EEVblog Electronics Community Forum
Electronics => RF, Microwave, Ham Radio => Topic started by: rhb on August 21, 2018, 02:49:51 pm
-
I'd like to do some physical modeling and VNA measurements of a dipole over ground in the 200-1600 MHz region. This naturally leads to the question of environmental effects. I can't afford to buy commercial absorbing materials for this, so I'm looking for alternatives. Iron and charcoal powder in a suitable matrix appears to be the way to go. For ease of handling, I'd like for it to be lightweight, which implies using foam as the matrix material.
@cdev mentions having done this, but made no comment about the foam he used. I need 80 sq ft and am at a loss to think of a material I could use to make foams with varying proportions of charcoal and iron. Assuming 3" thick sides, that's 20 cu ft of foam.
Any suggestions? At the moment the only thing I can think of is to mix the charcoal and iron powder with paint and then mix that with loose expanded polystyrene beads.
-
I don't know anything about RF chambers but I have worked with structural foam quite a bit, for creating super-strong carbon-fiber epoxy parts.
Structural foam is easy to work with but I can't recommend anything with my lack of knowledge of what you are trying to do.
A good vendor is Aircraft Spruce, of course, you have to wade through all the kit airplanes and trick aero stuff...
http://www.aircraftspruce.com/categories/building_materials/bm/menus/cm/foam.html (http://www.aircraftspruce.com/categories/building_materials/bm/menus/cm/foam.html)
I would generally stay away from the liquid foam products--they always sound like the perfect solution to any problem but getting them to actually do what you want in the time available (liquid for a few minutes) is (for me) impossible.
-
Thank you for your suggestion.
RF absorption materials embed carbon and iron, or similar materials in a matrix. A key requirement for suppressing reflections in an anechoic chamber is that the gradient of absorption be small. Large gradients will produce reflections. The square pyramid shape of commercial acoustic and EM absorption panels is motivated by this requirement. So long as the pyramids are small relative to the wavelength, it has the same effect using a medium of constant properties as a layer with varying properties.
Until you brought it up, I'd not considered solid foam sheet because I could not see a way to impregnate the sheet. However, one might well form wedges or pyramids of foam painted with an absorbing paint, though in this case, it might be difficult to make the dimensions small relative to the wavelength. But it could certainly be useful for suppressing corner reflections.
-
Some black anti-static foam is semi-conductive. The kind I'm thinking of is the stuff that you'd store through-hole ICs in by pushing the pins into the foam. I did a quick test of some pieces that were handy. Different types, thicknesses, etc. Size around 10x10 cm. Using two coins for contacts and pushing down on the coins, I got resistance values in the 1-100K range. I would think that would be a good RF absorber. The random nature of the fibers and spacing would ensure that there were no resonant effects. You'd have to try some to make sure that it would work for your purpose unless you could find some that was actually spec'd for use as an RF absorber.
Ed
-
It's my understanding from reading product datasheets that you need a conductor for the E field and a magnetic material for the H field. Hence the carbon and iron powder. I suspect the good stuff uses powdered mumetal.
-
There is a ready made graphite spray... Spray a layer, sprinkle with ground ferite. Dry, repeat.
-
There is a ready made graphite spray... Spray a layer, sprinkle with ground ferite. Dry, repeat.
I wonder if you used a spray paint gun and a bead blaster gun , how good a product you can make with very thin sprays. Both very cheap from harbor freight and you would not really care about the things you typically care about for paint job quality, and maybe a third spray paint gun with primer or some kind of filler compound if you need physical spacing. I can see myself building some kind of RF lasagna if I could wield two spray guns akimbo like a john woo movie, maybe with a foot switch on a heat lamp aimed at my mold to solidify it after each coat. It would take a long ass time but you can do it? Or maybe you can make a X/Y scanning laser to solidify it, some kind of UV cure foam binder maybe, i wonder if you can emulsify UV cure adhesive, it cures mad quick.
I wonder if you could grow trapezoids of the stuff using 3 spray guns and alot of time. Ferrite powder has to be cheap since you can buy inductors for so little. I don't think it makes sense to shield the really low frequencies at the foam level, you should just use mu-metal backing plates, they sell it alot for EMI crazy people that want to protect their houses from their junction box.
If you get the process down you could use solenoids to automatically trigger the guns, all taped together (like ripleys flame thrower pulse rifle), if you get the motion down, so aim it, and it gives you a few seconds of graphite loaded UV cure foam, then a few seconds of ferrite powder spray, then turns on a big UV led that you aim at your masterpiece before hand, all foot pedal operated, so you can do thousands of operation without destroying your hands, so long you get the rythem down. Or if the process dynamics are not too much of a pain in the ass, you might be able to mix it in the aresol phase by activating the sand blaster and the spray gun at the same time. It seems like you would need to try it to see if you can tune a good continuous distribution. Hell you can even easily do voice controlled solenoids, I imagine this would go on like 0.5mm at a time between cures, or slower... maybe throw in some stiffening strips of carbon fiber or some shit every so often so its strong.
You could actually make a big ass piece then cut it easily with a bandsaw to make your cones, rather then trying to grow it in cone molds.
Does anyone know the proper ingrediant? I can see something of very high quality being made this way without difficult shit like distributing everything in expandable foam to grow properly, it might be a pain in the balls if it starts sinking to form heavy layers. Unless you spray expandable foam real light, let it grow a bit and then spray it with the other stuff with layers...
I am convinced commercial pricing would hammer the hobbyist on buying this stuff. It might not even be that complicated.
I found that air-powder distributions are really good, even with large particles. If you are ever doing a garage epoxy floor from home depot, and you need to distribute those decorative flakes evenly (which will make it look great even if its all stained and fucked up from acid/solvent damage etc), I found the best way is to get a small leaf blower and put the flakes out in a cupped hand and just hit it with the leaf blower, works real nice compared to throwing them into the air, you get this nice evenly distributed cloud.
Maybe mixing UV cure adhesive with something like cabosil (fumed silica) would make a decent space filler that would kinda be foam like even if its not really foam. Cabosil is horribly voluminous for its weight. Or maybe aerogel powder of significant mesh can approximate foam where foam should not exist. I don't see commercial UV cure foam spray solutions.
Cabosil should be transparant to UV light some what, so maybe you can use a regular UV adhesive spray, then a dusting of cabosil mixed with the E/M conductors, if its not possible to spray the cabosil while mixed with UV goo.. might actually be more uniform then real foam without all the engineering crap to figure out foam behavior. Or maybe even using fiberglass 'powder' as a filler?
-
2N3055 and coppercone2 Thank you!!!!!!!!!!!!!
As it happens, I have a Marshalltown texture gun designed to shoot popcorn ceilings. I hate them, but bought it to repair some damage while painting a house I bought 25 years ago. Except for recently shooting orange peel in a bedroom and bath for my sister, I've never used it again.
Mix aerogel beads to reduce weight and carbon and iron powder with lightweight sheetrock mud. Vary the layers from a lot of carbon and iron for the base coat and lesser amounts in each additional layer to provide a gradual impedance transition. It's cheap, fireproof and easy to do. If I shoot a light coat of latex paint between layers of texture it should be quite durable and easy to repair if damaged.
I *love* this forum! I don't think I'd have ever thought of doing that, but it's the perfect solution for DIY RF absorber. It will be interesting to compare it to open sky with the aluminum sheet on top of a fiberglass stepladder in an open field. Figuring out how to make a box with no flat rectilinear sides will be interesting, but some 1/8" luan sheets with exterior formers should do the trick.
-
is iron powder the best tool for the job here? Won't ferrite be better and cheaper and more corrosion resistant (I think the sulfate might react with the iron powder).
How much of a spacing do you need between layers on this stuff? I thought it would be like fractions of a milimeter.
Can you detail your work? I can't do any of it right now. Cabosil is dirt cheap, I used to use it for pyrotechnics, and it flows amazingly.
What is a commercial solution going to do better? (seriously, I want to know what makes that shit so fucking expensive it rages me when I tried to estimate the cost of making a RF wall for an antenna analyzer so you can spin an antenna infront of it to measure radiation pattern). There is a serious chance of some kind of fat cat business man making all our lives difficult with something very simple, I never saw it explained in good detail. Unless those additives they use are like the price of gold, that stuff must have INSANE margins for manufacture.
You can probably make some kind of spray gun assembly to run back and forth on a rail with the most simple parts if you can find a process time constant that works, using the most simple of electronics, Hell you can mount a RC car to a 2by4 to make something wide set on the floor so you can cut it up later, like drive it upside down on a ceiling made of some particle board over a partical board mold lined with some saran wrap, maybe like let it dry every so often and remove the build up on the sides to ensure there is uniformity and it does not curve over, and then make it too wide so you can cut the edges off later so you get a nice lasagna
The tolerances for making some kind of oversized foam structure just seem loose as hell
Your idea should also be less flammable, since it uses gypsium, and there is no air voids to melt away since its filled with some kind of inert light crap
-
I also would like to know how a simple steel wool would perform.... Just attach it to surface and fluff it up ....
@coppercone2
I did suggest ferite... It shatters easy, and you shouldn't try to grind it too fine and uniform...
-
No I mean ferrite particles instead of iron particles, the stuff they sinter together to make inductors. Why would it have different performance in a matrix structure then iron powder? I think the domains would have a better frequency response then iron too. And it might be cheaper, more oxidation resistant, and lighter since its a alloy of zinc and nickle or whatever and its used in massive quantities, and I don't think its flamable like iron powder is.
I know if you use it solid its more fragile, but a powder is a powder?
-
can you sketch out a pyramid of what you have in mind to get me an idea of what the correct layer structure looks like (like food pyramid with distance of seperations etc)
-
My solution when I needed to do this was to conduct the test in an empty lot on top of a hill. The lot I used was not quite empty; I could measure the reflections from trees and lamp posts in the distance but they were small enough to ignore for my purposes.
-
No I mean ferrite particles instead of iron particles, the stuff they sinter together to make inductors. Why would it have different performance in a matrix structure then iron powder? I think the domains would have a better frequency response then iron too. And it might be cheaper, more oxidation resistant, and lighter since its a alloy of zinc and nickle or whatever and its used in massive quantities, and I don't think its flamable like iron powder is.
I know if you use it solid its more fragile, but a powder is a powder?
I use word ferrite here for a ground magnetic cores.
Ferrite is actually a name for an allotrope of iron. It is iron, pure one. Fine powder iron is mostly ferrite.
But, iron particles in cores are suspended in ceramic material. Which is good, because pure iron powder will become iron oxide very quickly.
Ceramic matrix in cores encapsulates and protects iron from oxidising. So it will keep it's magnetic properties. It also isolates particles from each other, preventing currents forming in core.
-
I have sandwiched a layer of the silvery static shielding plastic bag material under the lid of a 10 GHz amplifier to stop it from oscillating (which it did, in other bands, with the bare aluminum box lid. Not sure how it compares to proper RF absorber materials, possibly it was just good enough for these particular frequencies and the geometry of the box.
-
My solution when I needed to do this was to conduct the test in an empty lot on top of a hill. The lot I used was not quite empty; I could measure the reflections from trees and lamp posts in the distance but they were small enough to ignore for my purposes.
That is a valid technique and reason that I bought Signal Hound SA.. With a notebook, it is portable...
Problem is that where I live, it is increasingly hard to find a spot that has no RF activity...
-
I have sandwiched a layer of the silvery static shielding plastic bag material under the lid of a 10 GHz amplifier to stop it from oscillating (which it did, in other bands, with the bare aluminum box lid. Not sure how it compares to proper RF absorber materials, possibly it was just good enough for these particular frequencies and the geometry of the box.
Propper RF absorber is broadband, so it should work for (absorb) wide span of frequencies, preventing resonances and reflections..
-
All I know at this point is from reading datasheets for $$$ commercial material. In particular elastomer sheeting with carbon and iron powder in it. For low frequencies the magnetic particles need to be larger. So I think I'll start saving the steel chips from my bandsaw. It might well be that chopped up steel wool would be useful. There should be quite a bit of literature, though I'm sure a lot of it is classified because of the application to radar reflection suppression.
I've got a couple of WiFi feedhorns, so when I get my xaVNA I'll point them at some fluffed up steel wool and compare that to a plain steel sheet and also to test various spray mixes.
-
I still don't understand why iron powder instead of a ferrite powder?
http://www.comcraft.co.jp/products/temex/data/tem01_ferrite.pdf (http://www.comcraft.co.jp/products/temex/data/tem01_ferrite.pdf)
It looks like a MgMn ferrite would be good, look on page 11 of the document on the table that says FRQ range. I thought the dipole movement of iron is only good to maybe the low MHz range if that.
-
To be truthful, I have no idea. In this application losses are desirable whereas in transformers they are not.
In the end though, for this project, materials which are easier to source are more desirable. It's crazy enough even without spending a lot of money and time.
Once I get my xaVNA and start measuring the reflection from an aluminum sheet with various materials on the face I might actually know something. At present I'm drowning in a vast ocean of ignorance. Hopefully, I'll learn to swim soon ;-)
-
yea actually I am thinking about it and I am not sure what material is the best/where its lossy
-
Coppercone being coppercone... ::)
The normal way it's done is radar absorbent foam pyramids on top of ferrite tiles. I've seen hollow grid tiles and plain flat tiles. I suppose, preferably, the one on top of the other in that order.
I think the ferrite is a particular grade, but it's hard to come by -- probably because you'll only ever order a pallet worth at a time, building test chambers. Who needs just one or two, or a hundred pieces, right?...
You can probably do pretty well with a fairly large stockpile of NiZn or lowish mu MnZn plates, commonly sold for planar magnetics, or various EMI purposes (you can find perforated plates for connectors, and flat plates for near-field EMI).
Tim
-
To authors of "funny anechoic chamber build ideas": wheel was invented long ago :-DD
https://emcfastpass.com/anechoic-chamber-guide/ (https://emcfastpass.com/anechoic-chamber-guide/)
Basic Rules for Anechoic Chamber Design (https://www.nsi-mi.com/images/Technical_Papers/Articles/160101A%20-%20Basic%20Rules%20for%20Anechoic%20Chamber%20Design,%20Part%20One.pdf)
Im afraid that 4' x' 4' x 4' anechoic chamber is too small for 200-1600MHz antenna measurements. You would want to measure antenna quite into far field region (https://en.wikipedia.org/wiki/Near_and_far_field).
My suggestion: drop anechoic chamber idea and consider open field tests (get portable AC generator and maybe tent as well).
-
Well I mean alot of people are interested if someone works out a specification it might be economical to buy a pallet and distribute it to a few people that have the means and will to use the stuff to build their own chambers
The only RF lab I worked in had cardboard pyramids with two sides only that had some ferrite on em and were stuck to the wall with double sided tape. I was told it was the most economical, but I suspect that if you do something like antenna directional testing, you will want full 3d pyramids made of foam. I don't think that chamber went past like 2GHz.
-
Nice link! Thanks.
My goals are rather different than typical applications of an anechoic chamber.
My interest is in measuring the scattering parameters of a "fan antenna" as the element placement is varied, so a rather different case than typical. If needed the xaVNA goes to 3.5 GHz, so I can scale the antennae and/or build a larger box. I plan to compare it to an open area site with some basic 1/2 wave dipoles so I know what the distortions are. If they are low enough I should be able to correct for them.
Recording studios avoid rectilinear shapes to avoid resonant reinforcement of reflections from walls and corners. And suppressing unwanted reflections is a common problem in reflection seismology.
But this is all hobby science. Cheap rules and failure not important. The original question, what to use for an absorber, was answered. Aerogel or foam beads, lamp black, powdered iron and drywall mud are cheap and readily available. Ferrite powders are probably harder to source and likely pricey. I should be able to get a good idea of how effective materials are using the xaVNA and a pair of antennae pointed at a sample at a short distance.
The chamber size is dictated by what I can conveniently store. If I have to go much larger I'll just use cardboard and throw the thing away when I'm done.
-
I think it would be really worthwhile to look into ferrite powder prices to do a real comparison if you are going that far, maybe send some emails
g2g
-
I found iron filings for about $3.40/lb in the US delivered on eBay. Ferrite powder around $150/lb from the one place I found that had prices. But I need to research the materials. It might well be that a usable ferrite is being sold as sand blasting media or as sandpaper.
-
Old thread that may offer some tips and guidance:
https://www.eevblog.com/forum/testgear/emc-chamber-build-log/ (https://www.eevblog.com/forum/testgear/emc-chamber-build-log/)
-
Old thread that may offer some tips and guidance:
https://www.eevblog.com/forum/testgear/emc-chamber-build-log/ (https://www.eevblog.com/forum/testgear/emc-chamber-build-log/)
Very interesting report. Thank you!
You mention ferrite used in this post (https://www.eevblog.com/forum/testgear/emc-chamber-build-log/msg424583/#msg424583):
When its all complete, 1.5 tonnes of ferrite tiles will cover every inch of the inside.
After a quick google search I think I have these ones:
SFA600 or SFA600A
http://www.samwha.co.kr/SW_catalogue/catImage/37/Ferrite_Absorber.pdf (http://www.samwha.co.kr/SW_catalogue/catImage/37/Ferrite_Absorber.pdf)
Each individual tile is 100mm sqaure. They are mounted on a board in a 6x6 grid.
I'm told each tile costs $5 US direct from the manufacturer, don't know how much we paid for each.
I worked out (roughly) that we would need a little over 10,000 tiles to fill the inside of the chamber so $50k just on tiles would not leave me with anything for the equipment.
-
bulk powder of shady grade from china might be alot cheaper then sintered tiles with specifications.
You can just make the cones bigger I think with little consequence so long you stay in certain frequency bands. Ferrite is mainly designed for inductors I think, so using it as a general 'i dont want reflections here' might allow for loose specifications.
With a ferrite tile they probobly sinter it and measure it and there are rejects due to process errors, inspection costs of initial powder and finished product, chipped stock that does not sinter properly, bad distributions of ferrite, improper geometries, mold maintence costs, sintering cost, furnace upkeep costs, factory calibration costs, stock maintenance cost (they are kind of difficult to store as they are heavy and fragile) skilled ferrite technician costs, shipping cost and then multiply cost by 4-10 for company profit depending on how niche it is. If they sell a tile for 5$ that means probably ~1$ of raw material went into it (or maybe much less).
since we are spraying it in a garage with ceiling material some liberties might be allowed to reduce costs, I kind of imagine it showing up in a giant 'chemical bag' like you get seed, rocks or industrial chemicals in, the one that you basically lift into the air and cut it on the bottom made of nylon.
With some kind of home made antenna tester I think what you are looking for is a way to detect which signals are bogies so you can ignore them in your head rather then try to get some kind of universal 'every joe on the street will know what this graph means' thing going on, especially when you are using it as a calibration/reference standard against a hill you found outdoors.
I don't want to stifle discussion of high end solutions but I don't want to see this degrade to cave technology because someone found some price of some manufacturers product, we have yet anyone with experience in the ferrite tile or anechonic pyramid/cone manufacturing field to tell us how sane or insane we are, but I have yet to find some kind of encouraging ferrite price war between two companies undercutting so it might be a stagnant niche.
To put prospective on it, he is getting 3000lb of tiles for 50k, meaning that the material finished inspected certified all that jazz is 16$ a pound, vs the 160$ a pound of raw unworked material someone found.
That means 3000 units get you a bulk discount of 90% of finished product. Thats kind of strange imo. I figure that 150$ a pound is worth something like <<20$ a pound, 90% for 10,000 units is a bit of a steep curve for bulk buying IMO, 30-50% price would be a good deal @10,000, and thats finished product. I would like to know more about how those factories run so I can understand the cost reduction with volume that occurs. It makes sense to me because you get 16$ a pound finished for 10,000 units at say 50% discount, so a single unit might cost like 32-40 dollars, and thats a damn finished product, with 4x markup on materials say, so thats 8-10$ a single pound, but they wanna charge 150. Even if you multiply it by a really bad 400% small guy bullying/inconvenience cost, you still get like what, 40$ a pound?
150 a pound is cringe, even with a 400% bullying small customers cost. I bet you can get chinese stuff at 5$ (hell or less) a pound for 50 pounds.
The powder screening process is probobly throwing it into a ball mill and paying someone with a particle mask 12.50-15 an hour to beat a seive with a rug beater to get 'engineering particle grades' and some tech 17-22$ an hour to look at a sample of a 100kg bag of it under a microscope (they probobly get the raw materials from china too, off alibaba)
I think people are just willing to pay because the people that are typically interested in ferrite tiles would be rather well educated RF engineers that are all neat, understand mathematics, nice clean temperature controlled office etc. I don't see them wanting to fuck around with giant bags of dust and 'kiln conditions' that bring the movie 'deer hunter' to mind. It seems that even as far as metalurgical interest goes, making ferrite is the bottom of the barrel as far as work conditions.
Before someone says I am saying RF engineers are posh, I just mean its a dirty ass nasty job. My friend works in a factory that has a kind of department related to heat treating, kiln work, etc.. and it pays more to work in that location. Most people don't wanna do it, blue collar guys like mill wrights, stock workers, tool operators. Apparently it feels like walking into hell. But it means a hell bent individual can save alot of money.
-
Ferrites are probably more in line with geology. They are sort of between ceramics and metal. Before I strayed from the strait and narrow and went into geophysics, I was trained as an igneous petrologist and became quite familiar with phase diagrams of solid solid solutions of things like garnets and spinels. It is highly likely that there is an inexpensive alternative to expensive "optimal" materials.
The chief difficulty is likely to be that the purveyors of cheap materials will know little about the composition of what they sell.
-
I recently found out that you can do x-ray metal analysis at home very cheaply but it won't give you insight into the grain/crystal (what is it?) structure, though maybe you can sinter a small block of it and then polish it and do a acid etch and look at it on a metalurgical microscope to get some kind of idea. The videos on youtube show that you can get a elemental composition from a metal sample using a small home made benchtop device.
https://www.youtube.com/watch?v=5rTwOXVBOko (https://www.youtube.com/watch?v=5rTwOXVBOko)
https://www.ebay.com/itm/AmpTek-XR-100T-CdTe-Gamma-X-Ray-Detector-w-PX2T-CZT-Power-Supply-Amp-MAY-BE-NEW/132718107889?hash=item1ee69d9cf1:g:Ca4AAOSwBrlbMQld (https://www.ebay.com/itm/AmpTek-XR-100T-CdTe-Gamma-X-Ray-Detector-w-PX2T-CZT-Power-Supply-Amp-MAY-BE-NEW/132718107889?hash=item1ee69d9cf1:g:Ca4AAOSwBrlbMQld)
I thought it would run at least 50,000$ for one.
http://www.gammaspectacular.com/phpBB3/index.php (http://www.gammaspectacular.com/phpBB3/index.php)
Beyond that I think you would need X-ray crystalography right?
-
Barton garnet media is pyrope (Mg3Al2Si3O12) and almandine (Fe3Al2Si3O12). These form a continuous solid solution series with spessartine (Mn3Al2Si3O12). So it is highly likely that there is garnet sandblast media with a Mn,Mg,Fe composition very close to that of the expensive material at prices dominated by shipping costs.
A Von Lau camera will handle the X-ray diffraction albeit a bit tedious to interpret. That just requires an X-ray source. One could also use a petrographic microscope and immersion liquids.
At the end of the day though, what matters is the lossiness of the material and that is best measured electrically in the same manner used for core materials.
-
I used activated charcoal from several spent BRITA water filters and off the shelf "Great Stuff" foam. I tried to make it densest in the back where it touched the metal. It was used by being poured in layers into a round metal cookie box. Its very hard to work with because if it gets into any of your clothing or on your skin its soon hard as a rock when it sets and its very hard to get off, and it hardens quickly.
Even if you use a LOT of carbon its not that conductive. Iron filings would probably help there. (not ferrite, iron)
The idea was to make an RF absorber to go underneath a spiral antenna. I think I used four thin layers (15 mm each?) of the black(really black+pink) foam. Once it sets its not possible to change it. It was flat, not shaped. I still may have this thing somewhere. I'll try to find it, I haven't seen it in a while. My wife quite possibly may have thrown it out. (Understandably)
I've also used anti-static foam in a small plastic bag experimentally to see if LNAs were oscillating. (If it is pressing it down on top of the active device stops it, changing the current drawn.) thats a useful thing to know.
-
I could see using a ball mill to grind appropriate materials to be added to a binder and testing that for absorption but I have no idea how to extrude a consistent foam.
-
The foam is expanding as it leaves the nozzle and then you have to rapidly mix it and pour it into something else (or mix it in the container which is what I tried to do. This is extremely difficult to do fast enough to have it mix throughout the foam. As the goal of what I wanted to do involved needing some precision (I wanted to see if I could use this in a (receive only!) GPS antenna) I considered the whole effort not really at all successful. The signal strength was higher with the cookie box (because it was acting as a reflector- probably!) but I think the antenna was more accurate without it. Just in free space. The spiral antenna was a fairly good GPS antenna without any additional metal around it. (except for the feedline going straight down and to the side after a bit)
I think the earth in my back yard with its natural increase in moisture going down made a better RF absorber. Which makes me suggest. (I think the best testing environment thats likely available to you and the rest of us is some open space - like a field or a parking lot.)
And I found that the granular carbon I had was not that conductive. If you squeezed it the resistance would go down markedly (this is the principle that telephone mouthpieces used to use) but I think some kind of carbon fiber would likely work best, not granulated carbon like I had. What I wanted to try was many layers of different batches of conductive antistatic foam pressed together. I tried to do this with some carbon foam I had but I didn't have enough of it.
-
I think adding foam beads to a binder is more practical. In principle you can inject air bubbles into a binder. That's done with concrete. But cleanup with a non-water soluble binder would be a real headache and if you don't achieve the right viscosity, the bubbles just migrate to the surface.
I also think loss testing small samples is probably the most efficient approach. IIRC there was a thread in a VNA group which referenced an HPAK fixture for testing toroidal cores and a link to an article about solenoidal cores.
-
Yea I have serious concerns that trying to make voids without the use of some kind of filler media is gonna be a pain in the ass process. There is probobly a reason those hydrogen bomb plasma foams cost billions of dollars to develop.
Hmm, if you grind up foam, there is no reason you can't sinter a plastic mixture right?
Another binder idea is maybe using sand or anything else really soaked in sodium silicate, which is hardened by CO2 gas diffusion to make a glass like binder thats not time sensitive or nothing. You can store this stuff premixed pretty much indefinatly, put it into a mold and fill it with gas and it will begin to solidify, you just poke it with a fine needle to inject gas into it if its really deep. It makes for good metal casting molds. Also high temperature stability, inflammable and fairly hard but it will crumble if you start banging on it hard. If you set it right you need to basically use a chisel to break it.
-
Think about the wall absorbents as antennas.
If the wall should absorb and not reflect must it be impedance match.
Using ferrite/iron/coal powder will create a reflecting wall if wall impedance and wall structure not is matched to the space wave.
To achieve impedance match and not create a reflector is a certain deep relative lambda of absorber thickness needed as for most antennas.
That is why cones-shapes often are used as a kind of successive impedance matching, but these cones are still frequency and depending on angle of RF signal to it should be able to absorb well. If checking inside a chamber can it be seen that often different lengths of cones are used in middle of chamber compared to its ends. Size and angels and amount of absorbent in the cone decides its optimal effective frequency range and usable angles.
Ordinary coal is the most common material. In some cones is coal density different at different heights.
If only near-field magnetic field are needed to be absorbed can ferrite tiles with high resistance for actual frequency be used, no need to give them any impedance matching shape.
Open field measurement is so much simpler. Open field distance should have an length of 5-10 lambda. Reason to use open field is to avoid all none planar waves. That includes reflections in ground between Rx and Tx antenna, That is why a certain measurement height is needed.
VNA with gating function can also help a lot. Also using directive measurement antenna reduces unwanted reflections to be mixed with direct wave.
Wide-band horn antenna is often used for that purpose.
A very simple way to avoid ground reflections is to measure vertically in open field upward against open sky. This kind of measurements can be done even in a rather noisy urban environment as long as own signal can be identified, transmitting a relative short distance and using a directive antenna. In worst case, reduce measurement bandwidth to as low as possible, 1-10-100 Hz and let any frequency sweep be slow enough.
-
How do you work out the geometry with the cone shapes and angles based on distance from a radiator? The great thing is if we make big molds we can cut whatever cones we want out of them with simple tools like sawzall.
How can we categorize a cone that we make?
Also if you mix up a foam plastic/other mixture, maybe you can modify a 3d printer to grow them with layers using a infrared heat source to sinter it.
If its too hot you will smoke cones, or maybe you can smoke cones to your advantage because char is kinda conductive, add some saw dust in there and hit it with a laser, or pump smoke from an acetylene flame into it to paint it as its doing layers, so the cone will be smoked
maybe you can just set a tire on fire near the 3d printer and blow the fumes at it with a fan, who knows
-
Cones need to have a controlled and even density of coal and a thickness enough to be absorber at particular frequency.
See thickness as a kind of successive attenuation. If impedance is 377 Ohm, fine, then just let cone be thick enough to absorb as much of reflection that remaining reflections energy not interfere with main planar wave. If thickness is much less then Lambda or impedance is way of from 377 Ohm will it work less good.
Suppress unwanted reflections with more then 20-30 dB in whole chamber is more or less impossible but if chamber design is good will there be a zone with low amount of reflections, known as quite zone, where a dynamic range of 40 dB or more is possible.
Geometry of cone is depending on actual needs. Calculate its shape and impedance can be done in most RF design software such as CST..
-
How exactly do you define the impedance of the cone? 377 is the impedance of free space.
If the thickness is much less then lambda - is this why I sometime see the cones with the top cut off to make flat faces on top in EMI chamber pictures?
I have trouble relating what you wrote to a cone covered surface.
Can HFSS do what you want, if I modeled a foam wall in solidworks and imported to HFSS?
-
Can you tell us how to measure a foam sample to get the data relevant towards a HFSS or other program simulation? Can you wedge a block of particular dimensions into a VNA somehow? I just built my first heatsink in solidworks and it was remarkably easy, I found a way to import my solidworks models into HFSS, and youtube can probobly tell me how to run a 3d simulation, but I assume that HFSS will have some kind of table or graph import or something related to the material. How do you get this data from the laboratory. What equipment/setup do you need?
I assume it would be something like setting up a foam sample of particular dimensions connected to some kind of test equipment, either by direct connection or some kind of antenna system (like facing horns).
-
In CST/HFSS do you define each structure and shape before resulting RF fields can be analyzed.
It is you that provide material data such as foam conductivity and dielectric constants, same for coal and coal density.
Basic material data can either be found in books or internet or you have to measure it yourself.
How to do measure, search VNA+dielectric measurements. It is well described at R&S and Keysight websites
Some providers of absorbing material do have very informative data sheets that can be of help when calculating your own absorbents.
-
Does anybody know what the properties of carbon fiber are with RF?
-
Carbone fiber is a wide range of different materials. Fiber size and type of filler can vary in a rather wide range.
Measure a sample of your DUT is recommended. A lot of different types of carbone fiber are measured and results are published but results are nothing more then typical indicators. Some results here. (https://www.google.com/search?q=carbon+fiber+dielectric+properties)
-
My idea of testing material suitability:
Get basic directional antennas for the interesting range and go to the "hilltop" test site.
Make samples that will "block" the estimated main lobe. I would expect a good sample to be the one that doesn't act as a effective reflector for a "backfire yagi" and can get to the antenna proximity while changing S11 less than others.
When building your chamber, put your tiles/cardboard pyramids/whatever most dense in the expected main lobe, fill up when you can afford it. Consider a air gap behind the absorbers in your chamber, so reflected radiation can enter its demise from the backside too. Movable absorbers can be pushed around to see if they act better somewhere else.
-
Hi,
when I was young at university we hacked a chamber from egg cartons covered with a laquer made from shellack and powdered iron. After drying, we applied a layer of graphite spray normally user for the repair of CRT tube shieldings.
To our utter surprise, it worked quite well up to the 13cm band.
-
Some excellent suggestions. Thank you.
My interest is investigating multiband HF dipole designs by scale modeling in addition to using CEM tools. Not sure it will work, but I thought it would be an interesting experiment if I can keep the cost low enough.
I am a huge fan of the Amateur Scientist columns by C. L. Stong.
-
Hi. Did you accomplish building your anechoic chamber finally?
-
Sadly, no. I bought 4 racks full of T&M gear and overwhelmed myself dealing with all that stuff.