ultra fat dipole using steel drums? - Page 1
Forum section: electronics / rf-microwave

author: CopperCone
author_rank: Super Contributor
timestamp: June 14, 2017, 05:55:05 am
content: Could one make a ultra fat dipole using 55 gallon (or other large size) steel drums? Or do parasitic effects start messing with really fat dipoles (I can get them cheap).Perhaps one can weld something cheap out of scrap yard stuff (diesel tanks on trucks come to mind).or steel garbage cans with the lids welded on.You could drill a bunch of holes in them using a magnetic drill for increased wind immunity I think.Also, I wonder if a water tower held up on wood can be turned into some kind of desicone-like antenna.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 14, 2017, 07:03:39 am
content: I'ved seen wirecage dipoles of similar dimensions on pictures of old Soviet radar installations, so I guess the 'fat dipole' has been implemented before 😀 my biggest concern with drums is the heavy paint coating, but I dunno.

author: CopperCone
author_rank: Super Contributor
timestamp: June 14, 2017, 07:13:35 am
content: Perhaps the paint remover from home depot that is apple sauce like in consistency would work on this paint?This really seems like something that can be slapped right together.Do you have an example of this soviet radar?

author: CJay
author_rank: Super Contributor
timestamp: June 14, 2017, 09:45:05 am
content: Quote from: ChristofferB on June 14, 2017, 07:03:39 amI'm genuinely curious what concerns you'd have about the paint?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 15, 2017, 06:53:26 am
content: Quote from: CJay on June 14, 2017, 09:45:05 amI'm not sure. I just remember being told one shouldn't just use any paint on antennas, but that may well be ham superstition. The only reason I can think of is metal oxides in the paint acting as an RF absorbant, but that's pure guesswork.

author: German_EE
author_rank: Super Contributor
timestamp: June 15, 2017, 07:06:52 am
content: Oil drums? kids stuffBack in the 1970's I remember seeing in QST a guy who parked his station wagon and his wife's identical vehicle nose to nose. He then connected them to his ATU as a dipole and worked on the 80m band.

author: CJay
author_rank: Super Contributor
timestamp: June 15, 2017, 07:14:29 am
content: Quote from: ChristofferB on June 15, 2017, 06:53:26 amI've never been able to track down anything other than what could be called "old wive's tales" about antennae and insulating coatings, I guess a bulk insulating paint loaded with metal oxides or even particles of metal could cause an issue (stealth aircraft are rumoured to use paints loaded with ferrite for instance) but I and many many other operators have used insulated wires for antennae for years so it seems illogical that a coat of 'normal' paint would have some negative effect.I'd love to know the science behind it and see if it is a factor that needs to be taken into consideration.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 15, 2017, 08:53:32 am
content: Might be better used for one hell of a LW helical resonator, than an antenna element... it's an awful lot of metal for not a lot of electromagnetic value.Or cut it up into strips, and use those for a conventional cage style dipole, I suppose.If you want wideband, you need a tapered (cone or exponential horn) sort of structure. You can approximate a cone with a stepped structure, but then you need a geometric series of paint cans, and you still suffer from the response basically being the Fourier transform of that structure's deviations from ideal (i.e., a stepped waveform --> peaks and dips at frequencies the electrical length of the sub-elements).If you're okay with peaks and dips, you could use such a shape for essentially end-loading of a thin dipole, which will get you a nice strong, moderately narrow peak at the main resonance, and some blips and bobs at higher frequencies (with, I think, probably a strong peak at 2 wavelengths, if the end loads look reasonably "ground"-ly by then). As with any wire dipole, inbetween frequencies can be tuned to some degree of success, while nulls (anti-resonant) remain impractical to tune.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 15, 2017, 09:08:42 am
content: Quote from: CJay on June 15, 2017, 07:14:29 am"Loaded" materials will of course cause problems. Imagine looping some ferrite beads around the active element of a resonant wire dipole.Position matters: the closer the beads are to the feedpoint, the higher the current, and therefore the stronger the damping effect of the beads. Obviously(?), they have no effect right at the tips; actually, that wouldn't be true, they would have a slight end-loading effect, because the dielectric constant is fairly high and they act to make the wire ends a little fatter. Which might be considered an advantage instead!You get the same effect from any "loaded" coating, whether it's EMI tape*, ferrite beads, stealth coating or more. (Relevant product:http://www.surplussales.com/Wire-Cable/Wire7.htmlno idea where they got the stuff from, if you can find a place that sells it, new; it seems pretty boutique!)*You can get plastic sheets that are basically 'fridge magnets, but filled with soft ferrite instead of hard (permanent magnet) material. Permeability of 10 or more is available, not bad at all. It's pretty effective in the GHz, say to line a shield box, or stick over some offending ICs.Dielectric on the surface, has an effect identical to coating a fiber-optic (a waveguide) with a material having a higher index of refraction, thus causing total internal reflection. This is relevant if the coating thickness is ~1/8 wavelength or more. For very small amounts, it's nearly irrelevant**, and the most noticeable effect will simply be the line effectively being thicker, because the dielectric is more permittive than air.**However,Goubau linesdon't need much, apparently. Note that a lossy conductor effectively has a large imaginary component to its dielectric constant and/or permeability: just as inductance or capacitance is complex resistance, so too, complex capacitance or inductance is resistance again. Waves necessarily travel slower along wires for this reason, even if the penetration depth (into the wire, i.e., skin depth) is small, so this effect does not depend critically on the presence of a dielectric, or that it be very thick with respect to wavelength.So, that is to say, the same effect, just to different degrees.Tim

author: CJay
author_rank: Super Contributor
timestamp: June 15, 2017, 09:32:00 am
content: Quote from: T3sl4co1l on June 15, 2017, 09:08:42 amLots of interesting information snipped and there's some reading to do on my side so thank you (that reading list just gets longer all the time, I like it), what I read so far from your reply is that an insulating layer of any sort that hasn't been loaded with magnetic or conductive particles isn't an issue unless you're up in the mm bands or it's unfeasibly thick?

author: Codebird
author_rank: Regular Contributor
timestamp: June 15, 2017, 09:50:45 am
content: It would be really, really heavy and difficult to support above ground. That's why the cage dipole was invented. The problems may be structural rather than electromagnetic.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 15, 2017, 08:11:10 pm
content: Remembered where I saw the 'fat' wire-cage dipole: The famous Duga-3 OTH array 'round Chernobyl:And here's a pic of the feed:

author: Cerebus
author_rank: Super Contributor
timestamp: June 15, 2017, 10:56:10 pm
content: Quote from: CopperCone on June 14, 2017, 05:55:05 amA gentle reminder that welding anything that used to contain flammables canreallyruin your day.

author: CopperCone
author_rank: Super Contributor
timestamp: June 15, 2017, 11:19:42 pm
content: I'm surprised they don't use that to scan for b2'show abandoned is that place? can you go there with a pickup to poach bicones? or would putin show up on a horse when you started removing them?i'd sell those on ebay, just get a rope rig and a battery angle grinder

author: Bud
author_rank: Super Contributor
timestamp: June 16, 2017, 12:18:26 am
content: You obviously do not know what place Chernobyl is.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 16, 2017, 06:55:19 am
content: Apparently, there was a big fire in the transmitter after chernobyl, ander that's why it was abandoned. It's still in the exclusion zone, and everything is irradiated, so poaching is probably the worst idea I can think off.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 16, 2017, 01:31:08 pm
content: Quote from: CJay on June 15, 2017, 07:14:29 amMF AM Radiators are made of structural steel & are routinely painted for weather protection, with no effect upon their efficiency.The dipole arrays on older VHF TV installations were also painted with the same type of paint , but from what I've seen, they don't do that any more.

author: Cerebus
author_rank: Super Contributor
timestamp: June 16, 2017, 02:35:12 pm
content: Quote from: CJay on June 15, 2017, 07:14:29 amAs you suggest, the concerns about the pigments used in paints could be realistic; I certainly wouldn't want to use a paint full of iron oxide, or aluminium flake without proper evaluation first. But the concern only stretches to conductive, ferromagnetic, paramagnetic, or strongly diamagnetic substances.As far as the paint bases themselves go, most, if not all, are going to be dielectrics. Seeing as an antenna is already surrounded by a potentially infinitely thick 'coat' of air, with a dielectric constant of 1, and they also have to deal with coatings and sprays of water (dielectric constant 80), then I think a thin coat of paint, that most probably has a dielectric constant of 4, is going to make very little difference.

author: AG6LE
author_rank: Newbie
timestamp: June 19, 2017, 12:10:09 pm
content: Two radio clubs I am in gained access to the roof of a building on the north shore of Sydney, and had to powder-coat both the support poles and the folded dipole antennas in a dustygreen, except clamping points, to blend with the building. No problems, as far as I can tell. Bands are 2m, 70 cm, and perhaps some non-ham stuff, and 23 cm, although I am not sure if they have been treated.

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: July 03, 2017, 04:45:04 pm
content: In my experience, there aren't a lot of surviving "wives tales" in amateur radio these days. As physics have become better understood, technical knowledge more widely distributed and testing methodologies have become more accessible, much of the lore of the old days has died off.Paint on the drums should be analogous to insulation on a radiating wire. In that analogy, the insulation has a small effect on the net velocity factor of the radiator, moving the resonant frequency slightly. You would simply adjust the overall length of the drum antenna to compensate if you're trying to hit a specific center frequency. With wire antennas, we simply build it slightly long (maybe 3% to 5%), test and trim to resonance.This should be relatively easy to simulate using any of the many free (and not-so-free) antenna modeling tools out there such as EZNEC, 4nec2, etc. That'll get you close enough to decide whether it's worth building.It's an intriguing idea. There is a lot of information out there and it should be easy to get an approximate answer through simulation or through existing literature.This is hastily copied from the ARRL Handbook, 2014 edition, pp 21.3 - 21.4. Sorry the formulas & figure don't come out:Quote