A Lesson On Wavelength and Earth Science

I have been walking you through some basic “start slow and cheap before going fast and costly” steps to getting reliable off air reception. Now let’s get a little geeky to see if we can make more sense of this. As I wrote in the earlier posts, there’s no difference in the RF signal, and any antenna designed for the purpose will work even if it is 40 years old, as long as it isn’t rusted, broken, or bent, etc. All radio and TV waves behave the same way. They generally travel in straight lines at the speed of light. When you know the frequency and you have the speed of light, you can calculate how long the wave is for any frequency. Wavelength = velocity divided by the frequency. The speed of light is 300,000,000 meters per second or 186,000 miles per second if you want the wavelength in feet or inches. The Internet is a wonderful place and here’s a quick calculator that gives both units of measurements http://www.csgnetwork.com/freqwavelengthcalc.html Likewise we can find the visual frequencies for any television station at http://en.wikipedia.org/wiki/North_American_broadcast_television_frequencies so let’s run some numbers to see the full wavelength of several channels in the area!

Channel 4 visual frequency is 67.25 MHz and the wavelength is 14.63 feet.
Channel 11 visual frequency is 199.25 MHz and the wavelength is 4.93 feet.
Channel 17 visual frequency is 489.25 MHz and the wavelength is 2.01 feet.
Channel 40 visual frequency is 627.25 MHz and the wavelength is 1.57 feet.
Channel 55 visual frequency is 717.25 MHz and the wavelength is 1.37 feet.

It’s clear you wouldn’t want an antenna 15 feet wide on your house for channel 4, so they cut antennas in half or quarters to make them usable. There’s a huge difference in the VHF band since it is not a group of contiguous frequencies from 2-13, but once you get into UHF, the channels are more in-line and there’s smaller differences between them. If your antenna is 24” long, it would pick up 17 a little better than 55, but not very usable to pick up channel 4 in VHF, but you can see that VHF channel 11 is fairly close and can usually be received on a UHF antenna even though it is twice as long.

That’s going to be important in February, as channels from 52 to 83 will go away for television and be used for other services. Buying a good antenna concentrating on the lower UHF band of frequencies will be great next year, but I fear they are already being sold as the “digital antennas” in stores now, which means channels above 51 will not be as strong as those below it, and unfortunately that is where we are right now.

Since cable systems are closed to the outside world, they modulate the channels in order with no gaps between bands, and thus channels above 13 are different frequencies than over the air broadcasts. That is why televisions have an AIR /CABLE setting in the menu system. If cable goes out, your TV would only show over the air channels from 2-13 if you switch to an antenna until you change it to AIR. Cable uses a system called QAM that special digital tuners can tune in DTV stations without a cable box on their system, but we’ll talk more about cable later as we are mostly interested in free over the air reception at this moment. The DTV coupon eligible boxes may not include the QAM tuner to keep the costs down and will not pick up the digital signals on cable, but they will tune in the analog signals on cable which will not change in February. In order to get a set top box that does everything, you have to shell out more money than for the basic converter box.

At the beginning of this post, I said RF “generally travels in straight lines” but the lower frequencies will “bend” slightly over the curvature of the earth. The HAM radio band is in the gap between VHF channels, and operators exploit this phenomenon to talk to people beyond the line of sight from their towers on the 2-meter band. They also use the Earth’s ionosphere to bounce those signals thousands of miles around the world. Television is similar, and there’s even websites that predict when the atmospheric conditions are perfect for receiving signals far beyond line of sight. Quite a few people have captured a screenshot of our programming, and sent it in from far away to be verified with details of the event. Since the atmosphere is a bit unstable, these bounces may last several days or only a few minutes, but it is something only people with antennas can accomplish and enjoy. I remember many mornings watching a station in Baltimore, Maryland, and almost every morning we could watch a station in Richmond, VA.The practice of receiving these bounces or skip is also called Dxing http://en.wikipedia.org/wiki/DXing and is hobby in itself.

The Earth is not Flat!

The picture to the right is not a view of the Moon or Martian landscape, but is a look at our backyards. It is a “shadow map” generated from software that adds the terrain data for the area and overlays  RF signal patterns. It’s pretty plain to see the areas that will have a challenge getting DTV signals. It does not mean homes in the Northwest to Southwest quadrants can’t get DTV, but it’s a good indication those homes will need outdoor antennas and possibly an amplifier. Unfortunately, I was not able to overlay the county lines on this map so it is difficult to see if your house might be in the shadow of a hill. Buildings are not included, but can create their own shadow areas shielding your home from a direct line of sight to the tower. It’s clear to see towards the East and South there are no natural barriers and we seldom hear from viewers in those areas encountering problems. The antenna must be higher to clear any hills in the way, and attics and the roof top are natural places to install the antenna. Rabbit ears would be a hit or miss thing in the hilly areas, and getting the antenna higher in the air is the only option, such as the attic or rooftop.

So to close this Geek-A-Thon, let’s summarize a little to get back to topic:
Lower VHF can bend easier over the curvature of the Earth, skip, and bounce easier than UHF.
The long wavelength of channel 4 will pass through and around obstacles like Pine trees and hills better than shorter wavelength UHF signals.
There’s very little difference between UHF wavelengths, so a simple UHF antenna should work for most channels if it is peaked near the middle channels, and some antennas being sold today may not be tuned to pick up those “out of band” channels.
Channels above 51 will go away next year, but several Raleigh area stations are temporarily using those frequencies until next year.
A good UHF antenna should work fine for all TV stations in the Raleigh market after next year.
The lower VHF channels will not be utilized, as there are too many sources of terrestrial interference that will affect the signals. (60 cycle “sparkles” can often be seen on an analog set on those channels from power lines)
Cable frequencies are different, and contiguous so that channel 17 on cable is not the same frequency as 17 over the air.
Living near hills, valleys, lakes and surrounded by trees, most of the popular places people live, may create obstacles you’ll have to experiment with to overcome.

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