telescoping variable dipole/bicone antenna? - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: December 11, 2020, 06:48:55 am
content: the idea came to my mind to take two monopole telescoping standard radio antennas and put the back to back to make a telescoping monopole, perhaps with putting scribe lines (a rule) on the antenna to make it set well. I see them for sale.Are there guidelines for what a good telescoping antenna is? I figure to lubricate with deoxit but I wonder what kind of specifications can be looked at (i.e. resistance variation depending on extension).Just how precision machined are the telescoping antennas for sale? What are good parts to make them? What techniques are used to make them fit well? Tools for making them? (some kind of swager or something). I thought about a more robust one with set screws or some kind of collets.

author: A.Z.
author_rank: Super Contributor
timestamp: December 11, 2020, 09:13:26 am
content: Quote from: coppercone2 on December 11, 2020, 06:48:55 amForgive me but it's unclear (to me) if you want to buy the telescopic whips or to build them; given that they can be found for low prices, I think you may consider the idea of buying them; as for the configuration, an idea may be using two telescopic whips to build a "delta loop", in such a case, the two whips will be put in an "V" shape with the feedpoint at bottom and the tops of the whips will be connected using a piece of wire of proper length so closing them to form a loop, place a small transformer at the feedpoint and you'll have a small loop antenna which could be easily assembled/disassembled and used wherever you need it

author: radiolistener
author_rank: Super Contributor
timestamp: December 11, 2020, 11:09:40 am
content: Something like that?https://www.aliexpress.com/item/4001350143742.htmlNote: I suspect this DIY KIT lack of balun and RF choke.

author: A.Z.
author_rank: Super Contributor
timestamp: December 11, 2020, 11:17:06 am
content: Quote from: radiolistener on December 11, 2020, 11:09:40 amWell, almost, that's a dipole, what I suggested was adding a wire between the top ends of each whip forming an arm of the dipole to connect them together so turning the dipole into a "delta loop", then by the way we'll probably want to also add a balun to the feedpoint to mantain the radiation pattern; a simple 2:1 balun should probably do, the resulting antenna would have a triangle shape with the base at the top, the base will be the wire connecting the two sides (whips) together to create the loop, the feedpoint will be at the apex which will be at bottom

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: December 11, 2020, 01:35:48 pm
content: A video describing a portable telescopic dipole for 28 & 50 MHz.Use would also be possible on 144 MHz if the coil is removed and the whips shortened.

author: A.Z.
author_rank: Super Contributor
timestamp: December 11, 2020, 01:49:03 pm
content: Uh... so the idea is about a TX antenna ? I understood it was about an RX only one.

author: coppercone2
author_rank: Super Contributor
timestamp: December 11, 2020, 08:40:58 pm
content: I mean they are both interesting and I wonder what you can do to improve value and quality in either vs the purchased cheap ones and expensive ones with the key being bandwidth.What is interesting is if you make it really wide band it geometrically seems to resemble a bicone with the cones inverted (many many sections). Pyramid dipole. At least if you do it mechanically sound.If you do it mechanically unsound (say you use titanium that can handle it? with the stem first, rather then a thick section, I am really impressed with how strong titanium tubing is) you can make some kinda wrong impedance telescoping bicone thing (to keep the impedance near 50 for some fixed cone size, the sections would need to be incredibly wide if you keep the normal span of a telescoping antenna in mind).However I do wonder if you can do it by welding thin sheet metal into extra wide tubes/pipes.I mean like 100 nested sections per pole. I wonder how well that could be made.I have a good amount of stainless flashing, but I am not sure how to go about it, I could weld it together into circles and grind the weld seam flat, to make some kind of circle, but I doubt I could get them to nest well. Maybe I am wrong.. gas might work if you clamp them completely, at least if the geometry is kept large, and maintain the gap spacing with heavy clamping, by making sure they are cut square and spaced with a shim. Stainless does not seem to be that hard to weld in thicker sections and I figured out how to polish it nicely.btw that thing looks hilarious

author: coppercone2
author_rank: Super Contributor
timestamp: December 13, 2020, 09:19:36 am
content: fixturing is not easy for this, I easily get creases.

author: cdev
author_rank: Super Contributor
timestamp: December 15, 2020, 03:04:37 am
content: I have had several "rabbit ears" for decades that originated on TV sets. None of them can be made short enough to resonate at 70 cm but adjusted to the right length they can resonate at lower frequencies. one of them at its shortest length would likely make a good 220 MHz dipole antenna. Cleaning with IPA is about as far as Ive gone with cleaning. I also have tried to use rabbit ears to make a loop antenna (using a long, big pair of alligator clips between them and again this sort of worked, but I think you make a very good point about the inherent resistance of telescoping antennas not being so good. That's probably why - although it worked as a tuned loop with a variable capacitor for receiving, it was by no means a good one.I'd suspect there to be a lot of variability in telescoping antennas for any kind of critical use.If you want to make a biconical, (better pattern than a discone) the critical central piece they presumably would be attached to in quantity you'd either have to make it yourself or have custom machined, or repurpose some off the shelf item.I think you could likely do far better on performance as far as omnidirectional broadband antennas with gain out to the horizon than biconicals made the classic way with 16 telescoping antennas.Planar antennas in the "Planar disc" type work surprisingly well as biconical antenna substitutes, but get big quickly if you want low frequency coverage. But they can lie flat and arent too ugly, and aluminum foil is cheap and works for them. But getting the feed right and making them be quiet isnt as easy as one would think. feeding them at the middle with a balun and running the feedline off (sideways) at a 90 degree angle has (predictably) given me a quieter antenna but one with less of a top end response.Quote from: coppercone2 on December 11, 2020, 06:48:55 amYou know, there are all sorts of interesting rollable and foldable antennas. Some fairly well documented, others practically not at all.

author: coppercone2
author_rank: Super Contributor
timestamp: December 15, 2020, 11:38:46 am
content: fordable maybe, roll able is just compact with I assume bad variabilityWhat is the specification for a telescoping tube?

author: cdev
author_rank: Super Contributor
timestamp: December 16, 2020, 03:45:25 am
content: fed at the middle or at which end of the tube, thin or thick?Have you ever used a "planar disc" VHF/UHF antenna?

author: coppercone2
author_rank: Super Contributor
timestamp: December 17, 2020, 09:27:30 pm
content: I think a good one might be made by telescoping tubes that are covered with solder fingers soldered on to reduce mechanical design issues and bring performance up to microwave regions. Linear bearings can be glued on also to a tight fit with shims.I am imagining a cast iron drain pipe base with four elbows with a telescoping PVC mast. The base would double as a carry handle and weight while staying rugged (filling PVC with cement might also work but its flimsy and susceptible to positioning damage).I wonder how well it would perform if you keep the spec within RF finger deformation specification.It may be possible to make a cool design with no real machining and only parts from digikey and a hardware store.

author: cdev
author_rank: Super Contributor
timestamp: December 17, 2020, 10:02:03 pm
content: Thin wire or fine metal mesh and make some structure that could live inside a radome? Somebody on YT made a biconical using conductive glue for the two points feed area, and clear plastic to make the rigid enclosure it lived in so it would not flex. It looked a lot like a large hour glass, and had roughly the same proportions. He said it was the equivalent of a very pricey commercial EMC antenna.I dont understand what you are trying to sketch out, maybe if you draw it. Note that I don't have any EM simulation software which is what you would probably want for answering anything more than what is available in the literature.There are a lot of planar antennas which are easy to make and perform well. They are defined totally by their geometry and not at all picky about measurements as long as you get the angles right. They are the definition of broadband. Ive built a couple of them.User "superkuh" on Reddit has a web site where he goes into detail on the planar disk he uses for his continuous spectrum monitoring scripts.These broadband antennas are most useful for monitoring broadband noise over time. I'd be very surprised if the overall noise wasn't rising fairly rapidly.I don't understand exactly what you mean below, what it would look like.Quote from: coppercone2 on December 17, 2020, 09:27:30 pm

author: fourfathom
author_rank: Super Contributor
timestamp: December 18, 2020, 12:05:31 am
content: EMC (Electromagnetic Compliance) testing uses adjustable telescoping dipole antennas (and biconicals, and log-periodics), and the dipoles I've used didn't appear to have any special treatment of the telescoping sections. These standard antennas use a "W. K. Roberts" balun for matching and balancing, but I don't see why you couldn't use a ferrite balun for non critical purposes.Here's an example:https://www.ahsystems.com/catalog/FCC-1.phpWe used a tape measure marked in MHz to set the antenna length.

author: coppercone2
author_rank: Super Contributor
timestamp: December 18, 2020, 12:07:52 am
content: i also wonder if you can freeze stuff inside of tubing to expand it and nest themand if you can make a variable loop torroid out of dryer duct (well it would change shapes and be more like a expanding rectangle)

author: cdev
author_rank: Super Contributor
timestamp: December 18, 2020, 02:03:23 am
content: One definitely could use dryer duct to make a different kind of antenna.. a low band loop..it would be conductive, wide (which is how you want it because of skin effect) and very close to an optimal shape for a "larger than life" (electrically) loop antenna.It would be interesting to give that a shot. Its like another antenna Ive seen that was made with copper tape and pool floats.It needs a capacitor for tuning. The discussion elsewhere on tuned and untuned loops is all relevant to this variation.Suppose it works well, it would need a way to support itself that would add little loss, but lots of support and enough rigidity to keep it from moving too much.Otherwise if its outdoors the wind would destroy it. Also the wind would detune it if it moved around a lot making the resonant frequency and swr change, possibly a lot which would not be good. If you were transmitting it could easily lead to a disaster. Has to stay tuned.If that could be overcome, it might be a very good antenna because the wideness of the tape like aluminum strip might make the loop have very low rf resistance (how does it compare to litz wire, though?) It would seem to me it would have low inter turn capacitance, meaning higher srf frequency, right? and higher upper limit . The width would mean HIGH EFFICIENCY and the coil within the coil (especially if it was a big loop would mean possibly could achieve a much lower frequency than a similar sized plain loop possibly very low band, for its diameter. Does that sound right?Quote from: coppercone2 on December 18, 2020, 12:07:52 am

author: cdev
author_rank: Super Contributor
timestamp: December 18, 2020, 02:15:39 am
content: Do you know what kind of balun they used? i.e. design of balun used. You could use a ferrite binocuar core for a wide range of frequencies and the response could be quite flat. for those frequencies asfar as I know thats what works best. A very small ferrite binocular core, maybe #61 material #43 would work too. Not many turns. 1:1 impedance ratio would be best to get the flattest response. A mini circuits part I have would be perfect, its smaller and flatter than any of us could ever make one. Soldering it directly to a piece of coax or an SMA with very short leads would be really hard though, so need to make a small PCB for it or maybe could use stripboard. That would probably work fine if the frequency is not super high. An SMA mates well with a little piece of stripboard.They give out samples.Quote from: fourfathom on December 18, 2020, 12:05:31 am

author: coppercone2
author_rank: Super Contributor
timestamp: December 18, 2020, 02:36:03 am
content: ok you can do a few things to make this better possibly or worse1) coat the dryer duct with chromate conversion coating (high conductivity bonderite).. it would make it so when it bends there is no high impedance aluminum oxide blocking the path. You might be able to just dip it in a pot of bonderite.2) cut a hole in top, put a mesh over it and fill it with air like a modern halloween decoration to stiffen in. It might stiffen it a little. Otherwise you would need a telescoping structure with zip ties or something that can vary the shape of a rectangle (maybe 2 axis is enough) The idea is you put excess folded material in the middle of the rectangle sides and leave the bend the same then when you extend it you leave the bend radius the same but you extend the sides. Otherwise you bond it to a cross that telescopes. You can make the corner bends and pour a bunch of glue/fiberglass/carbon fiber over it to make strict bend radius with springs in the middle but obviously it would be a bastard to fix. Lashing to a curved structure might be the most repairable way.3) some kind of iris structure (can be made of wood or fiberglass) that literary varies a circle, harder to make but optimal shape. A bunch of cams on curved pieces. The complexity and readjustability of this antenna might be the best with precision parts compared to the other two because theoretically I can see it 'unfurling' itself if its pulled by cams that expand like a camera iris. The nice part is that all the parts can be flat so there is no need for 3 dimensional fitting structures only planar fit (a simple milling machine).I can see a big prototype being made easily with telescoping fiberglass brush roller sticks from home depot.It would need to be taken down to be shrunk though, because there is no way that stuff is easily collapsing again, expanding I think yes.

author: cdev
author_rank: Super Contributor
timestamp: December 18, 2020, 03:12:51 am
content: I think we are describing completely different antennas!Thats okay, though.

author: fourfathom
author_rank: Super Contributor
timestamp: December 18, 2020, 03:31:43 am
content: Quote from: cdev on December 18, 2020, 02:15:39 amHere's a link to a description the Roberts balun:http://www.glendash.com/Archives/Roberts%20-%20FCC%20Antennas.pdfThis has been enshrined in the various standards for measurement antennas, and is quite old. It is a coax-section balun and probably has a lower loss than a ferrite core. It also does the step-down from the 70-Ohm antenna impedance to the 50-Ohm coax. As long as you're not submitting measurements to the FCC, I see no reason not to just use a ferrite-core sleeve balun, or perhaps a ferrite-core transformer-style balun. You should be able to get similar (or better?) frequency response with the ferrites. For a receiving antenna you can use small ferrites. For transmitting you need bigger ferrites to avoid core saturation.

author: coppercone2
author_rank: Super Contributor
timestamp: December 18, 2020, 03:45:10 am
content: Quote from: cdev on December 18, 2020, 03:12:51 amthere is alot of options with expanding materials the other antenna I don't feel like drawing to explain its mechanically complicated, maybe later

author: cdev
author_rank: Super Contributor
timestamp: December 18, 2020, 01:30:19 pm
content: The bandwidth of the Roberts balun seems surprisingly wide for what looks like it would be a more frequency-sensitive balun at first look. (to me, right now)They will only accept measurements at the antenna's adjusted frequency, right? (within a range) Then there is a calibration curve that's applied in post processing? Is that how it works?

author: fourfathom
author_rank: Super Contributor
timestamp: December 18, 2020, 05:02:43 pm
content: Quote from: cdev on December 18, 2020, 01:30:19 pmThe measuring dipoles are adjusted in length for one specific frequency at a time, and a measurement is made. I think we might have applied a correction from a calibration curve, but honestly, it's been at least 40 years since I made those measurements myself (FCC type approvals and EMI mitigation). In practice, we would do a wideband scan with an untuned antenna first (or a broadband biconical), looking for likely offending signals, and then zero in on those using the tuned dipole. Above the frequency range of the various dipoles we would use log-periodic or biconical antennas. We would also factor in cable losses and if we used a preamp the gain curve of that as well.We also did swept measurements using calibrated biconicals to show that unintended emissions were below the compliance template levels, but I used a lab for that and I don't remember if/where any antenna calibration curve was applied.But I think you are asking if the dipole is used as a wideband antenna. It is not, it has to be adjusted in length for the frequency of each signal being measured.