Eugene, OR: OTA Reception strangely improves with 300/75 ohm combiner in line. - AVS Forum
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post #1 of 14 Old 06-14-2013, 10:14 AM - Thread Starter
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I was getting my local HD channels from Comcast QAM, but then they started encrypting everything, so I went back to my roof antenna.

For many years the roof antenna was working great, picking up all the channels without issue. Then one specific channel dropped out, channel 13 in the Eugene, OR market. I scanned and re-scanned, re-aimed the antenna, emailed the engineer at the station, no resolution to the problem. I knew it had something to do with either the antenna or the coax cable as I could plug in an indoor Silver Sensor antenna and be able to at least pick up the channel. With the roof I had zilch.

Now switching back to the roof antenna I was more determined to figure out what was wrong. Finally, after going over everything as I did before, I dug through my box of random RF gear and pulled out a 75-ohm / 300-ohm combiner that had a UHF hoop antenna attached to the 300-ohm terminals. I did this mainly to see what a more omnidirectional UHF antenna would do. And on a lark, I went ahead and screwed in the coax from the roof antenna to the combiner's 75-ohm jack.

With that, all channels, including 13 got picked up. That makes sense in terms of the combiner combining the signals from the roof and hoop antennae, but then why would 13 only show up when getting fed through the combiner? At first I thought the UHF hoop was picking up 13, so I removed the 75-ohm connection, only to see both 9 and 13 (the two stations in the VHF segment) drop out and all the UHF stations remain.

So it appears that the combiner is doing something to the roof antenna signal, but I can't fathom what as I don't know what's inside the combiner.

Here's the type of combiner I'm using: http://www.radioshack.com/product/index.jsp?productId=2062059&locale=en_US
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post #2 of 14 Old 06-15-2013, 09:07 AM
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Always include a location in antenna help threads. SEE THE STICKY AT THE TOP OF THE SECTION.

I also made your title more specific, as is the norm in these parts.

You'll also want to post your TVFool results, as also suggested in the sticky at the top of the section.

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post #3 of 14 Old 06-15-2013, 07:54 PM
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Quote:
Originally Posted by Cameron_Y View Post


So it appears that the combiner is doing something to the roof antenna signal, but I can't fathom what as I don't know what's inside the combiner.

Here's the type of combiner I'm using: http://www.radioshack.com/product/index.jsp?productId=2062059&locale=en_US

I entered Eugene OR into TVFool and it said Channel 13 is a very strong signal. My guess is channel 13 is overloading your TV. You reduced the signal level by using the combiner, which has insertion loss. You may want to try a variable attenuator. Solid Signal sells one, Model #1296F.
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post #4 of 14 Old 06-16-2013, 05:19 AM
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What kind of antenna is on the roof now and where is it aimed?

Ch 13 KVAL is actually VHF13, so if the antenna on your roof is UHF only, thats the reason why you are having issues. Also I noticed on your tvfool that you have other stations to the NE. So thats why I ask where is the antenna pointed now. It could be why you are getting the other stations fine, but Ch 13 is almost 180 opposite pointing SW.

Also I'm not sure how the combiner is playing a part in this since the UHF loop antenna should have no effect on Ch 13 being VHF.

And also even though you are 4 miles away from the tower, it looks as if there is a big mountain where the tower is on, so your signal is getting bent over the terrain, which is why it says 2edge reception. Post your exact tvfool results so we can get a better idea of reception in your area.
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post #5 of 14 Old 06-16-2013, 07:53 AM
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FWIW-1 some of the Radio Shack baluns of that physical design that were intended for FM radio signal use roll off brutally at UHF frequencies, so if you had some UHF stations that were strong, particularly the upper frequency UHF ones, then it may be helping by acting as sloping attenuator.

FWIW-2: There were Birdview satellite receivers that couldn't process Videocipher signals reliably unless there were back to back baluns inserted in the input coax. i never heard anyone advance a plausible theory why, but it worked for everyone that tried it.

FWIW-3: In previous threads that were addressing ground loops, it was observed that all of the baluns that look like that Radio Shack balun electrically isolated the input and output, whereas some of the more conventional looking ones that have 300 ohm leads rather than terminals had an internal physical connection between the coax threaded outer connector and one leg of the 300 ohm side.
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post #6 of 14 Old 06-19-2013, 02:05 PM - Thread Starter
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Quote:
Originally Posted by retiredengineer View Post

I entered Eugene OR into TVFool and it said Channel 13 is a very strong signal. My guess is channel 13 is overloading your TV. You reduced the signal level by using the combiner, which has insertion loss. You may want to try a variable attenuator. Solid Signal sells one, Model #1296F.

Thanks. I read recently about overloading as well, and that seems like the most plausible theory given my proximity. An indoor antenna can pick up 13, which would seem to corroborate the overloading theory.
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Originally Posted by mikepier View Post

What kind of antenna is on the roof now and where is it aimed?

Ch 13 KVAL is actually VHF13, so if the antenna on your roof is UHF only, thats the reason why you are having issues. Also I noticed on your tvfool that you have other stations to the NE. So thats why I ask where is the antenna pointed now. It could be why you are getting the other stations fine, but Ch 13 is almost 180 opposite pointing SW.

Also I'm not sure how the combiner is playing a part in this since the UHF loop antenna should have no effect on Ch 13 being VHF.

And also even though you are 4 miles away from the tower, it looks as if there is a big mountain where the tower is on, so your signal is getting bent over the terrain, which is why it says 2edge reception. Post your exact tvfool results so we can get a better idea of reception in your area.

The roof antenna is a Channel Master VHF and UHF, aimed about halfway between the two major broadcast locations (one to the east and the other to the south), approximately due east. The angle between locations is closer to 90 degrees, than 180.
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post #7 of 14 Old 06-19-2013, 04:53 PM - Thread Starter
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I had a Dayton VIT-1 Isolation Transformer laying around just to experiment as an attenuator. I have no idea how much insertion loss there is, but replacing the combiner with it and using only the roof antenna coax, Ch. 13 comes in, albeit at lower signal strength and signal quality compared to the others. With all the other stations, I get over 90% signal strength and quality, 100% symbol quality. With the VIT-1, 13 comes in at 70% signal strength and quality, but still 100% symbol quality. I'll probably get the variable attenuator just to have more flexibility though.
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post #8 of 14 Old 07-02-2013, 11:13 AM - Thread Starter
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I got the variable attenuator, but it arrived broken. The VIT seemed to be doing the job, so I let it be. Now 13 is acting up again and I've removed the VIT entirely. From watching the levels on the HD Homerun Config app, the signal strength hovers around 75-80% but signal quality is dropping below 50%. At this point the effort involved to ferret out the cause is outweighing the benefits, especially when I can stream a program off CBS.com. If it were all the stations having problems, that'd be another thing. However, I'll continue to keep an eye on the moving target that is my local CBS affiliate.
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post #9 of 14 Old 03-04-2014, 03:53 PM - Thread Starter
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After living without the CBS affiliate for about six months, I decided to try an Amazon Basics indoor antenna that's basically a clone of the Mohu Leaf. The antenna picks up all the stations in my area, including CBS. I also verified I was still unable to pick up the channel with either my roof antenna or a Zenith Silver Sensor indoor antenna.

So...I'm still befuddled about the cause as it would seem to be antenna hardware related. Nothing changed in the hardware over several years, so perhaps something changed in the station's broadcast that makes it incompatible with older antennas?

In any case, I won't question it too much as long as I continue to get reception.
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post #10 of 14 Old 03-04-2014, 05:03 PM
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Amazon Basics indoor antenna that's basically a clone of the Mohu Leaf.

Um, no. The two antennas, other than being thin, have little else in common. The Amazon Basics, a private label Winegard Flatwave, is actually designed to pick up some VHF which is out of the Leaf's performance zone. Solid Signal sells the same antenna under their own name, the D-Blade. The did a pretty good comparison back when it came out: http://blog.solidsignal.com/showthread.php/1641-Signal-Labs-tests-the-HD-BLADE

Quote:
Nothing changed in the hardware over several years, so perhaps something changed in the station's broadcast that makes it incompatible with older antennas?

Something changed, probably at or near your end of the signal path. Electrical interference, multipath, or some degraded connection somewhere in your system is the most likely cause.
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post #11 of 14 Old 03-04-2014, 05:11 PM - Thread Starter
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Originally Posted by ProjectSHO89 View Post

Um, no. The two antennas, other than being thin, have little else in common. The Amazon Basics, a private label Winegard Flatwave, is actually designed to pick up some VHF which is out of the Leaf's performance zone.
Something changed, probably at or near your end of the signal path. Electrical interference, multipath, or some degraded connection somewhere in your system is the most likely cause.

Thanks for clarifying on the Amazon product.

The Silver Sensor has a 3' coax cable plugged in straight to my HDTV. It also can't pick up CBS. So what gives the Amazon Basics the advantage?
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post #12 of 14 Old 03-04-2014, 11:21 PM
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Originally Posted by Cameron_Y View Post

Thanks for clarifying on the Amazon product.

The Silver Sensor has a 3' coax cable plugged in straight to my HDTV. It also can't pick up CBS. So what gives the Amazon Basics the advantage?

Are you using the Basics antenna in the same exact location as the Silver Sensor? The Basics has a 15' cable which would allow it to be further away from the television which may be causing interference. Plus the Silver Sensor is a UHF antenna. It can work for VHF, but here, a window facing correctly is mandatory.
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post #13 of 14 Old 03-04-2014, 11:36 PM - Thread Starter
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Yes, same location. Basically, my reception has often been a "fish in a barrel" kind of thing where I live. I've never had trouble picking up something of everything, even though the signal may be weak or inconsistent, but with Ch. 13 it effectively disappeared as tuners were simply unable to lock on to the signal with the antennas I was using. The Silver Sensor picks up the other VHF station (Ch. 9) in the area just fine, so UHF vs. VHF doesn't seem like it would be the reason.

In any case, it's all working (for now) so this is mostly about satisfying my own curiosity.
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post #14 of 14 Old 03-05-2014, 12:05 AM
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Quote:
Originally Posted by Cameron_Y View Post

Yes, same location. Basically, my reception has often been a "fish in a barrel" kind of thing where I live. I've never had trouble picking up something of everything, even though the signal may be weak or inconsistent, but with Ch. 13 it effectively disappeared as tuners were simply unable to lock on to the signal with the antennas I was using. The Silver Sensor picks up the other VHF station (Ch. 9) in the area just fine, so UHF vs. VHF doesn't seem like it would be the reason.

In any case, it's all working (for now) so this is mostly about satisfying my own curiosity.

Something of everything is exactly what I have experienced with indoor reception here. Regardless of the antenna employed, some channels work while others don't. However, moving and re-aiming the antenna can often solve the problem for a missing channel. It does have to be done one channel at a time. Before the transition, I had a Silver Sensor in one room that got everything when aimed east, which is the direction to the towers. When it got windy, one channel was best received when the Silver Sensor was turned 180 degrees pointing west. Height can be as important as location and aim. Stay curious.
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