OTA HDTV Reception Q&A

Updates on the DTV transition and how to receive over-the-air digital television for free.

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Location: Richmond, IL, United States

Hello there! I created this blog to share the information about over-the-air HDTV reception you have been wearily searching the Web or calling technical lines for, whether you have decided for less expensive means to get your favorite TV shows, or still adjusting from the over the air broadcast DTV transition that occurred on June 12, 2009. After working for a leading antenna manufacturer for almost 5 years, during which time I've shared my expertise with those who asked on the phone and by email at work, I decided to do the same in the Blogosphere! Confused about getting your local HD channels? Just click through the archives, some of the most useful information is in the early posts from 2005-06. If you want to get in touch with me with antenna related questions, just leave a comment anywhere on this site.

Sunday, October 18, 2009

OBSTRUCTIONS TO OVERCOME IN OTA DTV RECEPTION

Because my posts have become deep in the archives by now with the important information posted about 3-4 years ago, for the sake of navigation simplicity I thought I would list some common barriers to over-the-air DTV signals. Some would probably ask why it seems their DTV picture is outstanding on some days and blocky on others. Well, it has to do with the state of the area atmosphere, which varies on different parts of the day, or even the calendar year. Radio frequency (RF) signals ideally can travel a long distance in free space without any resistance. That is not the case in reality.

When RF signals travel through an environment, or a "medium" as engineers and scientists call them, they go through forms of resistance, or impedance, to their travel. To put it another way, when you drive a car, you may find yourself having to travel slower on foggy days or when roads are icy as compared to dry sunny days, making the travel difficult. Well, that's the same issue with RF signals. Sooner or later even on nice weather days, the natural impedance of the air will eventually reduce the RF signal power to the point where it becomes impossible to receive with even the most sensitive radios. How far the signal can go before that happens depends on the atmospheric conditions, and, what objects stand in the way.

The best conditions for any kind of RF signal reception are during mild nights with little wind and stable air. I say nights because in the daytime, an atmospheric layer is present when the sun is out that scatters radio signals, making reception better in some places and weaker when the antenna is moved by as little as a few feet. Windy days when the air is unstable, high humidity, and precipitation can also cause the air to have a higher impedance and make off-air reception difficult.

Also, there is the issue of obstructions. Non-metallic building materials like wood and masonry, and trees and foliage, are media with a higher resistance than for air, which is why reception in areas with a lot of trees or buildings is difficult, as well as indoor or attic reception as opposed to mounting an antenna on the rooftop. Metal building materials will reflect and/or load RF signals, essentially blocking reception, as well as hills. The ground can literally stop an RF signal in its tracks, which is why using an indoor antenna in a basement den is not worth the frustration.

So this is why that if you live in an area fairly distant from the TV transmit towers that is best to use the largest antenna that you can to ensure you can pull in the signals when the conditions are at their worst, and if you do use an indoor antenna, why should try to mount it as high above the floor as possible to allow the antenna to "peek over" any obstructions near the ground. Also as I have mentioned a long time ago, amplifiers are only good for overcoming signal loss through cables and splitters, and NOT for increasing the antenna range. Amplifiers can only boost whatever signals the antenna can receive.

If you use antennaweb.org or tvfool.com to determine the TV channels you can receive in your area, the calculations do tend to be conservative estimates, taking the varying atmospheric conditions into account to ensure the size of antenna recommended is in fact the correct one. So, if your existing reception comes and goes, try moving the antenna higher and left or right a few feet first to see if the DTV signal meter on your tuner suddenly jumps into the good range. If it doesn't help, you may need to consider a larger, stronger antenna to overcome the obstacles of off-air DTV reception.

7 Comments:

Blogger Billy Barule said...

Hey, Derek. great blog!

I live about 40 miles SW of downtown Chicago (New Lenox, IL). I've been going OTA since March of 2009. I initially built my own antenna using these instructions:
http://uhfhdtvantenna.blogspot.com/

I tried out the antenna in my Living Room, and only about half of the channels worked. I moved it into my attic and everything worked except channel 2 (CBS). I still have an old aerial on top of my house, so I hooked that up in addition to the homemade one. After that I got all channels stronger, but still no channel 2.

Now, sometime in the last six months or less (I forget when. I don't watch TV that often) I stopped getting channel 7. Nothing I do makes it come in, yet I still get completely full signal on Channel 5. Also, Channel 2 comes in now which I didn't get before.

I even tried buying a new HD antenna with a little amp thing in it, and that didn't work for channel 7.

Any suggestions? Anybody else having this problem?

Thanks!

Bill

2:56 PM  
Blogger twig said...

Hi Derek -

I need some help solving an OTA riddle.

I'm located on a hill in suburbia, 15 miles S.E. of the Sutro Tower broadcasting site in San Francisco. The top of that tower is >1800'; I am at 630'. There are some trees around, and there is a 1000' hill halfway between the tower and here, but nothing that should greatly attenuate the signals. I reckon it as pretty much line-of-sight conditions.

TVFOOL predicts -27 dBm signals at my house. I should be able to get a signal with a wet noodle, no?

But I get pretty rocky reception on most channels.

So I dragged a spectrum analyzer off the lab bench and hooked it up to my antenna (a Wineguard 7080p 10 feet above the roofline).

UHF signals measure in the range of -60 to -70 dBm. That is maybe 40 dB below expectation (net of antenna gain, cable loss, etc.)

I tried another antenna/feedline in the same location and got consistent results.

I have yet to try moving the antenna site. But is it likely that slightly perturbing the location will change such a deep null in the signals?

If you have any thoughts or suggestions, I'd love to hear them.

Cheers,

twig

9:41 AM  
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Blogger cris wat said...

I also agree that the best conditions for any kind of RF signal reception are during mild nights with little wind and stable air. Atmosphere air plays important role on the signals.

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