"coil" antennas for cheap 433MHz OOK radios - Page 1
Forum section: electronics / rf-microwave

author: gardner
author_rank: Regular Contributor
timestamp: March 28, 2021, 07:54:39 pm
content: I've been using crappy little 433MHz OOK radios for a bunch of projects. For some of them it's been practical to use a tuned dipole with good results. For some instances I need a more compact antenna and have been winding little coils of 0.8mm copper wire. I've been eyeballing the design of these "coil" antennas that appear in the usual ebay/alibaba/banggood/amazon places.I can't seem to find any reliable info on the specific design parameters for this style of antenna. Can anyone refer me to a description of how these are designed? What is the characteristic impedance? Radiation pattern? Polarization?It's probable that if I just knew what the formal name for this style of antenna is, I could find something via google, but I feel a bit stumped so far. Thanks.

author: nfmax
author_rank: Super Contributor
timestamp: March 28, 2021, 08:12:15 pm
content: Me too! I've been copying a design off a module I bought a while back, but I have no idea how good/bad it is, or if the transmitter/oscillator output impedance is detuning it.

author: rfclown
author_rank: Frequent Contributor
timestamp: March 28, 2021, 09:28:10 pm
content: This doesn't answer your question, but it's common to see an antenna made more compact by coiling the wire. The attached pic shows a LINX sleeve dipole with and without cover. (Pay no attention to the red wire soldered to the end to lower the frequency.) The sleeve side of the diplole is the full 1/4 wavelength, but the top end they coil to make the antenna shorter. I think it behaves similar to a straight wire (?). The one in the pic is a 915 MHz antenna.I don't know of any good way to answer beside measuring and comparing performance. I've had good results with sleeve dipoles, so I usually compare against them. I look at S11, and I compare range (RSSI at a distance). Nice thing about the sleeve dipole arrangement is the sleeve balun (if they build that in) which makes it much less sensitive to other stuff (like people) near the antenna.

author: E Kafeman
author_rank: Regular Contributor
timestamp: March 30, 2021, 03:38:35 pm
content: Actual wire is a monopole. That is a dipole but second pole is mirrored in your PCB ground.For best tuned antenna should PCB-length seen from soldering-point of the coil be a quarter wavelength. The coil is nothing else then a way to shorten a monopole length, at cost of reduced antenna performance.These coil antennas are not optimized at all for your needs. Unwind them at you will find a wire in range of 170 mm length.These antennas are always low performing antennas. It will work even better with a by yourself random bent wire cut to 170 mm total length or even better then that, do not bend the wire at all. A straight wire is always to prefer as antenna if space allows it.For even better antenna performance, connect your radio to a half wave dipole, a 2x170 mm long wire tuned for 50 Ohm. For low RF losses can outer braid of a coaxial cable be used as antenna elements in your diy dipole antenna.To tune a dipole antenna, balance it to measure output impedance 50 Ohm, can be simplest done with a reactive network, coils and capacitors placed close to antenna.You need RF tools to be able to measure antenna impedance and that way decide needed matching network topology and values.It is no real improvement to just block braid reflections with ferrite tubes. It is easier to use antenna as it is. Else a such tool to decide needed tuning values could be a nanoVNA, a cheap way to learn more about antenna design and antenna matching.

author: A.Z.
author_rank: Super Contributor
timestamp: April 08, 2021, 05:19:32 pm
content: hi, not sure how it may work on the 70cm band, yet this antennahttps://antennatestlab.com/antenna-examples/example-6-lhcp-miniature-fpv-omnidirectional-antennamay be worth a check, also since the design is open and the parts can be 3D printed

author: E Kafeman
author_rank: Regular Contributor
timestamp: April 08, 2021, 11:26:39 pm
content: That is an antenna designed for circular polarization => reduced performance in combination with linear antenna.As the antenna is designed for 13 times higher frequency (5.7GHz / 433MHz) must size be inverse scaled to fit 433 MHz resulting in an diameter of almost 300 mm.

author: A.Z.
author_rank: Super Contributor
timestamp: April 09, 2021, 09:11:22 am
content: Quote from: gardner on March 28, 2021, 07:54:39 pmSearch for "helical antenna" or "spring coil antenna" and you should be able to find some further informations, for example here are a couple of links resulting from a quick searchhttps://github.com/OpenHR20/OpenHR20/wiki/2.1)--433-MHz-and-868-MHz--Antenna-Design-Exampleshttps://www.instructables.com/433-MHz-Coil-loaded-antenna/notice though that such an antenna, while very compact, won't be so efficient, so if you'll need better efficiency and can afford a longer antenna, I believe that a collinear antenna built using some pieces of coax may be a good pick, here's an example, but you can find morehttp://www.rason.org/Projects/collant/collant.htmnotice that for permanent installation the above antenna could be enclosed in a PVC pipe with caps and connectors, the pipe will both support and protect the antenna and will also ease placing it on some supportHTH

author: E Kafeman
author_rank: Regular Contributor
timestamp: April 09, 2021, 09:08:03 pm
content: Quote from: A.Z. on April 09, 2021, 09:11:22 amIt may seem a bit picky from your view but it is absolutely not a helical antenna, it is wrong name of that antenna part.OP asks for definition of a monopole-part of an antenna that may have a helical winding but it does not make it an helical antenna which is something rather different:https://en.wikipedia.org/wiki/Helical_antennaI have already given correct technical description above. It is a very simple antenna structure. Please read and try to understand instead to convert wrong guesses to statements.Quote from: A.Z.It could maybe be interesting to know that linked collinear antenna is less effective then a typical simple dipole antenna.Guess it is due a confusion about basic antenna parameters. It is rather simple to understand what efficiency is, if it is about antennas.Compare with a handheld flashlight:Its efficiency is how much of consumed battery power that is converted to visible light and efficiency losses are mainly heat losses.Efficiency for an antenna is typical given as a value from zero up to 100%, if half of its feed power is radiated into space is efficiency 50%.Flashlight directivity gain is how well lenses focus light beam to a concentrated and narrow beam.For an antenna is its directivity typical measured relative another well known radiation shape, either dBi wherestands for isotropic antenna or dBd which is directivity gain relative an ideal dipole antenna radiation shape in fare field.Just setting for example 9dB as antenna gain is 9dB or 2.8 times relative an unspecified reference, just as wrong as entering a length of 9dB without specify what length unit that is used.Again, very picky but if correct information is of value is linked antenna not acollinear antenna.It is probably intended to be acollinear antennaarraywith unspecified top element.Assumed gain at this webpage is specified to 9dB... of what? That is probably not a well understood parameter by author of this page.It is an amateur who have posted this design. Antenna design and antenna description is likely copied several times between different websites. None have tried to build or verify promised result so the false claims are undetected.A blind recommends another blind.

author: gardner
author_rank: Regular Contributor
timestamp: April 09, 2021, 11:06:24 pm
content: Gah! I just typed up a long reply and then got a "bad gateway" on submit and lost it all.So some additional research reveals that this is a species of coil shortened monopole often referred to as a "rubber duck". These are generally a spring or coil inside a rubber tube. The polarization is linear along the axis of the coil and the radiation pattern is omnidirectional, except for nulls at the axis of the coil.Experts seem to be unanimous that the performance is terrible, exceeding only other similar sized antennas and dummy loads.There seems not to be much guidance on design. The main design methodology seems to be to make a coil, try it, and then cut little bits off until it works "best". "Best" can mean optimal impedance matching or optimal resonance, but it is very unlikely both will be achieved simultaneously.

author: E Kafeman
author_rank: Regular Contributor
timestamp: April 10, 2021, 09:46:04 am
content: >often referred to as a "rubber duck".No, it is not a rubber duck.Actual structure is a very well known structure.It is a part of an antenna called monopole.It is in this case shped as an coil but other shapes are ok too.Wire length is typical around 0.25 lambda.Important, I will not write it in this tread again => IT IS A MONOPOLE ANTENNA STRUCTURE <=Rubber Duck or stubby is not a name of an antenna structure, it is usually a description of a protecting cover, often hiding a monopole element or a dipole antenna.>Experts seem to be unanimous that the performance is terribleIt is rather easy to estimate its maximum possible efficiency as a result of its structure length compared to a full length 0.5 lambda dipole.Not much of an expert needed for that.>There seems not to be much guidance on design.It is too simple design job if you have basic antenna knowledge enough to recognized it as a simple monopole. If guidance is needed is it probably better to find a simpler antenna structure such as a full size dipole, which is easier to just refer to frequency for calculating antenna length dimensions, resulting in at least decent performance.As this is a monopole must also ground plane size be taken in account asequal importantpart of the design calculation.Its size is just as important as monopole wire length. These thing makes it a bit too complex to give correct advice for someone that not know anything about antennas, just want its dimension from a table.Leaving design parameters related to own PCB ground and own designed coil calculations to someone with a ruler as only antenna measurement tool and maybe with limited antenna knowledge is just impossible.My best guidance advise is to read real schoolbooks about these kind of antennas, to learn how to design it with correct parameters. It is what I have done.A table with ten rows at a webpage written by someone that neither know anything about antennas is not an good advise in this case.It is much simpler to design a dipole or full size monopole, then a coil-shaped monopole if both knowledge and qualified RF tools are lacking.These coil-shapes sold as antennas are sold more as an assumption that it will result in some radiation, good enough to give few meter coverage for an unknown gadget with unknown groundplane. It is cheap and simple with a more or less bent wire , at cost of less good performance.This antenna structure belongs to the group "linear polarized antennas". it it is described in all serious antenna books.It is basically a dipole (2 monopoles). One monopole is represented as an structure and the other monopole is virtually mirrored in PCB ground plane.If monopole is coil-shaped, for a part or whole of monopole length, does not change anything, it is just a monopole with a more or less complex shape.Wire can be bent in any direction and shape but antenna measured result will still be much the same.Classic way to reduce monopole length even more without change its resonance frequency too much is with bottom coil and top cap but a such cap will make antenna structure more complex and by then also more expensive to produce.Antenna resonance is when reactive load is 0 Ohm.A straight wire of 0.25 lambda length on an infinite groundplane will have an impedance of 36+j21 Ohm.It is not a 50+j0 Ohm but good enough in many cases, especially without measurement tools.Most monopole structures do needs some kind of matching to perform as good as possible.Impedance matching can be done by adjusting its shape or move feeding point.IFA is probably the most well known example of struture where feeding point is moved for best match.IFA do not need to consist of a straight wire, it can as well be shaped as a coil or whatever but too complex shapes and it can be hard to find a suiting feeding point which gives a reasonable VSWR. Total wire length also for IFA is 0.25 lambda, but now counted from ground-point to open end point.A maybe more common tuning alternative is to add stub tuning or tuning with reactive components, inductors and capacitors.Such tuning require a VNA and a lot of measurement technology to get correct implemented matching network.For real cheap electronic gadgets is mismatch and less optimal performance less important factors, low cost is more important and with some luck is antenna performance good enough even if its signal strength is maybe 20 db below what a good antenna can result in.>There seems not to be much guidance on designNo it is too simple design job if you have basic antenna knowledge so that wire shape is recognized as a simple monopole.As it is a monopole must also ground plane size be taken in account as a part of design calculation, as its size is just as important as monopole wire length.If ground size of own PCB is less then 0.25 lambda will it limit antenna performance just as when monopole structure is short.Even if groundplane is reasonable big will it not result in great antenna performance with such antenna structure.I have a lot of these antennas saved in my junk storage so it is no problem to show real world performance when placed on a big PCB.I placed a coiled wire in the middle of a PCB with size 300x300 mm.Connected a calibrated VNA. Using ferrite tubes around measurement cable to avoid extending ground.The structure was bought as a 433MHz antenna, have not winded it my self so it is pretty much a typical cheap antenna. Some are thicker and some are thinner, but it is pretyy much same performance and performance limitations.This antenna behaves better due to the big groundplane, which is bigger then for the typical 433MHz gadget but it makes it easier if someone will repeat same measurement.Photo of the measurement setup:Measured impedance and VSWR:This arrangement will radiate but with efficiency maybe as low as 2-3% and even less on a small PCB.It is however easy to at least compensate for the poor impedance matching which in this case will increase antenna performance with around 10 dB.Applied a tuning network, a L-network consisting of two coils from Murata and type LQPO02T. Else is nothing changed.As can be seen did it improve VSWR. Efficiency did increase to around 25%.As a reference value, a good designed dipole can have an efficiency of more then 90%. 100% is not possible.Several tries now, using both Edge and Firefox, jpg and png, but attached images refuse to be placed within text.This antenna type can also be a kind of tuned by just adjusting PCB size or adjusting coaxial cable length between antenna and PCB.A simple example. In this case is it a PCB with a size of 100x70 mm and at middle of this PCB, at 50 mm is coaxial cable soldered. Coaxial cable have a length of 80 mm until coil structure is soldered at coaxial center wire.This is a kind of stub matching and dimensions and resulting VSWR are specific for just this situation. A PCB with another PCB size and less ideal ground plane can change result a lot. Compare for example difference with infinite PCB size as above.Measured PCB:All is calibrated, port forwarding is correct and a big ferrit is used to avoid extending ground plane with measurement cable.PCB is placed rather isolated from surrounding, resting on plastic foam. It looks like coaxial cable not is soldered at PCB, but it is a minor blob at shaded side.Measured result:This is a very ideal VSWR. but without a VNA is it very hard to find correct tuning length. Shape of monopole and PCB size and losses in ground will all affect result. As PCB size and PCB ground losses only are known by PCB designer is it a bit complex to give advice how an antenna is best designed.Above was two examples, two different groundplanes and two different antenna placements (center, edge of PCB) where both was using same monopole structure. The very different measured result shows how important ground structure is for resulting antenna performance.Wanting to learn more about this or if you want to learn how to get good antenna performance can a nano-VNA be a good first start but if intention is to work effective and professional is more expensive tools needed.For these measurements have I used a Copper Mountain R60. It is a single port VNA, rather cheap ($50k).A for me important factor is that it is a small and noiseless VNA and takes minimal place at lab-bench.I also have a few old classic VNA's such as Agilent and HP 8752,8753,8719. Old but still very quality tools. It is shown in that they still, 20 years old, seldom are sold cheap even if broken.