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Background Trying to protect my home against the RF irradiation by cellular tower nearby. Read an article that proposes 2 things:

  1. Metal film, or fine metal mesh, which is grounded. This is used on all doors/windows which are the points via which RF energy manages to enter the home.
  2. An indoor antenna, with a matched load. This apparently reduces the residual radiation indoors, by absorbing and dumping the RF energy on a matched load.

Objective While I've already done the first thing, and observed close to 50% drop on my Electrosmog meter (a Cornet ED65) of the RF power inside house, I think I'd need to take the second step to bring it down further.

So, wondering if anyone can help me figure out what this 2nd circuit is and how to go about building/designing it ?

Addendum (April 22, 2012 - Earth Day!!) Probably a substantial portion of the battle-hardened scientific community (or rather, the more hands-on engineering community), might have a good laugh at the expense of this question (and I do not mind that), and brand folks like me a dumb tree-hugger. However, let me also say that there are many amongst us, who have a genetic predisposition of being more easily afflicted (than others), by often-untreatable life-threatening conditions, aggravated/hastened/catalyzed by EMF. For such people, doing what they can, to protect their loved ones and themselves, to the best they can, is important. There is sufficient quantity of very believable scientific literature, that is facts based, based on through, deep, long studies, that point to strong correlation of EMF and some of these ailments/conditions.

Just to answer some of the interesting questions like what do you do about the microwave oven, (or other such sources s.a. Wireless router, wireless broadband, cordless phone, wireless baby-monitor etc.), when living inside a faraday cage, let me state that I am not trying to reach zero radiation, but only bringing it to a level which, by today's international standards becomes acceptable, which it clearly isn't, as-is. In any case, I have already gone back to what might appear as a cave-man status. We do not use microwave, wireless router, cordless phones inside the house. It is a fully wired house, and has 12" thick brick-n-mortar walls, concrete roof. With the metallized film application, the level of radiation that "leaks in", apparently through the windows/doors has already come down, but not yet to be within acceptable limits.

So, if someone can please answer my original question, or feel free to ask for clarifications, i.e. is it feasible / possible to create an indoor antenna, that supposedly reduces the effects of cellular (850MHz/900MHz/1800MHz) EMF, by absorbing the RF energy and apparently dumping it on a "matched load".

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    \$\begingroup\$ Wearing an Al foil hat will also help. :) \$\endgroup\$ – Leon Heller Apr 20 '12 at 14:34
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    \$\begingroup\$ Make sure to do the roof too, else they'll just do a brain suck on you from orbit. Better yet, find a deep cave to live in. \$\endgroup\$ – Olin Lathrop Apr 20 '12 at 15:13
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    \$\begingroup\$ @LeonHeller I had always heard that Al foil was introduced to make it more difficult to obtain Sn foil haberdashery, which is significantly more effective in shielding. \$\endgroup\$ – W5VO Apr 20 '12 at 15:27
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    \$\begingroup\$ Hopefully the funny haha commentators do realise that shielding from EM radiation is unrelated to information transfer aspects and that there are numerous studies which prove and disprove both sides of the related arguments. \$\endgroup\$ – Russell McMahon Apr 20 '12 at 16:51
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    \$\begingroup\$ @icarus74 What is the name of this illness, or genetic predisposition, so that I can look it up? What did doctor say? \$\endgroup\$ – abdullah kahraman Apr 22 '12 at 8:51
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I have followed this subject for "many decades".
(I wrote an engineering paper on this just on 40 years ago!)

Working in close proximity to RF generating equipment may have some epidemiological correlation with provable hazards. (eg sucking your microwave oven when it is working is liable to not be wise).

But, due to inverse square law falloff and relatively low transmit power levels, at the typical distances away from cellular towers you are extremely unlikely to encounter hazard levels which are significant compared to any recognised standard.

While you may not trust this assessment it is important to note it because, if you want to be cell-site and other RF source "contamination free" you are going to have to exclude radiation from all similar energy level RF sources.

Screening doors and windows, but not walls, ceiling and floor is not going to produce the sort of reductions that you are liable to want. Normal non-metallic building materials are transparent to RF and if you place mesh across openings only then RF "sees" a see-through house with a few blocks at doors and windows. To do the job properly you need to create a "Faraday Cage". (A FC acts on the electric field but can be effective against RF if proprly built.) A Faraday Cage is achieved with a metal mesh or sheet covering of the whole house - although you can achieve reasonable results with conductors which are separated by distance small compared to the wavelengths involved. If you are dealing with say ~= 900 Mhz cellular, a wavelength of 300/F_MHz metres, and you probably want say 0.1 wavelength separation maximum so a metal grid should have a specing of not more than say 30/F_MHz. For 900 MHz spacing is <= 30/900 = 1/30 metre or about 30 mm. Use of 1 inch mesh or smaller chicken mesh may be reasonably effective. This could be laid in ceiling spaces if accessible. Treating walls may not be so easy. Wrapping your house in chicken mesh may be effective but may attract similar comments to those received when wearing tin-foil hats .

Use of a single antenna indoors is almost certain to NOT be very effective at reducing RF levels. Fastening chicken mesh under the floor is liable to be of more effect. You want to check results with a reputable meter as it would be possible to form a "beam" or waveguide which increased rather than decreased the field inside two planes of mesh - especially so if the transmitter is nearby.

Similar to your indoor antenna but more disributed - If you run multiple long conductors under the floor or in the ceiling you will alter and may well reduce RF fields inside the house. My house has a number of long steel girders running under the floor and it is particularly poor for RF reception. Wireless doorbells have much less range in this house than in others and cellphone operation is also affected. This arrangement "just happens" to work as it does and you'd need to dsign or experimentally produce a similar result.


Multiple low cost dipoles for energy absorption:

As I noted, use of a single antenna indoors is not liable to be highly effective - but also as noted, my underfloor steel beams have a significant effect on RF generally in my house. In your case the high target frequency means it would be easy to construct dipole antennas tuned simply by adjusting their length and with a resistive load - essentially just two wires in series of the correct length with a resistor across the gap between them. A significant number of these scattered about the house could have a significant effect as there is a finite amount of energy available.

You may be able to make these multiband but the simplicity of these means that having ones for each of the bands concerned would probably be easier.

Note that many resistors would be far from pure resistive at these frequencies due to reactive components. Long ago people used carbon composition resistors for RF for this purpose but these are likely to be increasingly hard to obtain and probably are questionable at UHF. There are no doubt materials that will work OK. It may be that a straight Nichrome link of appropriate resistance between two tuned wires would work OK. Dipole feed resistance is traditionally stated as 75 ohms. Will vary somewhat with conductor size and probably with length of interelement central gap.

Taping these on walls or in eg picture frames would probably work, but too close proximity to reinforcing steel in walls will detune them.

It is very easy to make up a few of these, and your field meter will tell you if or how well they are working. A spectrum analyser would probably provide a fae clearer picture of effects at a given frequency.


Biological effects of non ionising radiation:

  • The following may be felt to stray too far into the sociological and have little place in engineering design considerations. As long as we only base our engineering in such areas on only what we are sure we know, even when we may be wrong, is to court disaster.

RF Trolls are forever blinded to seeing Unicorns :-)

Whether low level RF or electromagnetic or electric fields constitute or cause or catalyse significant biological hazards has been the subject of much discussion, investigation, research and analysis over many decades.

As Olin notes, some research has indicated that placing a radio transmitter against the side of your head for up to hours a day for many years at a time can be a seriously bad idea. Few would doubt that this might be so and the surprise is more that the results are equivocal and that there is no consistent indication across all studies that low level RF frying of your brain is always demonstrably bad for you. (People who use cellphones for hours a day will often enough exhibit pathological societal behaviour, but the correlation is generally considered to be the inverse of the one of interest here :-) ).

However, the concerns and examination are far more widespread than just the above. As Olin says, the power densities are far (far) lower from a cell site than from a cranialy attached phone. Of greater concern would be higher power transmitters. However, in this area, too, there have been conflicting study results with no absolute certainty that there are not effects in some cases. I'd personally comment that in most cases what effects there may be are so much "in the noise" that prudent avoidance (as opposed to active mitigation) and being careful with your health generally is liable to produce much greater whole of life gains than concentrating on this one aspect.

In some cases there may be genetic predispositions that increase effects but there is a very very grave risk of falling prey to confirmation bias and quackery from people who claim to be experts but who bias their 'results' either unintentionally or with good intentions or to make a profit or just maliciously. Of these "confirmation bias" is the most insidious to deal with as you are trying to objectively examine your own thinking from within your own system. I understand Olin's perspectives of Unicorns and Elvis's - but he is as blind to the possible realities as those at the other extreme who imagine effects that are not there. Engineering and scientific rigour is essential - but this includes not dismissing, as of right, claims which appear to have no reasonable basis in reality. Essentially all new Scientific discoveries come from areas with "no reasonable basis in reality" - if it were not so we would be using them already.

In most cases where there is no good reason, based on scientific and engineering knowledge, that some effect should exist, then it probably doesn't. There will always be some who think such things exist regardless of proof or reasonableness, and there are large 'industries' built up around encouraging such false beliefs for whatever reason. Those involved in such promotions may be well intended or charlatans or conmen. BUT in some cases, regardless of all that we know and understand and rgardless of reasonableness and confirmation bias and conmen and the rest, there will be genuine effects which we have missed or are yet to discover. Amongst the over-unity magnetic motors, antigravity machines, magnetic bracelets, crystals, mainifestly swamped by noise biological effects, Unicorns and Elvises we may find with surprise effects which are unknown and real but usually swamped by noise or very well know effects that arrive by a different path.

An example of the later, which may or may not be significant but which helps illustrate how Unicorns may suddenly appear is "magnetic lensing" of incoming extra-terrestial ionising radiation by local em fields from power lines. It is widely suggested that the magnetic fields from HV transmission lines are not large enough to cause claimed increases in birth defects and cancer in adjoining residences. One plausible mechanism, no doubt by now both well proven by many scientiifc studies and completely discredited by an equal number of other scientific studies, is that the fields serve as a lens to deflect charged incoming ions and charged particles such that those that would have fallen within the area near the lines instead are deflected to fall in narrower areas outside the high intensity field area - ie just outside the line of the power lines. Whether in this case this does prove to be a significant factor in birth defects and cancer is not the point. What matters is that a mechanism does exist that may allow power lines to cause birth defects and cancer due to their 50 or 60 Hz magnetic fields, based entirely on a plausible main-stream engineering basis. Whethr it does then becomes a matter for investigation. Until such potentially feasible mechanisms are proposed it's all Uncorns and Elvis's.

I personally think that there is a high prospect that Icarus's concerns are not well founded and that, even given a high degree of apparent correlation with family health and genetics with electrosmog, that the correlation is illusory and more likely to be caused by some other effect. Example only! - eg the family may prove to all have low level gluten intolerance or some other such effect with accompanying health effects. BUT as long as it may be that low level RF fields are the cause of family health issues, rejecting them along with the Unicorns is doing not only Icarus and his family but society as a whole a grave disservice. Engineering prudence is allowable, as is even strong scepticism, but outright rejection, ridicule and rudeness goes beyond the pale.


Dipole antenna length calculator:

A dipole can be as simple as a length of wire cut in the middle.
The middle may be com=nnected to a feed line to a receiver or transmitter, or have a resistor connected between the two halves to received dissipate energy, or have a back to bak diode pair connected across the gap to, um, er ... .

To make an antenna resonant at a frequency of interest use two lengths of wire, each wire cut to about 120,000/F_Mhz millimetreslong. eg at 900 Mhz each wire = 120,000/900 = 133 mm long. (Or take a length of wire twice as long and cut it in the middle - instant dipole. These may be about 5% too long and wire diameter also affects length very slightly. Play ...

To receive and dissipate energy connect a resistor - ideally 75 ohms at the frequency of interest between the two halves. Nichrome wire is probably a good choice for the resistor as you can get a defined resistance with some ease. he wire can be twisted with the dipole ends but the actual resistive part needs to be straight.

Skin effect will probably play havoc with resistor calculations - discuss.
When RF flows in a conductor it tends to flow only in the outer "skin" in a thin layer. It will mean you probably need a shorter Nichrome link than expected. As the Nichrome wire is thin compared with most antenna elements the effect may not be as pronounced as in many cases.

For dipol length either use my formula above or one of these online calculators. Some pages also give extra information on dipole antennas.

Some extra comment
Some good extra comment
Basic but useful
Shows effect of diameter or wire - limited choice
Similar
Similar


Details:

Does the wire's gauge-size, material or insulated/naked matter ?
Any specific value of resistor to use ?
I believe nothing to be grounded ?

Some of those questions are covered in the text above or by reference material.

Wire does not need to be insulated if free space mounted. If mounted on a wall or similar then insulation may have some advantages if the wall was somewhat conductive.

As noted, the feed point is notionally 75 ohms.
75 ohm resistance at frequency concerned gives maximu power ransfer.
Note that many resistors will be far from a pure resistance at the frequencies concerned and so may be unsuitable.
As noted, use of nichrome wire seems a good idea.
Due to skin effect it will need to be less than 75 ohms.
Try varying lesser resistances to see effect.
Electric radiator element wire may be a source of Nichrome if otherwise unavailable.


What country are you in?

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  • \$\begingroup\$ What would you say about a person who builds a faraday cage around their house and then uses a microwave indoors? \$\endgroup\$ – W5VO Apr 20 '12 at 15:30
  • \$\begingroup\$ Is that sort of microwave power absorbed by the cage or reflected around like a bullet in a metal room? \$\endgroup\$ – AngryEE Apr 20 '12 at 15:42
  • \$\begingroup\$ @AngryEE - standing waves. There would likely be hot spots. Might have to relocate the fish tanks and the comfy chairs. \$\endgroup\$ – JustJeff Apr 21 '12 at 2:41
  • \$\begingroup\$ @RussellMcMahon, thanks for your supportive comment and very useful suggestions. For most parts, there are some practical constraints I am having to deal with, s.a. the fact that this is a condo, and I cannot do major structural modifications, especially on the structure's exterior, and only some minor, non-disruptive interior modifications. My research tells me that each concrete obstruction on the beam path, reduces the RF energy by a 10th. The measurements both inside and outside house, with my basic electrosmog meter, do confirm this fact (at least roughly). \$\endgroup\$ – icarus74 Apr 22 '12 at 6:29
  • \$\begingroup\$ Thanks Russell. Thanks for the very detailed & scientific response, once again. I hope at-least some here would agree that "scientific reports" can be, um... how to put it mildly... say "influenced", to present in-the-middle-of-no-where findings/results, obscuring reality. However, not being an expert in this field, one needs to base their judgement on the basis of something. In this case, mine was based on what might be seen as conspiracy-theory'ish reports, and also some simple experiments e.g. popping corns using 4 phones. There is no camera trick... <contd.> \$\endgroup\$ – icarus74 Apr 23 '12 at 6:41

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