Seeing the Unseen

Broadcast signals circle your head constantly following straight paths from point A to point B. They are everywhere and it would be extremely difficult to find any square foot of this planet that does not have some RF signal present. Think about it, satellites pretty much cover the entire planet, signals from radio and TV bounce around and into outer space.

But have you ever seen a TV RF signal?

I doubt it, unless your eyesight is tuned to microwave sensitivity and a lot better than mine! If you’re curious though, you came to the right place, because Digital Russ can show you what is floating in the air around Raleigh. And not just my signals, let’s look at a big portion of what’s in the airwaves, but first let’s go back and consider what has occurred in the past few years.

The FCC assigned each US TV station a second channel to temporararily place their digital signals until the February analog cut-off. Stations inside the core frequencies, or those that will remain open for broadcasting are set. They can stay where they are and do not have to spend another dime to be ready for 6-12-09. Channels above 51 are going away from TV broadcasters, and will be sold off to the highest bidder.

Some stations like WRDC and WRAZ “lucked out” and got assignments one channel below their analog channel. It’s called N-1 or NTSC minus 1 channel. The rest of us in Raleigh were assigned channels above 51 and therefore have no choice but to migrate to other channels in February. Some will move back to their original channel, like yours truly will simply flip a switch and turn off analog 17 and turn on digital 17. We will move from DTV ch 55 which should help those antennas with problems pull in better quality signals. The lower the channels, the easier they are to receive.

So first, let me show you some local RF! Yes, I am showing the competition. I’m an Engineer and we have unwritten permission to cross competitive lines to help the viewers. So here’s the N-1 pictures for channel 50 analog, and 49 their digital:

What you are looking at is a capture from an RF Spectrum Analyzer connected to a rooftop antenna at my station. On the left is their digital signal on ch 49, and on the right is their analog on ch 50. Both channels are 6 MHz wide. The tallest peak on the right side is the analog video carrier and the other peak is the analog audio carrier. Between those two humps is a lot of “wasted ” space. I put wasted in quotes, because it is necessary for analog TV to work due to the mathematics involved, where the video is 4.5 MHz from the audio carrier, but it does not carry any information other than the color just to the left of the audio.

The digital signal on the left uses the entire channel width. In order to get a great picture on the analog signal on the right, you need to peak those humps up as high as possible. Basically, the more signal the better and the picture quality goes down as the signal strength goes down. The DTV signal on the left is totally different. For good reception, you have to receive the entire signal from left to right, not top to bottom. As long as the top of the haystack is flat, your tuner will have no problem displaying it.

Flipping the channel up button, the next DTV signals are on ch 52 and 53. These are the DTV signals for WTVD and WRAL:

These stations are side by side with no gap between them, except a few hundred yards coming from different towers. As I have said, the important part of good reception is from left to right, not peak signal strength. You may have noticed already the WRAL and WRAZ haystacks are leaning slightly to the right. That indicates my antenna is not pointing exactly at them and is a few degrees off and dead on to the tower for WTVD. It’s very slight and maybe a degree or two off from the main tower, but a CECB would have no problem locking into either one as the haystack is linear from side to side, albeit a little tilted.

The next channel up is the WNCN-DT signal at ch 55, and next at the top of the Raleigh stations is WLFL’s signal at ch 57.

The haystacks are pretty even across the stations, and I will have to re-aim my antenna at some point, but antennas are not equal. Some have a very tight beam pattern, and some are sloppy and receive signals from several directions. Depending upon your location, you may have to tweak the antenna a few degrees to get all the signals correctly and sometimes you have to compromise and aim it between two stations and get an average from both. If you’ve been a consistent reader, and of course you are, you know I have said many times that multi-path is the killer to good digital reception, not signal strength and conventional assumptions on height and so forth.

I hooked up a really bad set top antenna, and took a snapshot of what multi-path does to a DTV signal:

You can see pretty dramatically what the rabbit ears have done to the nice flat haystack. It is no longer flat, and has dips and angles and was bouncing around. A receiver would have a hard time decoding that signal. Once in a while it would lock, but mostly just freeze, pixelate, and drop out completely. The signal strength is still pretty good, but looking at it from left to right, a box would be lost decoding anything from it.

I know this has been a long post, but it’s a good look at what’s floating out in the air around you, and hopefully with a visual picture of the RF signals, you’ll be able to “re-think” what you know from the analog world going from top to bottom, and think left to right from now on!

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