HF Radiation

Without giving you any specifics (since this looks like homework) - think about the problem a little.

1. What do "UHF" and "VHF" mean in relation to a standard TV set?

2. Which major component(s) of the TV Set would control your answer in #1?

3. Do computer monitors have that same component(s)?

Not homework, science experiment. has to do with algae and some other cool stuff, don't wanna get too specific. I was thinking of the cathode ray but I am not so sure. Thus I am asking experts.

VHF - Very High Frequency 30-300 MHz
UHF - Ultra High Frequency 300-1000 MHz

These are the carrier frequency bands used to transmitt television signals through the air. These frequencies are not generated in the television set. Rather the signal, the picture and the audio, are demodulated off of those high frequencies for use in the set. The power levels involved inside a home set is on the order of microvolts per square centimeter.

Audio frequencies used in the set will be less than the band from 20 Hz to 20KHz. The 20-20 range is considered the range of normal human hearing frequencies. So there is no need, and the cost is not spent by manufaturers to make, higher or lower frequencies. In fact most televisions won't even provide that wide a range of frequencies.

The video frequency is a bit higher, but I don't recall off hand.

What happens is that a stream of electrons is generated in the electron gun at the back of the picture tube. This stream of electrons flows, mostly in a small, straight line to the deflection plates. There are four plates, two horizontal and two vertical.

A sweep generator produces a variable charge on the plates in time with the received signal. The variable charge starts low and ramps up to a maximum value and then drops to zero again to ramp up all over again.

There are two sweep generators operating, for the two sets of plates. One generator, the slow one, is applied across the horizontal plates to drive the electron beam from top to bottom. This repeats about once every 1/8 of a second. That time allows for a pseudo frame rate of 16 frames per second on the screen because each time the beam is pulled down the beam passes between the lines drawn the previous pass. Called interleaving, like sliding your fingers together.

There is a much faster sweep generator, and this is the frequency I don't remember. This ramps from zero to max very quickly and is applied to the vertical plates to move the beam from side to side, "drawing" lines across the screen of the television.

Of interest is that the electron beam is "turned off" every time the sweep generator hits maximum, and remains off while the generator returns to zero. The beam is also turned off and on in synchronization with the incoming signal. In a black and white set there is just one kind of phosphor and when the electrons hit the phosphor it glows and you perceive it as a dot of white. If the electrons don't hit the phosphor then the phosphor doesn't glow and you see black. However, the dots are very small and there are a lot of them, so your eye and mind put it all together and you see a picture. Color sets use different phosphors and different technologies to generate the colors, but the basic beam steering is the same, although in Sony trinitron sets there are three beams and there for three sets of deflection plates.

Now, can any of this be measured? Yes. Usually oscilloscopes are hooked up inside the television and a saw tooth wave is looked for to verify that the sweep generators are working correctly. There are devices which can quantifiably measure the electron flow to the screen, but I don't recall what they are called, and they are seldom used in television, or monitor, repair work since if you can't get any kind of light at all out of the picture tube then you generally replace the picture tube as long as the high voltage power supply is working properly.

There are only two real dangers with CRT, Cathode Ray Tubes, displays. One is the very high voltage generated in order to generate the electron beam. That is on the order of 20KV or more, much more in some sets. The other is the very high vacuum inside the tube. If the tube is broken the air rushing in takes the glass shards with it, and then the glass shards bounce off the opposite side, inside, and ricochette back the way they came at extremely high velocities. Also the glass envelope, the tube, may shatter completely all the way around, in which case the particles of glass are blown into the tube, although some would say sucked in, and then they just keep going out the other side. High velocity fragments have been measured up to twenty feet from a broken television tube. I strongly urge anyone handling them to treat them with extreme care. And when transporting televisions or CRT monitors, keep the screen covered with something, and face it into the back or bottom of the seat while driving so that if something happens the shrapnel goes into the seat and not into people.

This probably shoots down your experiment to kill algae, but now you know more than you ever really wanted to about the inside of CRTs and televisions.

Kelly

In response to Fishin,

Computer monitors have fewer components, generally, than televisons. The monitors do not need the radio receiver, tuners, and demodulator equipment required to translate a television radio frequency signal down to television audio and video displays.

Also monitors, generally, don't have the audio circuts that televisions do. Yes you can get monitors with built in speakers, but you still don't have the rf section.

Kelly

I think we are hitting all aound the correct answers. The horizontal output wave form in older TV sets has up to about 25 watts of power, so it is not surprising that some of the harmonic escape the TV. Most of this escaped energy is between 15,000 hertz and 30 megahertz, but microwatts of VHF and nanowatts of UHF do escape the TV. I'm less knowledgeable on computer monitors and the new types of TV, but I believe magnetic deflection is not used, so the energy in the horizontal pulse is greatly reduced, allowing other leakage sources to become significant.
In modern computers, rise and fall time of pulses are often less than one billionth of a second = one nanosecond, while they remain much slower in most TV applications and/or much weaker. As a result computers (even with flat screen) have significant leakage in VHF, UHF and even higher frequencies. Neil

From where do they leak the most? Thanks for all of your answers!

magnetic deflection is used (to control the horizontal/vertical deflection plates) through induction on CRT monitors and TVs classically but obviously we have a lot of monitors and TV's nowadays that don't work that way anymore

When I open the newer CRT computer monitors, and CRT tellys I see essentially the same power circuits as fourty years ago. Only the control circuits has shrunk from vacuum tubes to printed circuit board assemblies.

One major change is that newer CRTs have built in automatic degaussing which helps to keep the picture square, and not as subject to twisting and shrinking in wierd ways.

Looking for power leakages will have to be done with a small circular antenna coupled to an analog volt meter, probably a millivolt meter. Then move the antenna around inside the case of the CRT to see where you get the greatest voltage indication. Next feed the antenna to an oscilloscope and determine the frequency from the displayed wave form. You can also check the magnitude more accurately than a meter. You will have to check the particular CRTs you have available to you because I suspect that each model will have different characteristics.

With all the concerns of the last twenty years about radiation hazards I seriously doubt that you will be able to detect very much radiation in any frequency band from modern CRTs, because the circuits are designed better and the shielding is very much better.

Kelly