My Answer to "Need Help with Combining Antennas" Post by 4.6 Explorer
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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:
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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. http://forum.tvfool.com/attachment.p...9&d=1442362452 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:
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Any further questions please ask or if I wasn't clear enough re-ask |
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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:
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. http://forum.tvfool.com/attachment.p...2&d=1441462888 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: http://forum.tvfool.com/attachment.p...8&d=1439497644 |
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When I click on the CHCJ callsign in your report I see the escarpment:
http://forum.tvfool.com/attachment.p...1&d=1442431709 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: http://forum.tvfool.com/attachment.p...2&d=1442432735 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.. |
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 :D 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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I did a profile for a location further north at Roxborough Ave and Cope St. The signal just clears the escarpment: http://forum.tvfool.com/attachment.p...7&d=1442606564 Quote:
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. http://forum.tvfool.com/attachment.p...9&d=1442620525 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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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. http://forum.tvfool.com/attachment.p...0&d=1442622539 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: http://forum.tvfool.com/attachment.p...7&d=1441052886 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: http://forum.tvfool.com/attachment.p...0&d=1441054170 This is what the same signals look like with a badly overloaded preamp: http://forum.tvfool.com/attachment.p...8&d=1441052947 This is what the same signals look like with a high quality preamp that isn't easily overloaded: http://forum.tvfool.com/attachment.p...9&d=1441053000 |
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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:
Most tuners will drop out a signal at about -85 dBm. 5.2 Multi-Signal Overload Quote:
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: http://forum.tvfool.com/attachment.p...1&d=1442623731 End of short course. |
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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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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