistical/wiring issues it'd create). The DB-8 lower on the pole toward Providence, for which we roughly have LOS (some trees), and the XG-91 at the top toward Boston at 55 miles, and received some Hartford stations off the back. Joined the two antennas with a combiner/splitter into the Channel Master 7778 preamp and the results weren't as bad as you might think. Stayed with that for a while. Added a CM 3044 distribution amplifier, which boosted signals ~ 10%. Then, replaced all the wiring with Philips Quadshield RG-6 and compression connectors (~$.75 a pop!). (Note: I had tied into an existing, properly grounded box right before entry through the wall and into the finished basement that the cable company had installed. We canceled cable ~8 years ago.) Reception with the new cabling improved a little (There was actually water coming out of one of the old cable wire section that wasn't in use). I then got a CM 7777 which I use for the Boston antenna, separated the downleads and added an A-B switch. This improved reception by not having the inherent 3dB signal loss and likely cutting out multipath... gained three Boston stations WBZ, WCVB, WSBK and WHDH, with varying degrees of reception. Finally, this week I put up another XG-91 I ordered and replaced the mast (adding ~ 2 1/2 feet) and it has noticeably improved Boston reception on WBZ and WCVB that would previously, throughout all this, spike between 90% - 40% - 0% - 60% - 90% - 0% ad nauseum. For Providence, WSBE is now coming in, but WPRI (real channel 13) is still registering at ~ 30-40% --- we used to get this station in crystal clear in analog. I've included a URL to a picture of my TVFool results. I'd love to've gotten pictures of the progression, as I'm not sure I've explained it as fully as pictures could, but....

Have enjoyed reading your column for the past couple of months since I found it. A couple of points I'd like ask and comment on:

1) For the Insignia box, would replacing it with the CM 7000 box improve multipath or overload problems with WPRI? I've seen ratings that the CM is an A+ box, but I'm just wondering about its sensitivity in comparison to the Insignia.

Throughout, I've noticed strong interference --- the box goes to "NO SIGNAL" with the slightest disturbance, e.g. my neighbor starting and stopping his lawnmower, a slightly loud truck going by, airplanes (this area is on the major NYC-BOS corridor). It's like another more than a loud fart puts this box off. To borrow Ben Linus' line from "LOST"... digital is a finicky b---h!

Why haven't most of the set-top box makers used silicon tuners, which literature going back to the 90s says is better than the tin-can tuners in the current boxes? I know, I know. Probably for the same reason that George Lucas kept releasing each successive little change of Star Wars on VHS even when DVD was long the new standard. Money. The U.S. market that wants things w/o paying a premium, doesn't get the best technology in its markets. It just seems wrong that digital has been around for 25 years now and the technology is still like this. Then again, I realize that most people's situations are not like this.

2) I'd like to comment on the relative availability --- more like the lack thereof --- of antenna products. You pretty much have to order everything online and pay shipping for anything of quality. Radio Shack these days is a shell of its former techie central. The home stores have wiring but lack many of the things for anything other than city dwellers. For the new antenna mast, I had a choice b/w a cheap-looking 5' goldish mast at Lowe's, nothing at Home Depot, and a nice CM 5' galvanized pole online for $6 each... with $30 for shipping. That's right. More than double the cost of the item for shipping. Improvisation: I bought a swedged 10' section of galvanized, 18-gauge fencing post in the Lowe's gardening dept, had them cut it in half to fit in the car, drilled some holes through and put 2 galvanized bolts at alternating angles in the swedge. I think it will hold, even with some of the strong winds we get here. Best part, $11 for the pole, ~$2 for bolts, washers and nuts, and I have a pole that won't rust. Might pass that along to other readers.

I appreciate your column, and I will continue to preach about digital against the costs of cable/sat that are getting even more untenable in times like these. In fact, I've done a set-up for a few other people in less extreme circumstances with good results (tho, I can personally vouch for the Magnavox box being quite a POS). For the cost of about 6 months of one of those, I have a set-up where we'll never have to pay for TV again. Seems I've about written a book here, so it's time to go.

Regards,
JD

JD in CT's Quiet Corner:
mary... the cat lady:

Wow JD,
Greetings from Texas to that quiet corner of Connecticut, Last Green Valley.
You seem to have done a completely outstanding and well thought out progression of upgrades to create a system that will serve you well into the future. I congratulate you.
I think you answered your own question about the silicon tuners. I am sure it is cost. To keep the boxes under the upper cost limit set by the government, that was of course a victim of budget restraints.
I have the Zenith box which is identical to the insignia box, but I have had no experience with the CM 7000, so I just can not answer your question. As far as multi-path goes, I would think that it was more of a factor of signals cancelling each other out, for which only an extremely directional narrow beam antenna would have much of an effect on. As far as the tuner overload, if too much "noise" is picked up by the antenna and then amplified, it loses it linear amplification, and just amplifies the noise, and that may be what is happening to you. Are there any power transmission lines between you and the transmitter? I have found that to be problematic.
I am not a fan of using top rail fencing for antenna masts. In my experience, once, it did not stand up to mediocre gusts of winds. I am not talking about tropical storm or hurricane force winds, but winds generated as a cold front came through. I will say that I had the last anchor to the house about ten feet below the rotor and another 2 feet or so up to the large antenna, so there was quite a bit of weight on top of the mast. I would recommend at least 1" or 1 1/4" galvanized pipe instead. But that is just me.
Yes indeed, it is harder to find good quality OTA products locally. I will say that I am quite lucky in that regard, as there is an electronics supply house near me that is a CM dealer, Ralph's Electronics, and they also carry telescoping poles and other antenna gear. I am very appreciative of that fact. They will bend over backwards to help me get what I need or want.
I thank you for the compliment of your enjoyment of the blog. That is always nice to hear. I will post back when I get a chance to study the results of your tvfool chart.
Oh and by the way, being a Lostie myself, I got a kick out of your Benjamin Linus line. I can hardly wait for the new season to start. To borrow a line from Hurley, "Dude, betcha didn't see that one coming..."

JD in CT's Quiet Corner:

Mary,

The fencing rail that I used was actually 1 3/8" galvanized, swedged, 18-gauge pipe. That's the same or better than all the other commercially available antenna masts I've seen. Is there some property of fencing poles that I'm not aware of? On my old reclaimed mast I used some good Rustoleum spraypaint, the kind where the ad copy says you can spray right over rust (I've had good results with it otherwise) tho I wirebrushed it first. By the time I replaced it last week, it was showing signs of rust on sundry scratches where antennas had been changed, but it had held up through some high winds (one with gusts 50-55 mph). You're probably not saying this, but the cost-benefit of thicker-walled (read: heavy) galvanized pipe probably hurts stability. Weight was also a contributing factor to getting the lighter XG-91s at ~6lbs to the 10lb DB-8 (which I'm going to be putting up for someone else; despite what you may have heard about tony CT, money does not grow on trees here either ;). Ten feet w/o an anchor does sound a little precarious especially in hurricane areas.

For WPRI (real 13) it only registers at ~30-40% even with the yagi-like XG-91. No power lines, and other stations on the same tower come in beautifully. (Anecdotally, one install I did with lines ~ 35 feet away right in the LOS didn't have problems, tho I know they definitely can). I've long held that you'd probably get this station in with a coathanger here. The XG is UHF-only, but I've seen the charts for its VHF-high reception on hdtvprimer and there's no glaring reason to me why it's not getting signal (tho, if I'm wrong, it's a concern b/c the Providence FOX, WNAC, is moving from real 54 to real 12 post-transition). I am flummoxed over it, and my only remaining explanation is overload.

I also forgot to mention another point. There are stations in Boston (WFXT) and Hartford (WTIC), both FOX, that are broadcasting digital on the same channel, 31. (FOX stations sure do seem to like channel 31, in my wanderings on avsforums). I get the same kind of hovering 30-40% on this as I do for WPRI. It's like the converter box can't make heads from tails which to receive so it just doesn't try. 'Course, that's what it's designed to do. What's disconcerting to me is that in this corner of CT, we get signal from both, and as I wrote before, I'm getting reception from the front and back, Hartford and Boston, from that antenna) Why does the FCC allow two stations to have the same channel in such close proximity/overlapping markets? This is also the case with channel 12, as I wrote above, as there is already a broadcaster on channel 12 in Hartford, WTXX. It's as if there's some channel groups that stations cluster around and other areas of spectrum they don't want to touch with a ten-foot antenna mast pole. To wit: Boston/Providence: 19, 20, 21, 22; Boston: 41, 42, 43; Hartford: 33, 34, 35, etc. It might be b/c these clusters of channels perform well, but it can raise serious adjacent-channel issues.

I've generally found that people who've stuck with LOST are smarter than the average bear.

mary... the cat lady:

Hi JD,
Please don't think I am ignoring you. I am having problems with my cable internet provider, and the service is intermittant at best. On and off again. I had a post almost completed a while ago when it my cable service dissappeared again. I wanted to let you know this in case I lose it again.

mary... the cat lady:

Hi again JD,
I talked to my best friend who is a welder by trade, and she said that the fence rail that you have is better than the standard fence rail that you get from most commercial installers, so you should be OK. You have taught me something today. So I thank you.
Again, my internet service was off all day yesterday, so I could not post back at all. Today, it was on, then off, and now on again. For how long, no one knows. I called them yesterday and the earliest they could get someone out the house is this coming Friday. FRIDAY??? Geez.
WPRI is transmitting an effective radiated power (ERP) of 18,000 watts (18 KW), but they have an application pending to go up to 30 KW ERP, so things may improve a bit as time goen on.
when the app. will be approved is anybody's guess. Our government at work for you kind of thing. But I am concerned about the reception of VHF on a UHF tuned antenna. Yeah, it may be able to recieve VHF in prime conditions, but this station for you is a 2nd edge according to your tvfool chart. That does not bode well at all. However, once again, with the maximization of power in the future, it may turn out OK. WNAC is a 2nd edge as well. I wish you good luck.
Usually when you get an overload condition, it blanks out the entile tuner. You get nothing at all, so I am hopeful that is not the case here.
Congress and the FCC sure have allowed an already crowded TV spectrum to become even more crowded, haven't they? And what is going to happend when they let the communication companies transmit their stuff in the "white spaces" in between the channels? More chaos, I would presume.
I looked to see if maybe WFXT or WTIC were going to possibly move to a different channel. Alas, 'tis not to be. Both are going to stay on channel 31 post transition. Sorry. That does seem awful close for these stations to be co-channel. From the chart is appears that roughly they are 93 miles apart from each other. That is about 1/2 to 1/3 the distance you normally see.
You are so right that the channel clusters you mention will cause adjacent channel problems. Probably not in the markets where the station are, because the signals are so strong that the tuner will lock in on them, but you poor veiwers caught in the middle, the tuners, like you said, the tuners can't make heads or tails out of them. It is like HAL in "2001-A Space Odyssey". He was given conflicting instructions, and look what happened there. "Open the pod bay door, Hal... Hal, open the pod bay door!"
Oh, and I think you just might be right Yogi.


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