# How safe is a digital-TV repeater?

1. May 11, 2012

### logics

I've read safe RF radiation for human in sleep should be below 1$\mu$W/m² and 0.1 for digital pulsating radiation. Is that true?

If the station on a roof has power: 30W
- at what distance radiation is below 0.1 microwatt through a thin wall or at sight?
- through how many storeys can it go?
- is there an effective way to screen it?
Thanks

2. May 11, 2012

### Staff: Mentor

3. May 11, 2012

### Staff: Mentor

You have to take into account the radiation pattern of the antenna. Most TV broadcast antennas concentrate the radiation near the horizontal plane so that they don't send a lot of energy uselessly into outer space or straight down to the ground underneath. At least this is true for full-power TV transmitters in the US; low-power repeaters (or "translators" as we call them here) might be more isotropic, but I don't know anything about actual practice.

As a starting point, you can calculate the average intensity at distance r as $P/4 \pi r^2$, but it will probably be greater in the horizontal plane, and less in the vertical direction; and it will be further reduced by building materials.

In general, wood blocks TV/radio signals the least, stone and brick block them more, and metal foil (e.g. some types of inside-wall insulation) or metal mesh (e.g. stucco walls) or plates (metal roofs) block them a lot. I would guess that wood walls might reduce the strength by a factor of 10, stone/brick by 100, metal by 1000 or more.

Also, I wouldn't think of digital TV signals as "pulsating." The binary data is not represented as a simple on-off switching of the signal, but instead as a complicated phase modulation. In the US, most digital TV transmitters actually emit less power than the analog transmitters that they replaced a few years ago, because the digital encoding allows good reception with a lower signal strength at the receiving antenna.

Last edited: May 11, 2012
4. May 11, 2012

### logics

... the station is about 100 meter from my window more or less on the horizontal plane (maybe a few yards above), can you find out the field strength?
Does that kind of RF pass through a wall?

Last edited: May 11, 2012
5. May 11, 2012

### Staff: Mentor

It has to be able to go through walls, at least partially, so people can use indoor TV antennas.

(I added some more stuff to my preceding post while you were posting yours.)

6. May 11, 2012

### logics

The walls are not stone nor bricks, but flimsy porous calcareous material, then put in a large window, can you make a rough general estimate if I get more than 1 microwatt, altogether (if it is not pulsating) ?
Thanks a lot, jitbell
Isn't it advisable to do some kind of screening, anyway? how, aluminim Venetian blinds, or what?

7. May 11, 2012

### Staff: Mentor

Are you sure there's no metal inside the walls? In the western USA, "stucco" construction is rather common, in which the walls are basically plastered over a wire mesh (like what we call "chicken wire") with some wood or metal support posts. People who live in such houses usually find it very difficult to get good TV reception with an indoor antenna, and have to mount an antenna outdoors on the roof.

As for the window, metal blinds would block the signal a lot, even if they're thin strips that let a lot of light through. Also, glass with very thin metal coatings for energy efficiency (what we call "low-E" windows) often also block a lot of RF.

8. May 11, 2012

### logics

I'm sure, here only the pilasters on the 4 corners are ferro-concrete and in between just blocks of porous calcareous material.
Can you now give me your verdict, can I (and the children) survive , or should we move?

If I had to use chicken wire, what is the most effective size to block those frequencies?

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9. May 11, 2012

### DragonPetter

Why are you concerned about this radiation in the broadcast spectrum? I don't think there are any known safety issues with UHF radiation, and I think this has been studied extensively. It is non-ionizing radiation, and I doubt your body is a very good antenna for absorbing this energy.

10. May 11, 2012

### skeptic2

11. May 11, 2012

### logics

Sure, but only for a few minutes, not 24h/day. The article I read said that expecially when asleep field strength should be lower than 1, for adults!
Can you work out the field strength, with those parameters?, thanks

Last edited: May 11, 2012
12. May 11, 2012

### DragonPetter

What damage do you think it could be doing to your body? Dielectric heating like a microwave?

Last edited: May 11, 2012
13. May 11, 2012

### logics

The article was in a newspaper, but I found same values here

14. May 11, 2012

### DragonPetter

That company is probably designing a tinfoil helmet too, because people will buy it. That same company says EM radiation causes:

"Neurological: headaches, dizziness, nausea, difficulty concentrating, memory loss, irritability,

depression, anxiety, insomnia, fatigue, weakness, tremors, muscle spasms, numbness, tingling,

altered reflexes, muscle and joint paint, leg/foot pain, “Flu-like” symptoms, fever. More severe

reactions can include seizures, paralysis, psychosis and stroke.

Cardiac: palpitations, arrhythmias, pain or pressure in the chest, low or high blood pressure,

slow or fast heart rate, shortness of breath.

Respiratory: sinusitis, bronchitis, pneumonia, asthma.

Dermatological: skin rash, itching, burning, facial flushing.

Ophthalmologic: pain or burning in the eyes, pressure in/behind the eyes, deteriorating vision,

floaters, cataracts.

Others: digestive problems, abdominal pain, enlarged thyroid, testicular/ovarian pain, dryness of

lips, tongue, mouth, eyes, great thirst, dehydration, nosebleeds, internal bleeding, altered sugar

metabolism, immune abnormalities, redistribution of metals within the body, hair loss, pain in the

teeth, deteriorating fillings, impaired sense of smell, ringing in the ears."

15. May 11, 2012

### logics

Bau Biologie makes no helmets, I suppose, but can you help me at all?
What is RFM?
What is the optimal size of a chicken-wire mesh?
You'll oblige me

16. May 11, 2012

### xxChrisxx

17. May 11, 2012

### DragonPetter

I'm not trying to mock you because I am not an expert in this, but I see demotivation in obliging you with time consuming calculations when I see what my efforts would be for, besides learning something myself.

As far as a wire mesh, you want the spacing in your wires to be a certain fraction less than the wavelength of the radiation you're trying to block.

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18. May 11, 2012

### logics

I do not know if you have children [around]. But I am not saying I believe anything, I only would like to know, roughly, what field strength might be. I'll decide whether to worry only afterwards.

19. May 11, 2012