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Old 23-Oct-2015, 2:10 AM   #1
astroantoine
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Join Date: May 2015
Posts: 2
Channels Good One Day Bad The Next

Hello!

So I installed an Antenna Craft HBU11 on the roof of my apartment building in San Francisco (about 70ft off the ground). I have a clear line of sight to the two broadcast towers I care about and they are 2 and 5.5mi from the antenna. So the signal comes in hot.

What's bugging me is that sometimes certain channels stop coming in clearly and will come in clear again a few days later. I know that is often caused by reflections but I just don't see how I could be getting reflections. There are two other antennas mounted behind my HBU11 for our building internet (plastic dish shaped antennas on steel masts) but I don't think those are close enough to cause interference are they? I can post pictures later if needed.

But what's even weirder is that all these channels except 11.1 are coming from the same tower, but not all of them consistently are good one day and bad the next. For example, I get channel 5.1 broadcasting from the same exact tower as channel 2.1 and 2.1 is the channel that comes in normally some days and really corrupted other days. BUT channel 5.1 always comes in rock solid.

Below is my TV Fool report:
http://www.tvfool.com/?option=com_wr...8e03f1716d4ed0

Can anyone offer any pointers to make these channels come in consistently?

Thanks!
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Old 23-Oct-2015, 11:26 AM   #2
Tower Guy
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The problem could be signals that are too strong. Try a 6 db attenuator between the antenna and TV set.
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Old 23-Oct-2015, 12:44 PM   #3
ADTech
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Keep in mind that UHF signal powers (especially) will be far below those predicted by TVFool when this close to the towers. The simulation assumes that all transmitting antennas radiate equally in the vertical plane which they do not. At only several miles from Sutro, the signal powers will be a mere fraction of those in the main lobe. For example, KPIX transmits about 686 kW in this OP's direction in its main lobe. However, the angle of depression is about 10. KPIX's FCC application (https://licensing.fcc.gov/cdbs/CDBS_...um=1&exhcnum=1) happens to include the calculated vertical pattern on page 9 of the engineering exhibit. According to that plot, voltage gain is about 0.1 the maximum at 10 below horizontal. Convert that to a power gain by squaring that and then multiplying by the ERP in the main lob and the result is ~6860 watts. This is a relatively extreme example as KPIX uses a very high gain antenna. VHF stations and FM are much lower gain with much more ERP at lower depression angles so their signals are going to be closer to the calculated (in TVF) signal powers.

Some attenuation is likely to be helpful as it will also reduce any undesired signals that might be present and causing issues.

Move the antenna several feet and try again. Repeat as needed.
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Last edited by ADTech; 23-Oct-2015 at 3:53 PM.
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Old 23-Oct-2015, 3:48 PM   #4
Tim
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Do you have just the HBU11 and coax running to one TV? Or do you have other components in the system?
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Old 24-Oct-2015, 1:17 AM   #5
rabbit73
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Multipath reflections are certainly possible, but it looks more like overload not only from TV signals, but also FM signals that are too strong.
Attached Images
File Type: jpg astroantoineTVF FM est.JPG (101.0 KB, 392 views)
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Old 3-Apr-2016, 7:14 PM   #6
astroantoine
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Thanks very much for your help. My apologies for the extremely delayed response.

TL;DR - Turns out antenna position, horizontal rotation (parallel to ground) and an unnecessary 3-way coax splitter in my apartment were all causing less than ideal reception. The adjustments now bring in a great and more importantly consistent signal.

Tim, so I should have more clearly stated the set up at the get go. From the antenna, about 80' on the top of my apartment building, coax runs about 100ft to a grounding block in my garage. From there I use a coax splitter (not amp'd) to send the signal to 4 units total. To my unit the TOTAL line run from antenna on the roof to my wall jack is probably about 120'. At that point I found out that the coax was then split AGAIN behind the wall in my apartment before going to all the wall jacks. I talk more about that below but basically I took that out as it's not needed and that seemed to help! I'm using a Tablo for viewing in my unit.

Tower Guy/rabbit73, I tried the attenuator as it seems that you guys were noticing the strong signals I'm getting. That didn't make a noticeable difference unfortunately.

ADTech, this was SUPER informative. I'm somewhat technical, but a noob when it comes to antennas so the links you provided were awesome. I knew from basic school stuff that there was a chance I was "underneath" the signal of the tower, but that doc and your basic equation REALLY illustrates that I could be getting bupkis when it comes to signal power.

ALLLLL that being said I was able to adjust the antenna's position and lateral rotation to greatly improve my signal reception. I also found that the cable coming from the garage to my apartment was split 3 ways before going off to various wall jacks in my place. So, took that out of the equation and boom now I'm getting consistent reception across all my channels (even the potentially much lowered KPIX signal)!

Thanks again for the help, I'll pay it forward!
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Old 3-Apr-2016, 10:27 PM   #7
rabbit73
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Ah, so; a lot of distribution losses.

Thanks for the update that adds to the knowledge base.
Quote:
I'll pay it forward!
You just did
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