e-field generator - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: January 21, 2019, 01:56:33 am
content: So Hwalker answered a ton of questions about e-field antennas in the other thread I made a while back, while I have not made one yet I thought that through fully and the next step is a e-field generator.What exactly are these things?https://www.arworld.us/html/13100.asp?S=4https://www.atecorp.com/products/amplifier-research/ate10k100mI kind of understand the h-field generator (which is a helmholtz coil).The idea is that I can test e-field shielding on stuff once I have a known e-field generated (measured by the e-field monopole antenna).I would have thought it would be like putting something between the plates of a air capacitor (equivalent to the helmholtz setup).Page 2 of the second link has a schematic btw, its bizarre to me.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 21, 2019, 02:25:58 am
content: Neat, it's terminated in the middle. Probably radiates quite well in the 10s MHz range, but at low frequencies it'll be largely static fields.Plates are fine at low frequencies, but at higher you either want something like this, or a TEM cell.Which one is chosen depends on the standard.Likewise, Helmholtz coils only work at low frequencies; a simplified loop antenna design is used at higher frequencies, for complementary reasons.The upper frequency limit (where it's still mostly E or H, and a mostly uniform field) is inversely proportional to the size of the antenna, of course. The uniformity within the test volume follows a similar scaling law.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: January 21, 2019, 02:53:04 am
content: do you terminate it to 50 ohms?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 21, 2019, 10:17:52 pm
content: Looks to be.VSWR max 2.5 suggests odd impedances, presumably at high frequencies (a folded dipole has a much higher driven impedance, probably a factor here). Without a plot of VSWR, it's not obvious what or where, but that's likely.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: January 21, 2019, 11:21:48 pm
content: are you supposed to just fit the power resistors in any way possible in the gap shown?How does thickness of the antenna element effect it? I assume thicker = more bandwidth?Looks like a copper pipe project. I happen to have a crappy pipe bender suited for bending copper pipe, I think I can make it out of 2 big segments without any junction pieces.If I just went with it based on picture, I think a 1/2 inch pipe would work.They look to be drawn off center. This is confusing to me.Can you tune it with a ghetto sliding load maybe?Can a thin version bent out of aluminum wire and some wooden zip tied on supports provide the termination placement? Or thinner refrigerator tubing. But I suspect the placement will change depending on dimensions?How should the junction between the termination resistors and the antenna look like? I had in mind to use a copper fitting that is basically smashed shut to make a taper soldered to the resistor parallel bank.And how should it be fed? Are those shown in the diagram as far apart for clarity or is there also some critical distance here? I am guessing if you use it with a low power gen there is no problem using a small non inductive resistor here ?I don't think I need power to test a shield, just frequency, I don't think my style of shielding (i.e. high silver soldered electrical copper sheet/plate) will be effected too much by power level. Maybe a crappy plastic metalized box or something.I am trying to think of how to test the spacing. I guess you can start in the middle, add a minimum distance copper-to-copper pipe fitting on one side, then slowly cut the other section away by say 1/4 inch at a time while monitoring on a VNA (reflection?) to get the proper spacing? Copper pipe is not really expensive.Then on the other side you can add 1/4 inch to it by extending a nested pipe after the opposite side is cut off.argghhh they dont tell you the top spacing either. So you have TWO unknowns to work out because you need to mess with both the resistor spacing and the damn Z axis spacing.Any ideas? Is there some kind of program that can find the unknown from the picture if you give it length (it figures out exact prospective based on X and Y dimensions provided)?or maybe this one is more accurate to the dimensions, that one looks square and impossible to figure out with the error?

author: coppercone2
author_rank: Super Contributor
timestamp: January 21, 2019, 11:53:53 pm
content: the description says74 x 41 x 102 cmthis would compare to71 x ?? x 96so 71/74 and 96/102so ratiometrically0.95 x ?? x 0.941Could you assume ?? is a ratio ~ 0.95, or ~39cm?then 71/2 = 35.5, so the middle is 35.5cm. Even if you shift it by 0.5cm per cut its only at most what, 60 cuts and solders? You could be done in like a few hours.The only thing is the input spacing.... you can't do two unknown at the same time. Any chance you can find one then the other (if the behavior of the input spacing could be approximated, the resistor spacing could be found first and then you can keep cutting away say 0.5cm at a time from both sides to get the response you want.

author: coppercone2
author_rank: Super Contributor
timestamp: January 22, 2019, 07:43:35 pm
content: is it possible the graphical artist was just lazy?im trying to think of why it would be unbalanced but nothing is coming to mind. All the magnetic equivalents are precisely balanced.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2019, 11:35:42 pm
content: It's a crude diagram, not a dimensioned drawing. A good bet the resistor is in the middle.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: January 22, 2019, 11:44:04 pm
content: that crude?in a manual? in a RF company ? and they want prob like 5gs for that.. imo thats worthy for internal documentation...anyway, how about the input spacing? that is the only variable then.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2019, 11:54:57 pm
content: As I've said before, and will likely say again...What you think, doesn't matter. What I think, doesn't matter. Physics gonna physic. What the fields do, matters.Regarding documentation, like I said, it's not a dimensioned drawing. It's not plans. You're the one trying to read them as plans. You're complaining about yourself, basically.Quote from: coppercone2 on January 22, 2019, 11:44:04 pmA very good balun (probably nanocrystalline core based), also in the middle.Incidentally, the balun can be a different impedance, which is probably a good idea,Quote from: T3sl4co1l on January 21, 2019, 10:17:52 pmin which case the terminator is the same value as the transformed impedance.The ratio should be easy to calculate from the electric field vs. input voltage/power calibration, and estimated distance between poles.If a balun is used, then one should expect DCR ~= 0 at the feedpoint, i.e., when they say 10kHz to whatever, they mean it, it's not down to DC. Or if the input were balanced, it could go down to DC, but I don't know of any test equipment that uses balanced cable, and they'd just slap a balun on it to use it with normal BNC cables anyway.But I doubt such tests need range down to DC; and the low frequency end offers possible power savings with an active, high voltage driver (which would also be killer for electrostatic loudspeakers, mmm).Tim

author: coppercone2
author_rank: Super Contributor
timestamp: January 23, 2019, 02:02:31 am
content: how should the layout of the resistors look like in the termination?I was thinking minimum impedance would be to put4 chassis mount RF resistors around a cube, but that leaves no area for radiators for 500W. say those 125W resistors that look like a diode block.like 5 10 ohm series resistors would allow you to put it on a bar, but it would be kinda long. How should the terminator size compare to the overall antenna size? I don't really know whats going on here because its shown to be part of the antenna. I never saw a design like that before.

author: coppercone2
author_rank: Super Contributor
timestamp: February 11, 2019, 03:34:24 am
content: The interior of the copper tubing should not matter right?Out of curiosity what does the oxide layer INSIDE the tube do on a metrological level, if possible to predict? If the exterior is polished, how many orders of magnitude down is the effect of the interior of a sealed pipe?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 11, 2019, 03:43:08 am
content: Like ten orders of magnitude? I find it hard to even begin to quantify that.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: February 11, 2019, 04:28:15 am
content: if you made the wall atomically thin would you begin to see something ? (skin depth related)?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 11, 2019, 05:38:46 am
content: You'd see nothing go out because the resistance is huge...Tim

author: coppercone2
author_rank: Super Contributor
timestamp: February 11, 2019, 02:56:49 pm
content: I am wondering a good way to stiffen up the pipe a bit since its going to be on a stand. Maybe fill with sand/glue mix or cement when ready. Based on how this device is to be used it looks easily damaged.

author: ejeffrey
author_rank: Super Contributor
timestamp: February 11, 2019, 05:12:16 pm
content: Quote from: coppercone2 on February 11, 2019, 04:28:15 amIf the thickness is smaller than the skin depth then the current is carried roughly uniformly across the cross section and the "surface" treatment is not terribly important beyond the effect on the total resistivity. Of course, the point is that at high frequency a typical rough oxidized surface is deeper than the skin depth, so a conductor 1 skin depth thick would be all oxide and perform extremely poorly.

author: coppercone2
author_rank: Super Contributor
timestamp: February 11, 2019, 05:20:43 pm
content: It has to do with mean path lenght at all? Are there any useful ideas relating to topology i.e. flattebing out the surface mathematically to compare aparant surface area to bounded stretched area?Does debrogaile wavelenght factor into it like with em reflectors?

author: coppercone2
author_rank: Super Contributor
timestamp: February 11, 2019, 05:22:13 pm
content: Like slice and line integral based on crosssection over strait line along the conductor?

author: coppercone2
author_rank: Super Contributor
timestamp: February 11, 2019, 05:25:05 pm
content: Any recommendations for a core? I think i have a copper pipe maybe and i got a pipe bender.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 11, 2019, 05:41:47 pm
content: What?At low frequencies, the current carried by the elements is very small, and they can be made out of anything moderately conductive. At higher frequencies, the resistance needs to be smaller, to keep the time constant below the driven period. At maximum frequency, inductance becomes significant, and resistance needs to be on par with (preferably lower than, I would imagine, so that the source and termination resistances dominate) the characteristic impedance.If the tube is thicker than the skin depth at any of these frequencies, then resistance of the inside of the tube will have negligible effect on the total, which already has a negligible effect on the total when its resistance is already low enough to be fine.I would imagine that thin stainless steel tubing could be used just fine.Tim

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 11, 2019, 07:19:45 pm
content: Quote from: coppercone2 on January 23, 2019, 02:02:31 amI also think that they simply left out the balun. Nothing else makes sense.And i don't think that the resistor in the middle should be 50R. For folded dipole, rather more.A figure of 240R comes suddenly to my mind, but I have to look it up - might be totally wrong.

author: coppercone2
author_rank: Super Contributor
timestamp: February 11, 2019, 07:53:49 pm
content: how do you measure the performance of something with a balun ? Do you put a directional coupler before the balun when its single ended, or do you need a special setup?I was going to bend the antenna and leave minimum spacing in the middle with some special pipe-to-wire terminations (braze a bit of wire or copper strip into a drilled pipe cap so a resistor can be soldered on with a soldering iron. When I find the right resistance I would expand to get the correct thermal size. At low power levels a 1/4 watt one should work?