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Old 4-Aug-2012, 5:26 AM   #1
Dan4328
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Brand new antenna and coax cable, random dropouts?

I recently bought an RCA Mini Yagi antenna for $44 on WalMart because of the high ratings along with the VHF Capabilities my other antenna didn't have. The antenna is fully assembled and ready to go in the attic tomorrow (It's sitting in an opened spot in my room facing towers) but I'm worried about these occasional dropouts that happen once every 10 seconds or more.

It usually only happens with any of my antennas during thunderstorms but this antenna seems to be getting it constantly. I didn't get these dropouts with my first antenna when I had it indoors and passing through 2 rooms, and I even kept it inside because it worked so well before I got it into the attic.

I do know that I live in a so called "signal shadow" that is caused by a mountain here in Kennesaw, Georgia, but this antenna -when it's not dropping out on me- gets me even better UHF and VHF signal than my first antenna. Is the mountain causing signal dropouts? I think the signals go over and bend back towards me or bend around the mountain and get to me, but would something be disturbing that signal? Airplanes aren't an issue even though I live near an air force base and a county airport because none of the airplanes go between me and the towers.

I got 25 feet of RG 6 coax cable (uninterrupted from TV to antenna, no amplifiers needed for now or FM trap anymore) which is still working fine (which isn't the issue because 1 or so db loss doesn't matter in my case) and better than one of my 3 FT RG 6 cables, which is a good thing.

The tower most TV signals are on the Westin Tower in Atlanta I believe but the FM trap from radio shack isn't doing any difference so FM interference is out of the picture.

I live by 75-100 ft. trees in all directions but that isn't looking like it's the issue because I've gone through wind and hail storms with no signal issues.

My test channel is the one that comes in best from Atlanta, WXIA 11.1 and 11.2. They get 45-50% signal in it's position and looks fine until it drops out on me for no reason. It's not a slow signal decrease like 40% 35% 30% 0%, it's 50% to 0% in a snap lasting for about 2 seconds each. I have noticed though that it happens more on VHF than UHF, but it still happens on both types of channels

I know I am writing this as I am tired and watching TV to get to sleep, but these dropouts are what got me out of bed and to type this post, so I apologize for the bad layout and such. Mail me if you need me to elaborate and have a nice day

On a side note, my old antenna said it had 10.4 DBi Gain or something, does that mean it could receive signals at -10.4 db?


SIGNAL Link
http://www.tvfool.com/?option=com_wr...990073d5537a43

Last edited by Dan4328; 4-Aug-2012 at 5:58 AM. Reason: Adding link to signal log
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Old 4-Aug-2012, 5:53 AM   #2
GroundUrMast
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It would help a great deal if you would post a link to your TV Fool report: Guidelines when asking for help
Quote:
On a side note, my old antenna said it had 10.4 DBi Gain or something, does that mean it could receive signals at -10.4 db?
Some antenna specifications are in units of dBi. Some are in units of dBd. TV Fool defaults to dBd. To convert dBd to dBi, add 2.15. To Convert dBi to dBd, subtract 2.15.

Example: 10.5 dBi - 2.15 = 8.35 dBd

Here is an explanation and several examples of how to use the information in your TV Fool report: http://forum.tvfool.com/showthread.php?t=109

Last edited by GroundUrMast; 4-Aug-2012 at 7:00 AM. Reason: Antenna gain
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Old 4-Aug-2012, 7:28 AM   #3
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Thanks for posting the link to your report.

Do you have the model number of the antenna? Is it this: http://www.walmart.com/ip/RCA-Suburb...-Mast/10828410 ?

When you look at the numbers for WXIA: Real CH-10, Noise Margin 26.6 dB, 2EDGE Path & Azimuth stand out as the most informative pieces of information.

More than the UHF band, the VHF band is affected by noise from motors, computers, engine ignition systems and similar sources. So, your report that VHF signals are affected more than UHF is not a surprise.

You say the antenna is in the attic. Attics can be fair locations for an antenna, they can also be quite troublesome. If there is a device in your home that's generating interference, the antenna in the attic is going to be influenced much more than if it were outside, above the roof, clear of obstructions. A roof can attenuate TV signals a few dB or more than 20 dB depending on what the roof is made of. A wet roof is going to attenuate more than the same roof that's dry.

Airplanes don't need to pass through your 'shot' to cause trouble. By reflecting the signal it can produce a phenomena called multipath interference. Your tuner has a more difficult time recovering error free data when the direct signal from the TV transmitter is mixed with another version of the signal that traveled to some reflective object (an airplane for example) then to the antenna... the two signal may add together, increasing the strength of the signal, but just as likely, they may be out of phase... working in opposition, thus cancelling each other. When the reflective object is in motion, you are guarantied the signals will rapidly fluctuate between adding and cancelling each other. This is because the length of the reflected signal path is changing, so the phase relationship also changes.

Then, there's the problems caused by terrain blocking your view of the transmitting antenna. The 1 or 2EDGE path is indicating this is the case for many of the signals. The signals are scattered much the same way sunlight scatters just before sunrise and just after sunset. (There is some light, but far less than when the sun is above the horizon, and the light is diffused, that is, light arrives from more than one direction.) The tuner is affected much the same way as in the case of multipath as described above.

The bottom line is, You could be seeing more stations, more reliably if you mounted a high gain antenna with a clear view pointed toward 135 compass. I'd suggest a Winegard HD7698P. If you have more than 20' of coax, or any splitters, add an Antennas Direct CPA-19 preamp.

The 7698 is rather directional so it's better able to receive more signal from the direction it's pointed at, while receiving less interference from the other directions. This addresses the problem of weak signals as well as multipath. Mounting above the roof uses both the additional distance and the attenuation of the roof deck to reduce the amount of interference from inside the house received by the antenna.

If you can't afford another antenna, the existing antenna will do better if mounted outside, clear of obstructions. The CPA-19 preamp like all other amplifiers is capable of 'pushing' signal through the coax and splitter that are on the output of the amplifier... Amplifiers can not 'pull' signal from an antenna or out of the air.

Last edited by GroundUrMast; 4-Aug-2012 at 8:05 AM. Reason: more thoughts
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Old 4-Aug-2012, 3:01 PM   #4
Dan4328
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Thanks very much on the detailed information you provided, and yes it is the antenna you linked in the forum, the suburban antenna. I'll see what I can do with getting that Antennas Direct preamplifier you mentioned. I got the antenna in a spot where no electricity is running too near to it or in front of it, and no cars are in it's path besides a street a mile in front of where the antenna's pointing (It does get congested with traffic every morning for rush hour but I think it's too far away to matter much) and our roof doesn't get much signal loss, so that's good for me, and it's away from the AC source so electricity won't be doing much damage to it.

To reply to your dBi and dBd's differences, does this mean that my old 10.4 dBi gain antenna would receive signals in -8.34 dBd conditions at best? And would that mean a clear picture, or enough for an amplifier to pull in?

We'll consider doing an outside installation but it would be on the side of our house and only up to 10 feet high (which is a good height for signal, nonetheless) so I can adjust, and if needed, switch out antennas because they are both compatible with the same type of rod/mast.

The only issue with grounding is that I have no experience on how to do it, I even left my old antenna outside (Not mounted, on a tether ball pole on the side of the house) for a few months without grounding and not knowing I even needed to ground it and it worked fine during thunderstorms or rain. It must've been safe because at least there were trees over 80 feet high up that were better off being struck than an antenna on the ground. I'll do grounding wire and stuff if we do an outdoor installation because now with a new antenna we can start over.

EDIT- Should I point my antenna towards 135 degrees or towards 128 degrees? 128 Degrees has most stations on it.

Last edited by Dan4328; 4-Aug-2012 at 3:19 PM. Reason: Added question
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Old 4-Aug-2012, 7:22 PM   #5
GroundUrMast
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An aim point of 128 or 135 is a starting point. Your existing antenna is an RCA ANT-751, it's not very directional so you may not notice much difference in 10 degrees. Try 5 degree adjustments in both directions, reevaluate the quality of reception each time, use the aim point that produces the best results.

You mentioned 80' trees. Trees can produce the symptoms you've described. Just like a roof deck, the foliage absorbs and reflects radio waves. If there is any way to get over or around the trees, the signal stability will be improved. Here in Seattle, strong local signals (with NM values in the +60 to +70 dB range) can be unreliable if the antenna is aiming through a Douglas Fir tree. Wind moving the branches will produce a multipath effect.

To use your antenna gain of 10.4 dBi, I would convert into dBd 10.4 - 2.2 = 8.2 dBd (yes, I rounded the conversion factor from 2.15 to 2.2... hundredths of a dB are insignificant when much of the values used are accurate to half a dB at best.) Next consider NM (noise margin) value of a station you want to receive, I'll use WDTA-LD as an example. Your TV Fool report shows an estimated NM of -0.8 dB. When I add the gain of an antenna with 8.2 dBd forward gain I get, -0.8 + 8.2 = +7.4 dB NM at the antenna output terminals. This is theoretically above the needed level for reception. But the tuner is not at the antenna terminals so we need to consider the effect of the cable and splitter losses, then the imperfections of the tuner. Lets say you have about 50' of RG-6 coax when you total the sections from the antenna to a splitter and then to one of the TVs. That much RG-6 will have about 3 dB loss. Let's also assume you have a 2-way splitter in the system as well, which has 4 dB loss. From the antenna terminal where we start with a NM of +7.4, we must subtract the cable and splitter losses of 7 dB so we get +7.4 - 7.0 = +0.4 dB NM. It looks as if we are still at the edge of reception, just 4 tenths of a dB above the theoretical limit of a perfect receiver. No tuner is perfect so we need to apply a correction factor called tuner noise figure (Tuner-NF). This is a difficult specification to find for consumer grade TVs but generally runs between 6 and 9 dB, the lower the number the better. To use the Tuner-NF, take the intermediate NM at the end of the cable connecting to the TV (+0.4 in this case) and subtract the Tuner-NF. Using a good tuner (low NF) we get, +0.4 - 6.0 = -5.6 dB Net-NM.

A properly specified preamp can overcome the effects of the cable and splitter losses, and possibly some of the Tuner-NF. My experience has been that if the Net-NM is below +10 dB, you won't have enough "fade margin" to deal with intermittent noise and interference.

The CPA-19 preamplifier is specified to have 17 dB Gain and a noise figure of 2.1 dB. In the above example, by applying the amplifier at the antenna terminals, we can discount the cable and splitter losses of 7 dB and substitute the Preamp-NF in place of the Tuner-NF. If you plug all those numbers into the spreadsheet I've posted, you get a Net-NM of +5.3 dB. That certainly doesn't leave much room for multipath, trees or other sources of interference. You would want to look for an antenna with more gain or look at the signal level at higher mounting levels.
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Old 4-Aug-2012, 10:37 PM   #6
Dan4328
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Thanks a whole lot for everything you said and the time you put into what you said- I'll consider everything you said and I will try soon to get the amplifier you suggested and will see what happens. Thanks very much and I hope you can help others as well as you did help me.
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