self adhesive foil versus 2.4ghz reception ?

[Dave Hopkin]

I am aware that faraday cages are not totally clean environments (I once worked for a UK computer company and they rented a section of Salt Mine near Natwich about 1200' underground to build a faraday cage down there to try and get a totally clean environment)

The question is: How mush attenuation does a aluminim covering give

This afternoon I wrapped a XT in kitchen foil and it worked perfectly - I will be taking it to the field tommorow to see how the range is affected with and without the foil wrap....

[John Olsen 1]

A Faraday cage can be made of any conductive material, and aluminium, while not as good as copper, is a pretty good conductor. Any openings in the cage should be small, less than 1/10 of the wavelength you want to exclude. Any wires entering, for instance to bring in mains power, need to be filtered to prevent unwanted signals getting in. Even a properly designed Faraday cage will allow some signal in, although the attenuation should be pretty high. The cage itself does not actually require to be grounded, although it usually would be for electrical safety. But a flying airframe wrapped in aluminium foil is likely to make quite a good impromptu Faraday cage, which is why I think they make more sensitive receivers intended for such applications. It is better if you can get the antennae outside the foil. Note that often the antennae is a few inches long, it is only the bared piece on the end that really matters.

We could make quite a suitable antenna by making a slot in the foil and connecting the feeder to either side in the middle. This is actually done on full size aircraft, the slot being closed with a suitable dielectric like epoxy fibreglass.

It is not necessarily strictly true that 2.4GHz only goes in straight lines. It will reflect nicely from metal objects like fences, shed roofs, cars, and so on. This gives rise to multiple signal paths, which leads to a thing called Rayleigh fading....the signal level will flutter up and down. You may have heard this on an FM radio while driving in areas with poor reception. This is one reason why most receivers have two antennae, or even a satellite receiver. The modulation schemes used are also designed to help cope with rapid fading. (fluttering) This sort of thing is why you can get in building coverage with cell phones, even though the concrete is full of steel reinforcing.

As for not giving 100%...100% only means that you have met the standard required. It should be possible to attain it, if not necessarily common. I passed a mechanics examination once with 100%. If you perform all the calculations correctly, show all the working, and answer all the long form questions clearly, what else can they do? It does not mean that perfection has been achieved. If I had sat the same subject at the next level up, which would have been university, I would have been unlikely to do anything like as well.

John

[Erfolg]

I am bragging, that I have managed 100% in a materials exam, as a HNC subject. I was also told that it was not possible, by one of the tutors. I am bragging again, but, why bother, lots of people will have done better, there is always some one more successful.

One of my daughters had a summers job marking "A" level papers, who said that 100% is possible, if the candidate answered all the marking schedule requirements, fully.

Perhaps a little more information on the criteria with respect to Faraday cages design. I am not a electrical engineer, although having worked in the electrical engineering industry where generators, transformers and ancillaries were manufactured. I saw many Faraday cages, that seemed to be mainly fresh air, which seems to be at the other end of the spectrum to what is being discussed. Why?

 

Edited By Erfolg on 02/02/2015 22:47:31

[Codename-John]

Because electric always chooses the path of least resistance to a lower potential, An electrical faraday cage just has to be between you and the supply really, it will channel the voltage around the metal as its easier than travelling further through high resistant air and you, for RF it has to totally shield the signal to not let it through, think of it like a metal sieve, for normal electric it will hit a point closest to the supply then travel through the sieve as easily as it can to the other side, RF would do it to an extent but also leak through the holes

[Fatscoleymo]

OK - this morning thought I'd do a similar test as Buckboy did but using a Frsky Taranis and X4R receiver.

This on a range check (reduced output) can give RSSI - receiver signal strength indicater. Telemetry sends the figure back to the tranny to see.

I used a little cardboard box just big enough for the rx, battery and switch harness and totally covered it in one layer of 40 microns thick self adhesive foil tape.

Results: 1st figure a couple of paces away, 2nd figure 55 paces away (bottom of my garden)

Rx out of box totally open- 80, 51

Rx in box two antenna ends poking out 80, 51

Rx in box one antenna end poking out 80, 45

Rx in box tape just cut open on 3 sides of lid  62, Signal low warning 42 then critical 38 at 15 -16 paces

Rx in box totally sealed with tape 45, Signal completely lost in 6-7 paces

 

RSSI is an exponential figure.

Would all confirm the farady cage principle. But as soon as there is even a very small break in the foil the signal finds a way in.

Make sure that if you cover in foil tape the area of the fus where your rx is located you get the ends of the antenna outside for maximum range.

Steve

 

 

 

Edited By Fatscoleymo on 03/02/2015 09:44:21

Edited By Fatscoleymo on 03/02/2015 09:48:24

[Phil Green]

Posted by Codename-John on 02/02/2015 19:11:38:

Umm, don't know what happened there, but my post seems to have thrown a wobbler

John - copying the format as well as the content is a dead giveaway that you simply quoted Wiki !
The formatting can be removed by either pasting temporarily into notepad (which is a purely text app) then copying from there, or by using 'paste special', 'unformatted text' where an application supports it.

Cheers
Phil

Edited By Phil Green on 03/02/2015 09:53:14

[Codename-John]

Posted by Phil Green on 03/02/2015 09:52:55:

Posted by Codename-John on 02/02/2015 19:11:38:

Umm, don't know what happened there, but my post seems to have thrown a wobbler

John - copying the format as well as the content is a dead giveaway that you simply quoted Wiki !
The formatting can be removed by either pasting temporarily into notepad (which is a purely text app) then copying from there, or by using 'paste special', 'unformatted text' where an application supports it.

Cheers
Phil

Edited By Phil Green on 03/02/2015 09:53:14

the 2 big paragraphs I copied and pasted, but the last I typed out but it turned the same, that's what I thought was strange

[Daithi O Buitigh]

John - when you do a straight 'copy/paste' without pre-formatting it, the remainder of the text will adopt the same format as the pasted block

[Dave Hopkin]

Posted by Fatscoleymo on 03/02/2015 09:39:10:

OK - this morning thought I'd do a similar test as Buckboy did but using a Frsky Taranis and X4R receiver.

This on a range check (reduced output) can give RSSI - receiver signal strength indicater. Telemetry sends the figure back to the tranny to see.

I used a little cardboard box just big enough for the rx, battery and switch harness and totally covered it in one layer of 40 microns thick self adhesive foil tape.

Results: 1st figure a couple of paces away, 2nd figure 55 paces away (bottom of my garden)

Rx out of box totally open- 80, 51

Rx in box two antenna ends poking out 80, 51

Rx in box one antenna end poking out 80, 45

Rx in box tape just cut open on 3 sides of lid 62, Signal low warning 42 then critical 38 at 15 -16 paces

Rx in box totally sealed with tape 45, Signal completely lost in 6-7 paces

RSSI is an exponential figure.

Would all confirm the farady cage principle. But as soon as there is even a very small break in the foil the signal finds a way in.

Make sure that if you cover in foil tape the area of the fus where your rx is located you get the ends of the antenna outside for maximum range.

Steve

Edited By Fatscoleymo on 03/02/2015 09:44:21

Edited By Fatscoleymo on 03/02/2015 09:48:24

Cracking test Fats.... thanks

Getting the RX ariel out from behind the foil is a must then, I think I would advise using a satelite aerial in foil covered models, because at some point in the flight the the foiled area is going to be between the TX and RX - better safe than the walk of shame with a bin liner!

Makes you wonder about the tests Flite Metal did doesnt it........