effect of steel shield on rod antenna? - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: November 28, 2019, 01:21:37 am
content: on a ferrite rod antenna, if the electrostatic shield is replaced (typically nonferrous) with a magnetically conductive shield, so long its split, what is the effect? I happen to have a steel tube that will offer a 1.5x coil diameter spacing between the antenna and the exterior of the shield.It is thin (1/8th inch) steel.Or at least, if not optimal, are there any situations where a steel shield could benefit the antenna, so that a interchangeable shield system is an option. Otherwise I am inclined to toss the tube because its taking up alot of space and I don't really like it too much.With experiments using a multimeter and magnet, I notice it behaves like a sense coil used for mechanics, in the sense that only the magnet traversing infront of the exposed section causes any offset, but does nothing if moved along the side (where it does cause an offset in the bare coil).In really nasty environments maybe?

author: coppercone2
author_rank: Super Contributor
timestamp: November 28, 2019, 08:03:33 am
content: and, when I see the shield designs I see various implementations, one where the core is sticking out (they say to wind the antenna so that the core is bigger then the winding), and one where it only covers the coil, even in the case of electrostatic shields (with plastic endcaps to handle the rest).What is correct?And is the figure of 6dB of peak tuned signal loss for a electrostatic shield used with a rod correct? So I can compare the steel shield to the 6dB figure of a proposed electrostatic shield to see what it does? I can bend an aluminum shield of the same diameter as the steel one and make a bracket for the coil so its consistent. (perhaps just aluminum corregated dryer duct as a test ?forgot the inductance changes, does go down about 25% so you can't really do a swaparoo test without retuning it

author: radiolistener
author_rank: Super Contributor
timestamp: November 28, 2019, 08:44:03 pm
content: Quote from: coppercone2 on November 28, 2019, 01:21:37 amshielding effect depends on many factors.It depends on frequency, field type (near field or far field), shield material and thickness.If you are trying to make shield against near field, you're needs one solution.If you are trying to make shield against far field, you're needs another solution.If you are trying to make shield against another frequency, you're needs third solution.For example, in some cases iron shielding works better than the same shape silver shielding.But for another case silver shielding will works better than iron shielding.So, on the first step you're needs to determine for what frequency you're want to build shielding.Then you're needs to determine what kind of field you're want to attenuate (far field or near field).If you're trying to attenuate near field, you're needs to determine what kind of field needs more attenuation - magnetic or electric.etc.

author: coppercone2
author_rank: Super Contributor
timestamp: November 28, 2019, 09:19:10 pm
content: ok, for a rod antenna on a radio receiver, what have you seen?I was hoping for useful cases for a ham antenna VHF-LF tunable set with what I have available to clear out space. I don't know what a useful spec is for home made radio equipment with no real use. Its for fun. If its a cool capability I keep it, if its not then it will be thrown out. Everything online about loop sticks says a non ferrous shield might be useful in many cases. I have seen nothing about a steel shield.and whats the deal with coverage area, do you need to let the non-wired ferrite be exposed?

author: coppercone2
author_rank: Super Contributor
timestamp: November 28, 2019, 10:03:56 pm
content: better design I think, I am just going to hide it inside of an old spectrum analyzer chassis. its aluminum so I will slit the top, put a dielectric to maintain integrity and mount it upside down and put the thing sidewaysI can also fit the lead acid battery in there and it fits nicely on a shelf. thank you IFRrange will be 100-300KHz and rugged design + it gets rid of ALOT of trash, let the spectrum fall where it mayAnd I can bend a single aluminum bracket + rivet/glue it to the top of the chassis, then put the slit on the bottom of the chassis where the feet are and glue a plastic plane down there to give it rigidity. I want to hit myself every time I go to the attic until a design idea like this comes along

author: coppercone2
author_rank: Super Contributor
timestamp: November 28, 2019, 10:09:49 pm
content: inside the chassis, should a twisted pair be used to carry signal from the LC tank to the reciever? (differential op-amp)

author: floobydust
author_rank: Super Contributor
timestamp: November 28, 2019, 10:58:42 pm
content: I would say you can't use ferromagnetic material for an antenna E-field shield, it must be diamagnetic or paramagnetic, like copper or aluminium. VLF loop antennas are usually wound in copper tubing.I have seen ribbon cable used as a shield wrap or trough around the rod antenna. Or PC board copper clad, or copper or aluminium foil but tight wrapped it adds capacitance to your coil and can upset tuning.Quote from: coppercone2 on November 28, 2019, 10:09:49 pmYes twisted pair is best, it will pickup less interference within the enclosure.

author: coppercone2
author_rank: Super Contributor
timestamp: November 29, 2019, 12:13:08 am
content: how big a gap should I put in the chassis? This was discussed on the forum before, with the answer being 'very small' for a loop antenna for HF, but in pictures I see anywhere between 1cm to 2-3x coil diameter (missing side).I want to just cut a slit in the box with a saw. I want to cut a slit, throw plastic sheet with glue in there, fill the gap with epoxy, paint and call the chassis done. Prefer to have a solid chassis , but I cannot rivet thick plastic... I would like to not drill 16 bolt holes, a big slit will require bolts because glue area will be less.I have to leave the sides facing the coil face plastic? or can the slit go all the way around the box?Also, the slit seems to be on the top most of the time. Is this in case you place the box on a metal surface? Not sure if I should mount brackets on the side so the coil is torquing the brackets, or on top so the coil is just pulling on them with gravity.. I would prefer top mount to solid bit and leave the slit on the bottom so its stronger.*ignore geometric violations its in Klein space or something*in position 2 the feet of the box would be flipped, i guess so a metal surface its resting on does not form a capacitive shunt to close the shield? on a wooden table I would be OK? so long a electronics shield box is not bridging the gap? can the height of the feet negate these at LF?I suppose I should just reinforce it and not gamble on standoffs working if i toss it some where, brackets are made of bent aluminum with plastic extensions

author: coppercone2
author_rank: Super Contributor
timestamp: November 29, 2019, 02:18:40 am
content: or I can turn it upside down, I get stuck with the idea that it will have a unsealed lead acid battery while it can happily run from some alkaline cells

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2019, 03:43:16 am
content: In the same way that an electrically conductive shield must be slitted lengthwise to prevent shorting out the electric field, a magnetically permeable shield must be slittedcrosswiseto prevent shorting out the magnetic field.Use a nonferrous shield and don't worry about it. It's literally a magnetic loop antenna, youcan'tmagnetically shield it.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: November 29, 2019, 04:01:20 am
content: Quote from: T3sl4co1l on November 29, 2019, 03:43:16 amI have a sensor that screws into something (has a bandwidth of 50Khz) to sense fields, it is basically built like a electromagnet. If you put it near a laptops ports while connected to an amplifier you can hear whatever. It's built with a fully ferrous body with a 'nib' poking out maybe 0.5mm. (electro brand)https://sensing.honeywell.com/3010AN-passive-speed-sensorsThis is the reason for my confusion. I wanted to build it the same way, where the ferrite is sticking out (1.5 inches per side), like the nib on the elector probe.Someone (jim williams?) also has some probes that are built in a brass tube, one of them is slit lenghtwise but the other is not (using a figure 8 coil), the one that is not split supposedly is more of a 'pinpoint' probe. (I built both of these, but did not test them too much because I need to make a medium bandwidth RF amp for them), however nonferrous. This one splits the shield only with a ultra fine jewelers saw (you can barely see the gap, as per the document iirc).This made me think it could be made into some kinda 'pinpoint' antenna.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2019, 04:21:29 am
content: Right, near field probes are closed to reduce coupling from a distance.Unless that's what you wanted, but I don't get why you'd call it an "antenna" in that case.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: November 29, 2019, 04:45:12 am
content: so, can it be slit around like a clamshell, or does it need to be a open hotdog?Thats what some CIA document does (one of the few resources which I find).https://www.cia.gov/library/readingroom/docs/CIA-RDP78-03424A000500010007-0.pdf

author: coppercone2
author_rank: Super Contributor
timestamp: November 29, 2019, 04:53:27 am
content: online they haveDoes it work in any different assembly?a) what happens if there is slit sidewalls on the end?b) or if the rod pokes through a hole in the aluminum?c) or if its a wrap with a slit that covers the whole rod, but it has open ends (not a open top like the pictures). ?for me, A is preferred, C is doable, B would have me pissed off because of my bracket arrangement (riveted) and is not attainable 100%, and it would be alot of work with a cutoff wheel and dremel. I made them at right angles to the face, that shit is going to end up being sheetrock grid tape covered with bondois it the case that if the slit is narrow (rather then a missing top), you need the rod to protrude? (thats the conclusion I get from the limited data)I can't find any explanation of this online.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2019, 06:19:10 am
content: Ends don't matter much, again, as long as they are slitted and nonmagnetic.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: November 29, 2019, 06:23:49 am
content: but.. what do they matter from a prospective of hard to measure theory?seems like the radio would be most robust with biggest shield coverage and minimum use of plastic.. simpler design just bend a piece of plastic around a slit cut made with a saw (2 seconds with a metal cutting hand saw).. vs drilling holes etcDoes anyone have examples of ferrite reciever radios that are made for scientific use? (if they exist)

author: coppercone2
author_rank: Super Contributor
timestamp: November 29, 2019, 10:55:35 pm
content: would the air capacitor benefit from being shielded from the coil? I have roomsince its operating at 100-300KHz now, unless I get a bigger capacitor

author: coppercone2
author_rank: Super Contributor
timestamp: December 05, 2019, 09:23:17 am
content: when in its slitted aluminum box, when the coil is measured with a LCR meter the impedance went from 12.7mH to ~10.5mHthe box has a end cap with hinges (nice for batteries) on the side. The hinges will short out the shield on the end cap. Is this OK? There is a gasket isolating the endcap, but since it latches it, its almost like putting a helmet on the side bonded with 4 wires, despite main part of the chassis being cut and strengthened with a plastic plate.Do I need to slice this in half also?annoying to seal since its curved, I would need to put fiber glass (or maybe just cut a buncha dowels up and pour epoxy on it to fill the round), then carefully slit into it with a grinder (m12 die grinder might be nice). But keep in mind, it would be like 2 strips being attached by wire on 1 and and steel dowel pins on other end (hinge), unless I made a conductive gasket (also annoying because the rubber offers nice barrier). So it would be like 'strips' kinda bonded to the chassis, not really solid conductor like the chassis.

author: floobydust
author_rank: Super Contributor
timestamp: December 06, 2019, 06:43:12 am
content: I've only used ferrite rod antennas for VLF lightning detection and pipeline locators.Blitzortunguses ribbon cable for the shield.You can use a foil shield, as long as the foil is insulated so it does not form a (shorted) turn, or leaves a slot. But the extra capacitance can shift tuning or upset differential balance.edit: I can't see a way to have a rod antenna in a metal case. You're still making a shorted turn from end to end, note flux lines make a right angle turn at the end.

author: coppercone2
author_rank: Super Contributor
timestamp: December 06, 2019, 08:57:33 am
content: how is the box different then this? is the diagram just wrong?that's why I am asking about the endcaps. I guess they need to be split then, I did not know how to consider it because its protruding from the box, I guess if its attached to the box but offset from the coil its just a turn with significant inductance/resistance (poor electrical joint)? Vs a split shield which has a pure capacitive coupling to the other side (femtofarads I guess).. and I would prefer metal endcaps with a slit then plastic endcaps if its possibleI don't see why the damn split end caps make a difference unless it has to do with the shield thickness along the depth axis, since along that 1mm despite the split in the middle its like 8cm thick?? in that case you would need a taper so it looks like a sand crawler?? and does the ferrite log need to stick out like a oversized hotdog?But now you are saying brontosaurus bones (ribcage) are the optimal structure? so what would the optimal endcap be, metal concentric split rings? or a metal tapered cone made of concentric split rings? thats not too hard to make out of flat bar aluminum or copper reinforced with fiberglassAny idea how much difference it would make? I want to make 2 chassis now, a standard cheap one and try to make a alternative one with this ribcage design, I can use the original chassis as a mandril to make the new one.or I can slap a buncha fiber glass or plastic in there and then turn it into a rib cage with a die grinder to optimize/fix it? and seal the gap with epoxy or bondo so it looks like a normal box?