New roof -- new attic antenna?

dgs · 10-16-2009, 12:11 AM

First a little history on my setup.

I live in Edmond, OK. I've run the tvfool analysis and most of the stations we watch are in the 5-8 mi range and all within about a 20 degree band. They were very high power stations broadcasting from central Oklahoma to provide coverage throughout the surrounding rural areas. I don't know if their power was reduced when they changed frequencies for DTV. The local area is hilly, but I live about halfway down the backside of a hill with mostly flat land between the house and the TV stations. This should be ideal for TV reception, but it hasn't worked out that way.

I installed a large Radio Shack yagi in the attic 20 years ago and pointed it in the direction of the large antenna complex. (I intentionally bought more antenna than I thought I needed because I knew I was going to put it in the attic and lose some of its effectiveness.) I used a splitter to send the signal to 3 TVs and a VCR. The antenna is just below the "TV room" with all my gear. TV #2 is in the bedroom on the other side of the wall and TV #3 is in the kitchen (about a 50' cable run). Signal was acceptable, but weak on some of the UHF channels, particularly in the kitchen.

A few years ago (pre-digital), I decided to try to improve the signal in the kitchen. After a bit of trial and error with some amplifiers, I installed a Radio Shack 1-to-4 Bi-Directional distribution amplifier that claims an 8db signal boost. I set the adjustable gain to just above the minimum level. Picture quality was significantly improved on some of the weaker analog UHF channels.

After the DTV conversion I got excellent reception on most channels. Occasionally I'd get a weak signal that resulted in pixilization, but it was rare.

About a month ago we replaced the wood roof with a heavy duty asphalt composition shingle and foil sided roof decking. I can hear the groans now, but a good roof was my top priority. I expected I would have to deal with the loss of signal quality when it was all done. Fortunately, it was not as bad as I anticipated. On most channels I still get a good signal. However, the station transmitting on VHF-hi channel 7 breaks up some of the time, and the station transmitting on UHF channel 40 rarely comes in. According to tvfool, the signal strength is 68 and 73 dB for these stations at my location. I know I'm getting some attenuation with the new roof and cranking up the gain on the distribution amp to just under max helped a bit. What puzzles me most is that I still get good reception of stations transmitting on frequencies on both sides of the spectrum from the signal I've lost (UHF channel 40).

I'm trying to figure out what to do next to get reliable signals. I've experimented with various indoor antennas, and despite the fact that tvfool says I should be able to get all these channels with an indoor setup, the signal is still not reliable.

I added a fourth coax cable to a computer tuner. When I did that I cut out all the other splitters I was using for VCRs, DVRs, converter boxes, etc. I'm trying to keep the signal as pure as possible in case one of the splitters or other cables was creating problems.

I'm wondering if something like a Clearstream C2 will help. Although I doubt I'll get any help in the VHF-Hi range (and it may be worse than my current yagi), I wonder if it will do better for the UHF frequencies. Can I tie the antennas together to get a better combined signal? The other alternative is to move the current or a new antenna to the roof, but I'd rather not expose it to the severe weather we get in OK, so that's a last resort.

Does anybody have any thoughts about my configuration and what I could do to improve signal quality? I love the digital picture, until I start to lose it or when I can't tune a channel at all. I want to watch broadcast TV because I have no desire to spend money for cable or satellite TV with a bunch of channels I don't care to watch. I'm hoping some of the experts on this site can help me get it back. Thanks for your help and comments.

Don_M · 10-16-2009, 09:11 AM

You have some troubleshooting to do before deciding whether a replacement antenna is warranted. I have a hunch the RS antenna is still suitable in that situation. There are two likely issues here:

1) ... did you say, "foil-sided roof decking?" Radiant barriers are great for energy savings, but their aluminum surface blocks just about all signals. Asphalt shingles aren't an issue; if anything, they're a bit better than wood due to lower average moisture content. If it's on the underside of the roof deck, consider creating a signal aperture by removing the barrier in front of the antenna. This area should be several feet wide and probably two feet above and below the plane of the antenna. If doing this is out of the question, there's no alternative but to install the antenna outside, above the roof.

2) An amp is needed in most situations to feed three or four sets, and you had to crank up the amp to make up for signal strength lost at the barrier when it was installed. This may be problematic on two counts:

• Overload, in which the amp is either overwhelmed by strong signals, or it has boosted the signals beyond the tuners' ability to decode them.

• Many Radio Shack amplifiers are infamous for being highly noisy, and noise is also probalematic for digital reception. Increasing noise levels make it progressively harder for DTV tuners to reproduce good video and audio from the signals; once beyond the dreaded "digital cliff," most tuners give up and present nothing at all.

A quick experiment would provide strong hints as to the more likely culprit. Try running a coaxial cable straight from the antenna to one tuner -- no splitters, no amps, nothing in the line except maybe a barrel connector to splice two lengths of coax together -- and see how it performs. If you see improvement, the amplifier is your main issue; if not, it's the radiant barrier.

I'm thinking that only the amp needs replacing. Once you know how the antenna performs with one tuner, we'll have a better idea for suggestions on how to proceed.

Tim58hsv · 10-16-2009, 07:38 PM

Interesting post, dgs because we have similar roofing material and I also have a very old Radio Shack yagi antenna. Like yours it's at least 20 years old and while I'm not sure what it was called, it was the largest yagi (with 30 or so elements) that Radio Shack sold.

It should work good even in an attic but it doesn't. The 2-bay and 4-bays I use work far better up there so the old RS yagi just lies there in the insulation unused.

Not sure if bow tie type antennas would be an answer for you but they sure work great here.

EscapeVelocity · 10-16-2009, 07:42 PM

Well, the antenna itself should be in good shape...not being exposed to weathering and elements, unless someone fell on it in the attic.

And newer antennas arent necessarily better.

My 2 cents.

Tim58hsv · 10-16-2009, 08:35 PM

Quote:

Originally Posted by EscapeVelocity

Well, the antenna itself should be in good shape...not being exposed to weathering and elements, unless someone fell on it in the attic.

And newer antennas arent necessarily better.

My 2 cents.

The yagi has always been inside so it's in relatively good shape minus half of a director that was broke off and one missing reflector element. Still it should work really well but it doesn't. Compared to the 2 and 4-bays, the yagi's performance is really pathetic. Maybe the roofing material doesn't mix well with that sort of antenna design?

dgs · 10-17-2009, 01:37 AM

I did a few things today to try to improve the signal and test the results.

First, I took out all the extraneous wiring for DVD players and video switching. I went straight from the antenna to the 1-4 amp with only 4 outputs.

Second, I went up to the attic with a compass and checked the antenna alignment. My stations are all between 194-208 degrees. It was pointing at about 195, so I moved it over as close to 201 as I could read with the compass I had. With azimuths that tight, I didn't expect much and couldn't see any difference in the signal strengths. The antenna is in perfect shape, but the insulation on one of the leads to the coax connector is slightly cracked. Although slightly exposed, the wire itself looked fine.

Finally, I took some signal strength measurements with the signal meter on my DTV converter box. It may not be as precise as real test equipment, but at least it gives me some idea of relative signal strength. I took the readings from the TV at the end of the longest run in the kitchen. Here are the results.

Channel_____Signal Strength
Disp_(Real)__Direct__Amp Lo_Amp Hi
_4.1__(27)____72_____50_____80
_5.1___(7)____73_____88_____89
_9.1__(39)____87_____90_____89
13.1__(13)____75_____77_____76
25.1__(24)____48_____45_____63
34.1__(33)____84_____72_____90
43.1__(40)_____9______7_____15
46.1__(46)____23_____16_____45
52.1__(51)____48_____46_____67
62.1__(50)____56_____55_____75

As you can see, at the low setting (a bit above minimum) the noise in the amp actually cuts the signal by an average of 3 points. Then I set the gain a bit below maximum, which provided an average 11 point gain. Unfortunately, Channel 43.1 (broadcast at 40) is still below the threshold to provide a picture. At the min amp setting I also lost channel 46.1 (46).

Now here's the puzzle for all the really smart antenna guys. TVfool shows that the missing Channel 43.1 (40) is my third strongest station and channel 9.1 (39) is number 9 on the list, but it provides the strongest signals! Why is there such a disparity between frequencies that are ajacent on the spectrum. Could it be interference between the two stations or is the roof creating a frequency notch in the reception? Is there a hole in the antenna's spectrum? Would a better UHF antenna fix the problem?

My wife has some programs she really likes and regularly watched on channel 43.1 before the new roof. Since my overall signal strength readings are pretty high on most of the channels, I'm thinking that I should still have enough roof transparency to provide adequate reception if I can figure out what's causing the notch.

I'm still wondering if I need a new antenna or if need to move it out of the roof. Again, is it possible to couple a better UHF antenna with this old yagi to get overall better UHF reception?

EscapeVelocity · 10-17-2009, 10:00 AM

More likely its multipath interference....or each particular signal interefering with itself.

Tighter beamwidth antennas or perhaps atenuator(s) may help.

But Im not the expert, Don, Eureka, and Piggie are.

Don_M · 10-17-2009, 10:46 AM

Quote:

Originally Posted by dgs

Why is there such a disparity between frequencies that are ajacent on the spectrum. Could it be interference between the two stations or is the roof creating a frequency notch in the reception? Is there a hole in the antenna's spectrum? Would a better UHF antenna fix the problem?

The transmitting antenna's height has a lot to do with how strong and reliable a signal is. I just looked up the stations' data on file with the FCC. Sure enough, channel 9's antenna is about 55 feet higher than channel 43's.

The roof's construction may be at issue by causing multipath interference on 43. Multipath causes ghosts in analog signals, and it can blank out digital reception completely if it's bad enough. Attic-installed antennas are frequently vulnerable to multipath, which also causes low, erratic readings on signal-level meters, just like overload does. BTW: An amp can't differentiate between direct and reflected signals, so it amplifies them all, exacerbating multipath issues.

Any antenna gain "notch" at channel 43 (40) would also effect channel 9 (39). Antenna gain curves are rarely that abrupt and basically never occur at mid-band frequencies. So it's safe to rule this out as a possibility. That doesn't mean a different antenna wouldn't perform better for you, though.

Quote:

I'm still wondering if I need a new antenna or if need to move it out of the roof. Again, is it possible to couple a better UHF antenna with this old yagi to get overall better UHF reception?

I'd try troubleshooting it first as suggested earlier. While you're at it, replace the coax transformer at the antenna terminals: That cracked insulation and exposed wire may mean the transformer is just as worn out as the wires. You'll definitely need a new "balun" if the antenna is headed outdoors.

If these issues persist, then it's time to consider moving the antenna outdoors. This has a way of solving all kinds of persistent signal issues. Being that close to the transmitters, you may discover that you don't need any amplification, even for four TVs! I have heard of others doing just that within 10 miles of the transmitters, so it is possible.

Two antennas may be combined as you suggest, but that's advisable only after the antenna is outdoors.

dgs · 10-17-2009, 12:04 PM

Quote:

Originally Posted by Don_M

The roof's construction may be at issue by causing multipath interference on 43. Multipath causes ghosts in analog signals, and it can blank out digital reception completely if it's bad enough. Attic-installed antennas are frequently vulnerable to multipath, which also causes low, erratic readings on signal-level meters, just like overload does. BTW: An amp can't differentiate between direct and reflected signals, so it amplifies them all, exacerbating multipath issues.

Is there an easy way to test for multipath to see if that is the culprit? When I'm looking at the signal strength meter from my DTV conversion box, the signals fluctuate on all channels by a few points, but only Channel 43 (40) is cut out completely.

Quote:

Any antenna gain "notch" at channel 43 (40) would also effect channel 9 (39). Antenna gain curves are rarely that abrupt and basically never occur at mid-band frequencies. So it's safe to rule this out as a possibility. That doesn't mean a different antenna wouldn't perform better for you, though.

Thanks for confirming my theories about that. I didn't see how one could perform so much better than the ajacent channel.

Quote:

I'd try troubleshooting it first as suggested earlier. While you're at it, replace the coax transformer at the antenna terminals: That cracked insulation and exposed wire may mean the transformer is just as worn out as the wires. You'll definitely need a new "balun" if the antenna is headed outdoors.

I did the troubleshooting as part of my experimentation. Those are the numbers in the "Direct" column. Based on the improvement of the signal strength with the amp gain turned up, I think it is helping my signal overall. However, I would also consider buying a higher quality amp if I knew where to get one. I'll try to get a new "balun" ASAP. I want to rule out as many causes as possible.

Quote:

If these issues persist, then it's time to consider moving the antenna outdoors. This has a way of solving all kinds of persistent signal issues. Being that close to the transmitters, you may discover that you don't need any amplification, even for four TVs! I have heard of others doing just that within 10 miles of the transmitters, so it is possible.

With as close as we are to the transmitters, I suspect putting it outside will provide amazing results, but I expect the antenna won't last long with the severe weather we get here in OK every Spring. Maybe strapping a 4-bay to the chimney would work.

Quote:

Two antennas may be combined as you suggest, but that's advisable only after the antenna is outdoors.

Why does it need to be outside to combine them--to avoid making the multipath in the attic worse? Is there a thread here somewhere that talks about combining antennas?

Thanks again for all your help in trying to solve this perplexing mystery for me. I'll let you know how the new "balun" works.

Don_M · 10-17-2009, 01:35 PM

Quote:

Originally Posted by dgs

Is there an easy way to test for multipath to see if that is the culprit?

It used to be so easy: Just look for ghosts in the analog picture. Today, most signal-level meters are great at letting you know you have a problem, but they're not so good at suggesting what it might be. In most situations:

• Multipath interference is frequently behind a meter that fluctuates wildly -- 30 points, 40 points or more on a 100-point scale. These wide swings may be either constant or occasional (especially if it's windy or gusty, even on an attic antenna).

• A low but fairly steady meter reading means the tuner can't lock on to the station. Failure to lock occurs because 1) the signal is too weak (thanks to an insufficient antenna, or one with too many sets connected to it without an amplifier; 2) the signal is too strong and has overloaded the tuner; 3) the signal is too strong and has overloaded the amplifier, which increases noise, which in turn causes the tuner to fumble the digital lock.

Quote:

... I suspect putting it outside will provide amazing results, but I expect the antenna won't last long with the severe weather we get here...

I can understand the reluctance to subject an antenna to that weather. OKC also has a reputation for getting bad ice storms at times. Freezing rain is also mighty rough on delicate aluminum elements!

Quote:

Why does it need to be outside to combine them--to avoid making the multipath in the attic worse? Is there a thread here somewhere that talks about combining antennas?

Apologies for the murkiness there. Take two: Antennas for different bands can be combined either outdoors or in the attic, as long as there's enough space to separate them by an absolute minimum of three feet; the more separation, the better. But if the attic is exacerbating multipath, combining antennas in there isn't very likely to resolve this issue. Hence the suggestion to put the antenna you already have outdoors before buying a new antenna that might turn out to have been unnecessary after all.


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