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Old 16-Sep-2015, 12:44 AM   #1
rabbit73
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My Answer to "Need Help with Combining Antennas" Post by 4.6 Explorer

Quote:
Originally Posted by 4.6 Explorer View Post
I have two 8-bay collinear arrays stacked on top of each other aimed east to capture the Buffalo and Grand Island stations about 50 miles away. Decent signals for the most part with occasional drops outs due to weather time of day (or whatever). Those two are also using a preamp CM 7778;

one simple 4bay to Capture Toronto towards the north and another Yagi that aims west for a single station.

Presently the single 4bay is not hooked up, yet

Because I'm below an escarpment the western transmitter is hard to capture and it's only about 4 miles away. I do get it but a very weak signal.

When I bought a used 8 bay a weeks ago the mechanical properties of the used 8 bay were in terrific condition but the simple combiner that came with had fried due to a light lightning strike or eddy current. No burn marks anywhere but the coax wires inside the combiner popped like fuses.
I simply rewired as original and put it back in action. I do have more gain and adding the preamp certainly helps even more so.

I copied that concept to make a second combiner by stripping an existing two-way splitter and wiring the same as the original one.
So, right now, I have the western facing Yagi into one side of the combiner and the two eastern facing 8bays with a preamp to the other side of the combiner down by the TV set it self.

That's all that's active so far. My plan is run a third coax line from the 4-bay and create a 3 way combiner and hopefully capture an almost 180 degree signal pattern.

yes? No?

What would you advise?

Thanks thus far to all you guys.
We need some more information to make a good analysis.

Please tell us the town or city where you are located and give us a tvfool report using your exact address (which will not show) or coordinates (which will be shortened) using this:
http://www.tvfool.com/index.php?opti...pper&Itemid=29

If you have a problem with that page determining your location, use the interactive map browser with exact coordinates, or move the cursor to you antenna location and generate a report by clicking on Make Radar Plot >> at the upper right corner of the map and give us the URL link in bold type near the top:
http://www.tvfool.com/index.php?opti...pper&Itemid=90

Quote:
I have two 8-bay collinear arrays stacked on top of each other aimed east to capture the Buffalo and Grand Island stations about 50 miles away. Decent signals for the most part with occasional drops outs due to weather time of day (or whatever). Those two are also using a preamp CM 7778;
Does that mean you have combined the two 8-bay antennas into a 16-bay that is connected to the 7778? Is it the old 7778 with separate VHF and UHF inputs or the new 7778 with only one antenna input?
Quote:
and another Yagi that aims west for a single station.
You have more than one yagi? Is it a VHF or UHF yagi? What is the callsign of that station?
Quote:
Because I'm below an escarpment the western transmitter is hard to capture and it's only about 4 miles away. I do get it but a very weak signal.
The transmitter is 4 miles away or the escarpment is 4 miles away?
Quote:
So, right now, I have the western facing Yagi into one side of the combiner and the two eastern facing 8bays with a preamp to the other side of the combiner down by the TV set it self.

That's all that's active so far. My plan is run a third coax line from the 4-bay and create a 3 way combiner and hopefully capture an almost 180 degree signal pattern.... yes? No?
Combining two antennas aimed in different directions, using a splitter in reverse as a combiner, doesn't always work because when the same signals arrive at the combining point they will interfere with each other if they are not in phase.

When the coax lines need to be the same length

If you have two identical antennas, aimed in the same direction, and are using a splitter reversed as a combiner, the coax lines must be the same length for maximum gain. You will be able to get up to 2.5 dB more, 3 dB because of doubling the signal minus the 0.5 dB internal loss of the combiner.

When the antennas are aimed in the same direction, the incoming wave front arrives at both antennas at the same time, and the signals arrive at the combiner at the same time, so they add in phase.

This only works if the wave front is uniform across both antennas. If the wave front is not uniform across both antennas (like thru trees), you don't get the gain you expected. This explains why a 4-bay bowtie antenna sometimes works better than an 8-bay bowtie, like 4221 VS a 4228, because it has a smaller capture area.



And when they don't need to be the same length

If the two antennas are NOT aimed in the same direction, the incoming signals do not reach each antenna at the same time, so it is not necessary to have the coax lines the same length, because the same signals aren't going to arrive at the combiner at the same time anyway. This means that they might interfere with each other because they aren't in phase.

It is possible to adjust the lengths of the coax lines to different lengths so that one desired signal arrives at the combiner in phase, but that often harms the other signals that might have been OK before adjusting the lengths.

My previous post sending you here to solve your reception problem:
Quote:
Originally Posted by rabbit73 View Post
What started out as an antenna theory question has turned into a reception problem question. I quoted your question and have given my answer on the Help With Reception thread:
http://forum.tvfool.com/forumdisplay.php?f=7

Please go to:

My Answer to "Need Help with Combining Antennas" Post by 4.6 Explorer
http://forum.tvfool.com/showthread.php?t=15747

You will get more help there, and ADTech might have a better answer that I have overlooked. He works for Antennas Direct and has a lot of experience with 8-bay antennas because they make the DB8 and the newer DB8E.
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File Type: jpg Non-uniform fields HDTVPRIMER snip2_1.jpg (142.5 KB, 1860 views)
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Last edited by rabbit73; 16-Sep-2015 at 11:20 AM.
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Old 16-Sep-2015, 12:15 PM   #2
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Quote:
Originally Posted by rabbit73 View Post
We need some more information to make a good analysis.

Please tell us the town or city where you are located and give us a tvfool report using your exact address (which will not show) or coordinates (which will be shortened) using this:

I live in Hamilton Ontario. As seen below CHCJ is the one western transmitter I'm after. Its Tx Total distance: 4.49 km (2.79 mi) miles away but the escarpment is 850 meters (Google maps).
On my north are all the Toronto stations; no issue there. I even pick many of those up off the side of my eastern facing two 8 bays

http://www.tvfool.com/index.php?opti...pper&Itemid=29

http://www.tvfool.com/?option=com_wr...8e03f6c65e440a

If you have a problem with that page determining your location, use the interactive map browser with exact coordinates, or move the cursor to you antenna location and generate a report by clicking on Make Radar Plot >> at the upper right corner of the map and give us the URL link in bold type near the top:
http://www.tvfool.com/index.php?opti...pper&Itemid=90

Does that mean you have combined the two 8-bay antennas into a 16-bay that is connected to the 7778? Is it the old 7778 with separate VHF and UHF inputs or the new 7778 with only one antenna input?

That is all correct. I created my own 16 bay from two 8 bays and it's a CM 7778 with one UHF input and between the preamp power inserter and the TV is my combiner
You have more than one yagi?
No just one UHF Yagi and the station is CHCJIs it a VHF or UHF yagi? What is the callsign of that station?


Combining two antennas aimed in different directions, using a splitter in reverse as a combiner, doesn't always work because when the same signals arrive at the combining point they will interfere with each other if they are not in phase.

When the coax lines need to be the same length
All the 8 bay connection lines to each combiner (on their own antenna) are carefully made the same length, In essence 4x4 bays with identical lengths. I DONOT recall how exact the feed from the two 8-bays antennas to the preamp is. Is that crucial? If you have two identical antennas, aimed in the same direction, and are using a splitter reversed as a combiner, the coax lines must be the same length for maximum gain. You will be able to get up to 2.5 dB more, 3 dB because of doubling the signal minus the 0.5 dB internal loss of the combiner.All the combiners that I used are copycats from that original combiner that I found with the one 8-bay. They do seem to work, how well needs to be determined

When the antennas are aimed in the same direction, the incoming wave front arrives at both antennas at the same time, and the signals arrive at the combiner at the same time, so they add in phase.

This only works if the wave front is uniform across both antennas. If the wave front is not uniform across both antennas (like thru trees), you don't get the gain you expected. I do have a tall 80ft poplar tree about 50ft away from the 16-bay [2x8 bay combination] in direct line of sight for the eastern facing NOTE: Full foliage now no leaves in the winter. I tried to take all that into consideration. Consider my tower is in a fixed location and I'm not cutting down that beautiful tree This explains why a 4-bay bowtie antenna sometimes works better than an 8-bay bowtie, like 4221 VS a 4228, because it has a smaller capture area.



And when they don't need to be the same length

If the two antennas are NOT aimed in the same direction, the incoming signals do not reach each antenna at the same time, so it is not necessary to have the coax lines the same length, because the same signals aren't going to arrive at the combiner at the same time anyway. This means that they might interfere with each other because they aren't in phase.

It is possible to adjust the lengths of the coax lines to different lengths so that one desired signal arrives at the combiner in phase, but that often harms the other signals that might have been OK before adjusting the lengths.
My answers are highlighted in red
Any further questions please ask or if I wasn't clear enough re-ask
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Old 16-Sep-2015, 12:44 PM   #3
rabbit73
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Thanks for the tvfool report and more details. I will try to make some useful comments.

http://www.tvfool.com/?option=com_wr...8e03f6c65e440a
Quote:
I live in Hamilton Ontario. As seen below CHCJ is the one western transmitter I'm after. Its Tx Total distance: 4.49 km (2.79 mi) miles away but the escarpment is 850 meters (Google maps).
On my north are all the Toronto stations; no issue there. I even pick many of those up off the side of my eastern facing two 8 bays
I'm not sure if you know, but there is a Canadian forum with a reception thread for your area. It might be helpful if you also asked about your problem there:
OTA Reception Results
http://www.digitalhome.ca/forum/129-...ption-results/

ON - Hamilton, Stoney Creek, Brantford, Haldimand
http://www.digitalhome.ca/forum/129-...haldimand.html

The first thing I see is that you have two very strong transmitters that might overload your preamp. Fortunately, you picked the 7778 that tolerates overload better than the 7777. Even if the overload is only moderate, the IMD (Intermodulation Distortion) from your two strongest signals can create spurious signals in the preamp that raise the noise floor to damage your weakest signals.

CITS has a Noise Margin of 76.1 dB even before adding the preamp gain.



Interpreting Noise Margin in the TV Fool Report
http://www.aa6g.org/DTV/Reception/tvfool_nm.html

When I was first learning about Noise Margin, I made this chart to help me understand the concept:

Attached Images
File Type: jpg 4.6 ExplorerTVFmap.JPG (63.7 KB, 1227 views)
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Last edited by rabbit73; 19-Sep-2015 at 2:19 AM.
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Old 16-Sep-2015, 8:28 PM   #4
rabbit73
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When I click on the CHCJ callsign in your report I see the escarpment:



I then did another profile using different software based on your estimated location (I don't have your exact coordinates) which puts you about 1 block SSW of Shanghai Chinese Food and Rosedale Sports Bar and Eatery:



The escarpment is definitely in the way, and UHF signals can't make it over a peak by diffraction as well as VHF.

Tell us about your yagi. I wonder if tilting the front end up would help?

And then I did an FM FOOL report which shows some very strong FM signals that might interfere with TV reception, if the filter in the 7778 isn't sufficient; see attachment 3. Canadian DHC forum member roger1818 told me that the FM Fool database isn't up to date for Canadian FM stations.
http://www.mcmelectronics.com/produc...-FM-88-/33-341

I suggest you make some tests with each antenna in its respective direction without combining. If it can't get what you want alone, it can't do it combined.

Try the yagi for channel 35 with a UVSJ (high and common ports) between the antenna and the preamp. If that doesn't give you CHCJ try inserting an attenuator between the UVSJ and the preamp to reduce any possible spurious signals created by IMD. Then try a similar test with the 8-bays.

https://www.antennasdirect.com/store...ttenuator.html

http://www.3starinc.com/drop_in-line_attenuator.html

http://home-automation.smarthome.com...=&w=attenuator

http://mjsales.net/collections/atten...ant=1083705673
•Attenuation values 1, 3, 6, 8, 10, 12, 16, 20dB (FAM)
click on 1 dB for other values; the up and down arrows are faint

If you want a short course on overload and IMD, ask me.

73,
rabbit
W4...
ex-W2...
ex-DL4..
Attached Images
File Type: jpg 4.6 ExplorerTVFprofile1.JPG (60.6 KB, 1219 views)
File Type: jpg 4.6 ExplorerTVFprofile2.JPG (124.6 KB, 1592 views)
File Type: jpg 4.6 ExplorerTVF FM est.JPG (107.1 KB, 269 views)
__________________
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Last edited by rabbit73; 16-Sep-2015 at 11:11 PM.
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Old 18-Sep-2015, 12:48 PM   #5
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That profile of the escarpment spells out exactly my situation, which explains why my son can pick up this station better than me and he has "almost" the same OTA set-up as me but he's about 8 blocks north east of me. Truthfully after your previous post to this one, I did some more research on my own and your graph of the escarpment says it all.

I thought I might be in the Fresnel zone to pick up CHCJ and probably am, hence is why my reception is spotty. Maybe add 200 feet worth of tower and life would be good.

My real challenge is climbing that tower. I'm almost 68 and while I have a waist belt and a body belt (used together) I'm still not as young as I used to be. As silly as it might sound, I wonder about what might be involved with respect to azimuth in order to capture RF within the Fresnel zone. (as you suggested) If you think that might work, what degree of tilt would you suggest?
One thing in my favor is the dreaded winter that I hate so much. But, when the leaves start falling it could reduce natural attenuation (yes, no?)
Plus, in the cold, certain things, wires and RF can become super conductors and might enhance my cause.

I remember last year I borrowed a friend's MFJ antenna analyzer and one would think I would write down the resonant frequency of my Yagi.(guess what Thought did?)

I found that virtual channel 35.1 is 596 to 602 MHz and I don't recall the Yagi's center frequency. I might borrow it again and try from my desk. Hopefully the coax won't affect it's accuracy, as I'm sure it might.

This transmitter that I am trying to capture is a repeater transmitter from a station further up north CKVR on one of our many Canadian networks CTV. Their news base often reports the more northern news and other events. It's near that area where we go camping, traveling and peace of mind turf. Even their local advertising enlighten our sprits with boating and RV stuff.

You suggested CITS as being too strong and an overload. The structure of their program is religious and not my thing (no disrespect to others)
I'd be happy to trap that out entirely. I checked the signal level this morning and while 36.1 is not full scale it's up there at 82% (TV's own meter)

36.1 is 602 to 608 MHz. close enough?
Yes, anything you have to offer for my education and learning is an absolute asset. For me and everyone.

I'm going by this Digital Television Physical Channel Frequencies http://www.hmtech.info/av/dtv-channels.php These channel assignments are at the very bottom of the chart.
Sorry, my copy and paste below didn't turn out the way I expected.
Virtual channel left most number(s). SPACE next 2 numbers Space next 2 numbers are the lower and upper frequencies for each channel


Channel

Lower
bound
MHz

Upper
bound
MHz

1 44 50
2 54 60
3 60 66
4 66 72
5 76 82
6 82 88
7 174 180
8 180 186
9 186 192
10 192 198
11 198 204
12 204 210
13 210 216
14 470 476
15 476 482
16 482 488
17 488 494
18 494 500
19 500 506
20 506 512
21 512 518
22 518 524
23 524 530
24 530 536
25 536 542
26 542 548
27 548 554
28 554 560
29 560 566
30 566 572
31 572 578
32 578 584
33 584 590
34 590 596
35 596 602
36 602 608
37 608 614
38 614 620
39 620 626
40 626 632
41 632 638
42 638 644
43 644 650
44 650 656
45 656 662
46 662 668
47 668 674
48 674 680
49 680 686
50 686 692
51 692 698
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Old 18-Sep-2015, 9:02 PM   #6
rabbit73
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Quote:
I found that virtual channel 35.1 is 596 to 602 MHz
Quote:
36.1 is 602 to 608 MHz. close enough?
Well, yes, but when you design an antenna for a certain channel you should use the real channel number and not the virtual channel number. The real channel frequency is what the transmitter uses. The virtual channel number is a holdover from analog days to maintain the identity of the station for the viewer. In this case, the real channel numbers and virtual channel numbers are the same, but it isn't always that way.

I did a profile for a location further north at Roxborough Ave and Cope St. The signal just clears the escarpment:



Quote:
I wonder about what might be involved with respect to azimuth in order to capture RF within the Fresnel zone. (as you suggested) If you think that might work, what degree of tilt would you suggest?
I estimate the tilt at +6 to 7 degrees. Hams that do satellite contacts use an AZ/EL mount for their antenna. The AZ is for azimuth aim and the EL for the tilt.

There aren't many antennas that have a tilt adjustment, but the Antennas Direct 91XG does.

Keep in mind that the wavefront presented to the antenna might not be uniform. Sometimes making a small adjustment in the location of the antenna can make a difference, like raising the or lowering the antenna a 6" to 12". When I was doing a temporary setup across the street I setup a 2-bay UHF antenna, my Sadelco DisplayMax 800 signal level meter, and a preamp. I was able to get a nice scan and a stronger signal with the antenna aimed at the transmitter for CH42.



Interestingly, when I moved the antenna a few feet left or right, without changing the height or azimuth, there was a big difference in the signal strength and scan quality. Another example of layering in a non-uniform field. This is most likely because of the tree line in front of the antenna about 200 ft away.

When I was doing another temporary setup with a 4-bay UHF antenna, I noticed that it was necessary to tilt the top of the antenna back a little for max signal.

When I adjust the aim of an antenna, I use my signal level meter (SLM) for max signal. I then adjust the antenna aim for max signal quality as defined by SNR and errors; they don't always happen at the same azimuth. When I was first using my Apex DT502 converter box just after the transition to digital TV, I noticed that difference. The Apex has two signal bars, one for signal quality and one for signal strength. I now use my Sony TV that has a Diagnostics Screen that gives signal strength, SNR and errors.
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File Type: jpg CH42setup.jpg (132.5 KB, 2896 views)
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Last edited by rabbit73; 19-Sep-2015 at 11:54 PM.
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Old 19-Sep-2015, 1:29 AM   #7
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Quote:
Yes, anything you have to offer for my education and learning is an absolute asset. For me and everyone.
OK, now the short course:

THREE TYPES OF OVERLOAD

There are three types of preamp or tuner overload, in order of increasing signal strength:

1. The strong signals almost cause enough intermodulation distortion (IMD) to interfere with the reception of weak desired signals, but the spurious signals are at or below the noise floor of the weak signals. This is the point that holl_ands uses in his preamp charts to obtain max SFDR (Spurious Free Dynamic Range). No damage will happen.

As the strongest signals continue to increase in strength, more of the weaker signals are damaged until you reach:

2. The strong signals cause overload to the preamp or tuner that makes it impossible to receive any signals. No damage will happen. The strongest signals are still there, but they can't be decoded because the IMD products have damaged them so that they contain more errors (high BER....bit error ratio/rate) than can be corrected by the FEC (forward error correction).

3. The signals are so strong that the input transistor is toast. You are not likely to encounter OTA signals that strong, unless you live next door to a high power transmitter and you have your high gain antenna aimed at the transmitter's antenna.

As a general rule, tuners can tolerate stronger signals than preamps before overload. The difference in strength is approx. equal to the preamp gain.

Forum member holl_ands has made a preamp chart that shows the maximum input signal for preamps. Many of the preamps are no longer available, but it gives you the general idea.
http://imageevent.com/holl_ands/file...=0&w=1&s=0&z=4

Intermodulation Distortion

The IMD creates new spurious signals within the preamp (or tuner) itself that can interfere with the reception of your weakest desired signals if the spurious signals are stronger than the noise floor of the weakest desired signals. The spurious signals are caused by the interaction between two or more of your strongest signals.

IMD is not the only distortion that can be created within the preamp; you can also have distortion caused by signals so strong that the top of the strong signals are clipped, which causes compression of the signals. This can be seen if you increase the input to the preamp by, for example, 10 dB and the output increases by less than 10 dB.

Spurious Free Dynamic Range

The Spurious Free Dynamic Range needed is the difference in strength between your strongest signal and your weakest desired signal, plus 16 dB for the SNR of the weakest signal, using the dBm Pwr scale on your tvfool report. This difference is expressed in terms of dB, not dBm, because the original units are the same. The difference between the strongest signal and the weakest signal is the Signal Dynamic Range/Dynamic Range, which is 16 db less than the SFDR, because it doesn't include the SNR of the weakest signal.



Another way to think of SFDR is from the top of the strongest signal down to the bottom (noise floor) of the weakest desired signal. The top of the spurious signals must be at or below the noise floor of the weak signals if they are not to cause interference. The Signal Dynamic Range is from the top of the strongest signal down to the top of the weakest desired signal.
You can also use the NM scale for your calculations, but I prefer to use the Pwr scale because most tuners drop out around -85 dBm.

ADTech did some interesting tests with a spectrum analyzer that show how weak signals are damaged when a preamp is overloaded by strong signals:

Just the antenna, no preamp:



This is what the same signals look like when the preamp is moderately overloaded. Notice that the weakest signals are damaged first. The noise floor rises from the spurious signals created by the IMD. This reduces the SNR of the weak signals to less than the required 15 dB minimum:



This is what the same signals look like with a badly overloaded preamp:



This is what the same signals look like with a high quality preamp that isn't easily overloaded:

Attached Images
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Last edited by rabbit73; 19-Sep-2015 at 2:35 AM.
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Old 19-Sep-2015, 1:48 AM   #8
rabbit73
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And now to signal strength.

TVFOOL reports give signal strength in the dBm column, but it's more properly called signal power, because the dBm value is a ratio of the power of the signal as compared to the reference level of 0 dBm. 0 dBm doesn't mean no signal, it's just the reference level. Signals less than the reference level are given a minus sign; signals greater than the reference level are positive.

ATSC has suggested dBm values for TV receivers in ATSC A/74:

5 RECEIVER PERFORMANCE GUIDELINES

5.1 Sensitivity


Quote:
A DTV receiver should achieve a bit error rate in the transport stream of no worse than 3x10E-6 for input RF signal levels directly to the tuner from -83 dBm to -5 dBm for both the VHF and UHF bands.
To make an estimate of your situation, you can add the antenna gain and the preamp gain to the dBm value in your report.

Most tuners will drop out a signal at about -85 dBm.

5.2 Multi-Signal Overload

Quote:
The DTV receiver should accommodate more than one undesired, high-level, NSTC or DTV signal at its input, received from transmission facilities that are in close proximity to one another.For purposes of this guideline, it should be assumed that multiple signals, each approaching –8 dBm, will exist at the input of the receiver.
Tuners can tolerate much stronger signals than preamps. The difference is approx. equal to the preamp gain.

To give you an idea of how strong a signal needs to be, take a look at the dBm values in the signal strength calibration chart that I made for the Diagnostics Screen of my Sony KDL32R400A:



End of short course.
Attached Images
File Type: jpg SSCHART KDL32R400A.JPG (118.4 KB, 1639 views)
__________________
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Last edited by rabbit73; 19-Sep-2015 at 2:30 AM.
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Old 22-Sep-2015, 5:11 PM   #9
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Thank you for the large pile of info. I will be studying this info. I noticed my Yagi does NOT have an adjustable boom for the azimuth, but I might try creating a little adjuster plate, if only to use as a temp solution so to answer those questions. Yes, the escarpment is my biggest issue, hence that azimuth to capture the Fresnel zone of the signal. My attitude is maybe, maybe not.

In the mean time thank you, and the others, for all the help and guidance.

As a ham you and I both know that even though I retired from the cable company and I too, am a ham, there is a different world again with OTA. Understanding the basic RF in any of these is crucial. Audio is FM and video is AM and those signals operate differently especially under "weird" conditions.

Thanks
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Old 22-Sep-2015, 8:55 PM   #10
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Quote:
Originally Posted by 4.6 Explorer View Post
Understanding the basic RF in any of these is crucial. Audio is FM and video is AM and those signals operate differently especially under "weird" conditions.
That changed with the introduction of Advanced Television Systems Committee (ATSC) standards for digital TV. The modulation is Eight Level Vestigial Sideband (8VSB) and consists of digital packets representing the audio and video. There are a couple of articles on Wikipedia that explains the basics...just click the hyperlinks.
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Old 22-Sep-2015, 9:51 PM   #11
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From what I recall during my CATV days AND it being analogue way back when the Analogue signal was 6 megs wide per channel and the Vestigial sideband was sort of the protective guard band between channels.
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