Chattanooga TN: Got Atlanta But Few Locals - Page 3 - AVS Forum
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post #61 of 89 Old 12-16-2011, 04:21 PM
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Congratulations on your "anti-ghosting" UHF antenna, Trip!

Good detective work, tests, and construction.

Looking forward to the VHF version for your locals; maybe a folded dipole or loop?

73,
rabbit

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post #62 of 89 Old 12-16-2011, 04:26 PM
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Quote:
Originally Posted by Trip in VA View Post

Success!

I bought a metal trash can from ACE Hardware on my way home this evening and in my bedroom, I stuck my Silver Sensor inside it aimed in the direction of the transmitters. Voilá, 100% signal quality on WFLI, high 80's on WDSI, WTCI and WYHB-LD. The Silver Sensor is too small for the VHFs but it's a start!

Wait, wait, wait, did you say metal trash can? If your neighbors weren't laughing at you for using a TV antenna, they will now!

Any engineers want to explain how this supposedly works? Wouldn't the trash can scatter and reflect the signal into multipath oblivion?

Max Vrany
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I make no guarantees as to the accuracy of any information given.
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post #63 of 89 Old 12-16-2011, 04:49 PM
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Yes, trash can.

I'm not an engineer (I started out at Rutgers to be an EE but the draft got me), but maybe this explanation would help: If it worked for Bill Naivar at Georgia Tech, I don't see any reason why it wouldn't work for Trip. Did you look at the links that Trip previously posted?:

www.prism.gatech.edu/~wn17/
www.prism.gatech.edu/~wn17/Web%20bill%20page%202.htm

Bill grounded the can and used a feedthru for the coax so that there would be no chance of interference being picked up on the outside of the feedline.
Quote:


Wouldn't the trash can scatter and reflect the signal into multipath oblivion?

On the contrary, the trash can narrows the beamwidth to ignore off-axis reflections and benefits from the internal reflections to enhance the direct signal; Bill mentions an increase in gain from the internal reflections that are close enough in time sequence so that the tuner can handle them. The ability of a tuner to handle multipath reflections is determined (in part) by the time difference between the arrival of the direct signal and the reflected signal. The greater the difference for acceptance, the better the tuner.

The British DVB-T system handles multipath better than 8VSB, but it requires a SNR greater than 21dB, which means more transmitter power for the same coverage area.

The term "anti-ghosting" is a holdover from analog days. We saw a secondary image on the screen from multipath reflections. The distance between the primary image and the ghost image is an indication of the time difference between the direct signal and the reflected signal.

The depth of the shielded enclosure is what narrows the acceptance angle. A yagi would require a deep enclosure, but a reflector type antenna doesn't need to be much than 1/2 wave deep (1/4 wave for the antenna to rear plus maybe another 1/4 in front....to be determined by experiment).

Using a long yagi for its narrower beamwith to fight harmful multipath reflections is a traditional approach, but using a "shrouded" antenna is even more effective.

Another technique previously used is to substitue a CM4228 for a CM4221 antenna for UHF multipath problems. The 4228 has a much narrower horizontal beamwidth because of horizontal stacking even though it doesn't have much more gain.

In Trip's location his local VHF signals are strong, so he doesn't need a high gain antenna, but he does need a very narrow beamwidth to reject off-axis multipath reflections, which is why I suggested a folded dipole or a fullwave loop as the active element. Either of these would be smaller than a fullwave bowtie so that the shroud could be smaller. A more expensive off-the-shelf solution would be an AD C5 in a can.

If you can not measure it, you can not improve it.
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post #64 of 89 Old 12-17-2011, 11:04 AM - Thread Starter
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Well, the reflections must be way too strong on the deck to overcome, as an hour of waving around my garbage can yielded nothing but a few good positions for WELF (which I don't care about) and a few decodes of WFLI. Nothing else.

I did find a position in my bedroom where it decodes almost everything, though that position is on my bed. According to the Pinnacle tuner in my desktop, which shows nothing below its minimum decode value of 60%:

WTVC-09 No decode
WDEF-12 66-70%
WRCB-13 70-72%
WELF-16 No decode
WTCI-29 78-84%
WNGH-33 0%-94% (Wild fluctuations lead to frequent drops; works on the outdoor antenna anyway)
WDSI-40 94-96%
WFLI-42 100%
WYHB-44 74-78%
WPXA-51 64-70% (Drops occasionally; works on the outdoor antenna anyway)

- Trip

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post #65 of 89 Old 12-17-2011, 11:16 AM
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Quote:
Originally Posted by rabbit73 View Post

Yes, trash can.

The depth of the shielded enclosure is what narrows the acceptance angle. A yagi would require a deep enclosure, but a reflector type antenna doesn't need to be much than 1/2 wave deep (1/4 wave for the antenna to rear plus maybe another 1/4 in front....to be determined by experiment).

I'm not sure if the depth of the shielded enclosure affects the acceptance angle. In my radar antenna days, what affects the antenna beamwidth or acceptance angle is the aperture area at the mouth of the trash can. The trash can is like a very large circular waveguide and I would think the antenna waveguide theory applys here. The length of the waveguide does not affect beamwidth.

There is also a cutoff frequency which is defined as the lowest frequency that will enter the waveguide.
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post #66 of 89 Old 12-17-2011, 03:08 PM
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Which one of your tuners is best able to handle multipath, the DTT900?

If you can not measure it, you can not improve it.
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post #67 of 89 Old 12-17-2011, 03:13 PM - Thread Starter
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Probably either the DTT900 or the Aero-M, but the former requires a TV set (which I don't have) and the latter I haven't actually tried yet with this configuration, not that it did any better on the outdoor antenna than my HDHR does.

- Trip

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post #68 of 89 Old 12-17-2011, 05:42 PM - Thread Starter
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I think I finally have a relatively convenient position in the room where I get the Chattanooga locals, using the DUV-Vii instead of the Silver Sensor with the trash can.

WTVC-09 68%
WDEF-12 74%
WRCB-13 82%
WELF-16 No Decode (No loss)
WTCI-29 74%
WNGH-33 82%
WDSI-40 100%
WFLI-42 100%
WYHB-44 70%
WPXA-51 No Decode (Fine on outdoor antenna)

I am having serious trouble balancing WTVC, WTCI, and WYHB. The position it's in makes WTVC flaky; I'm seeing it vary from 64% to 74% with one or two odd drops every few minutes. I'll need to play with it, but it's much closer to ideal than I've had from any setup I've tried up until this point.

- Trip

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post #69 of 89 Old 12-18-2011, 11:02 AM
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Quote:
Originally Posted by Trip in VA View Post

I think I finally have a relatively convenient position in the room where I get the Chattanooga locals, using the DUV-Vii instead of the Silver Sensor with the trash can.

WTVC-09 68%
WDEF-12 74%
WRCB-13 82%
WELF-16 No Decode (No loss)
WTCI-29 74%
WNGH-33 82%
WDSI-40 100%
WFLI-42 100%
WYHB-44 70%
WPXA-51 No Decode (Fine on outdoor antenna)

I am having serious trouble balancing WTVC, WTCI, and WYHB. The position it's in makes WTVC flaky; I'm seeing it vary from 64% to 74% with one or two odd drops every few minutes. I'll need to play with it, but it's much closer to ideal than I've had from any setup I've tried up until this point.

- Trip

Looking at the Georgia Tech setup, it looks like he used a small trash can since the DB2 antenna barely fits. The smaller can helped him to receive only that one UHF channel.

For your UHF reception only, I would try the smallest trash can that can hold a single UHF bowtie antenna (around 12" to 13" diameter). This proposed setup would not work at VHF since a larger trash can is required. Good luck.
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post #70 of 89 Old 12-18-2011, 12:25 PM - Thread Starter
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The trash can I got is about 20" in diameter, give or take. I haven't measured to be sure. I do know that the DUV Vii does not fit in it, though my current successful antenna position has the antenna sitting immediately in front of the trash can.

I've attached two photos of the current setup. Certainly not ideal in terms of space, appearance, or anything else, but I am receiving all of the locals.

- Trip
LL
LL

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post #71 of 89 Old 12-18-2011, 02:38 PM
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Quote:
Originally Posted by Trip in VA View Post

The trash can I got is about 20" in diameter, give or take. I haven't measured to be sure. I do know that the DUV Vii does not fit in it, though my current successful antenna position has the antenna sitting immediately in front of the trash can.

- Trip

Try plan B then, the Silver Sensor (HDTVi) is about 13 inches wide so try a trash can that the Silver Sensor barely fits into.

A little theory here. We want the TV signal to propagate down the can to the antenna in the back end. A too low frequency will not enter the can, a too high frequency will enter but will not propagate efficiently. I refer you to the 'Cantenna' which is being used at 2.4 gHz for more details. I am scaling it so that it works over some of the UHF band.
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post #72 of 89 Old 12-18-2011, 03:55 PM - Thread Starter
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What I'm thinking about doing is building an antenna (dipole maybe?) approximately the size of the can I have while I'm home next week. Since I have three VHFs I'm trying to pull in in addition to my much stronger UHFs, I'm hopeful that the size is close enough to make it work. Only one way to find out, really.

- Trip

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post #73 of 89 Old 02-23-2012, 05:29 PM - Thread Starter
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Well, this weekend I have in my possession a Rohde and Schwarz ETL, with all the options, that I can use until Monday when I have to ship it back. I've got it hooked to the antenna on my deck at the moment, and since it has a much higher resolution than my Sencore, I do think I better understand what is going on now.

I can compare WSB-39 and WDSI-40 and the difference becomes evident. I've attached a photo.

Unfortunately, it seems that the ETL won't give me an echo plot until it decodes a signal, which won't decode in this case due to multipath which would be displayed by the echo plot. I can see, however, that my Atlanta signals have weak echoes at about 35-40µs, so it is not a stretch for me to believe the locals have strong echoes at that range as well.

I also have two Mobile DTV phones until Monday. Got WPXA at my desk with both of them and will try for the rest of the Atlanta M/H signals on my deck tomorrow and over the weekend.

- Trip
LL

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post #74 of 89 Old 02-23-2012, 06:14 PM
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Trip, quick question is WOOT broadcasting from the old AT&T microwave tower on Signal?
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post #75 of 89 Old 02-23-2012, 06:18 PM - Thread Starter
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No, Henry put up a tower at Montlake.

- Trip

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post #76 of 89 Old 02-23-2012, 08:27 PM
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Trip, can you do a single sweep on WDSI and see if the amplitude really fluctuates by 10 dB, Thanks.
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post #77 of 89 Old 02-24-2012, 07:25 AM
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Trip - Thanks for the PM heads up. Looks like a wonderful piece of equipment. I watched most of the YouTube videos on it. I had to laugh when he said the ETL showed an MER of 49dB. Only on the bench, never in the real world!

Maybe you could post some images showing what different multipath looks like. I've learned that it's not possible to see multipath with my spectrum analyzer.

How many tens of thousands does that thing cost?

Chuck
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post #78 of 89 Old 02-24-2012, 10:45 AM
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Well, you're going to learn what it's like for a guy who drives a sedan to get a Lamborghini for the weekend!

Have fun!

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post #79 of 89 Old 02-25-2012, 04:36 PM
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Trip, your trash can antenna reminds me of someone in my local HDTV discussion using a pizza size sat. dish as a reflector back when dtv first got started. haha
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post #80 of 89 Old 02-26-2012, 10:53 AM - Thread Starter
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Quote:
Originally Posted by Calaveras View Post

Trip - Thanks for the PM heads up. Looks like a wonderful piece of equipment. I watched most of the YouTube videos on it. I had to laugh when he said the ETL showed an MER of 49dB. Only on the bench, never in the real world!

Maybe you could post some images showing what different multipath looks like. I've learned that it's not possible to see multipath with my spectrum analyzer.

All of my multipath looks basically the same here, just like the image I posted. All of my signals either look like WSB or like WDSI, just at varying power levels.

WNGH and WPXA have good looking echo plots. WKSY has almost no echoes, so no wonder I have no issues decoding it. The Atlanta signals all show one large-ish echo around 37µs and another around 41µs, but not severe enough to damage reception, nothing closer. I wonder how far away that puts the two reflections.

Still no luck with Chattanooga on the deck.

Quote:
Originally Posted by Calaveras View Post

How many tens of thousands does that thing cost?

Several!

Quote:
Originally Posted by retiredengineer View Post

Trip, can you do a single sweep on WDSI and see if the amplitude really fluctuates by 10 dB, Thanks.

It looks like it does.

- Trip

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post #81 of 89 Old 02-26-2012, 01:25 PM
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My comment:

Trip, can you do a single sweep on WDSI and see if the amplitude really fluctuates by 10 dB, Thanks.

Your response:

It looks like it does.

I believe your multipath problems are due to the nearby freeway to your north that lies right along your local stations LOS (as seen from TVFool). Multipath signals are reflecting of the freeway. If they were static reflections than you would not see any problems. However the multipath signals are constantly changing due to freeway traffic. Dynamic multipath signals cannot be processed by your TV.

May I suggest using the two-antenna trick which can be found at:

www.hdtvprimer.com/ANTENNAS/silver.html#TAT

Your goal is to put a null in the LOS direction thereby minimizing that multipath signal and hoping you can find a signal reflecting from a non-LOS direction with minimum multipath interference.

Good Luck.
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post #82 of 89 Old 02-26-2012, 02:09 PM - Thread Starter
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Here's a set of photos. If it starts with "IN" then it's using the garbage can. If it starts with "XF" it's using the antenna on the deck.

http://www.rabbitears.info/gallery3/...php/ETL-Photos

- Trip

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post #83 of 89 Old 02-26-2012, 02:50 PM
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Quote:
Originally Posted by Trip in VA View Post

Here's a set of photos. If it starts with "IN" then it's using the garbage can. If it starts with "XF" it's using the antenna on the deck.

- Trip

I compared the WDSI IN and XF spectrums and at first I thought the IN spectrum looked pretty clean until I saw you were averaging the IN spectra. Can you also post a single scan WDSI IN spectrum. Thanks.
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post #84 of 89 Old 02-26-2012, 02:56 PM - Thread Starter
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Oops. Yes, I will. I'm using that setup at the moment though, when I'm done I will capture corrected images from that antenna system.

- Trip

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post #85 of 89 Old 02-26-2012, 04:03 PM - Thread Starter
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I have added the requested photos. They are IN-SS.

- Trip

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post #86 of 89 Old 02-26-2012, 04:09 PM
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I'd love to see my echo plots here. I'll bet some are pretty ugly. Are all those many peaks reflections or is most of that noise?

I can't see that much variation in amplitude unless I set the RBW to 1KHz and any video filtering reduces it.

I see you have a number of LP co-channel problems just like here.

Chuck
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post #87 of 89 Old 02-26-2012, 04:24 PM - Thread Starter
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I am pretty sure they are reflections. I certainly have enough things around to cause them.

One of my co-channel problems is rather annoying. WTNB-27 is the bane of my existence, since it allows WAGA to come in solidly about 90% of the time but only if I'm aimed off a bit from Atlanta. It's a careful balancing act to keep enough signal for a decode of WAGA while pulling down WTNB enough.

WDGA-43 is in the same direction as WUPA, so there's not a lot I can do about that one except wait for the conversion to digital on 47.

If I cared about any of the programming on WATL I would probably be more concerned about WPDP-25.

- Trip

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post #88 of 89 Old 02-27-2012, 08:12 AM
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Quote:
Originally Posted by Trip in VA View Post

I am pretty sure they are reflections. I certainly have enough things around to cause them.

I suspect that most people have plenty of things around them to cause reflections. Why else would there be so many reports of signal qualities in the 70's and 80's when by all other indications the signals should be strong enough to be close to 100%?

So I take it that the Echo plot would show nothing except the main signal if there were no reflections?

Nothing like a new piece of test equipment to give you new insight into what the typical problems are.

Chuck
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post #89 of 89 Old 02-27-2012, 05:28 PM - Thread Starter
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Quote:
Originally Posted by Calaveras View Post

So I take it that the Echo plot would show nothing except the main signal if there were no reflections?

Yes. You can see some of my signals are really clean, while others are pretty bad. On the XF, most wouldn't decode at all, which I assume means the echo plot would look pretty vicious.

Quote:
Nothing like a new piece of test equipment to give you new insight into what the typical problems are.

Indeed!

- Trip

N4MJC

Comments are my own and not that of the FCC (my employer) or anyone else.

RabbitEars

"Ignorance and prejudice and fear walk hand in hand..." - Rush "Witch Hunt"

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