1/4 wave spiral, vs. 1/4 wave straight, vs. 1/2wave straight etc - Page 1
Forum section: electronics / rf-microwave

author: Jester
author_rank: Frequent Contributor
timestamp: February 02, 2022, 02:40:39 pm
content: I'm using a pair of HC-12 modules to send a few bytes of data and I'm now trying to maximize the distance between modules. The HC-12 is operating at 433MHz and set to 100mW. Best so far is about 300m with the supplied spiral antenna in my test site mix of space and thin forest.I'm planning on putting this in a small 3" x 3" plastic enclosure so no ground plane to speak of. Perhaps the power supply wire might provide an inkling of a ground plane?I'm wondering how much effect I would get if I replaced the 1/4 wave spiral with any of the following:1) a straight 165mm wire (1/4 wave)2) a straight 329mm wire (1/2 wave)3) a Dipole with 2x 165mmAny wild guesses on any improvement I might see?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2022, 05:37:39 pm
content: If that's everything in the picture, module and element, no wires... well, I guess there must be a battery and cable or something, and whether it's also connected to anything else via, whatever, serial maybe, also matters.Well, say it was by itself. The antenna element is a single conductor, a slow-wave structure. The board acts as a top hat (load) on one end, and the rest of the element acts as the bottom load. It's an asymmetrical system, not a clean dipole, but it's also electrically short (432MHz ~ 17cm quarter wave) so this doesn't really affect the radiation pattern, it's a basic dipole. The small size means it's resonant in a narrow band, and there will be some losses associated with the element, having somewhat higher insertion loss than an ideal one. It will be linearly polarized, in the plane defined by the symmetry of the board-element system (which again, is somewhat asymmetrical, so, not quite at the base of the element, probably).Assuming that antenna is well matched to the radio, and that ERP (effective radiated power) is the limiting factor, and that there's no other means of improvement (e.g. trading bandwidth for range), then there are only two ways to improve reception:1. Reduce antenna losses2. Increase antenna gain#1 can be done by using more conductive materials: a silver-plated coil perhaps. A structure can be used, that is less critical of element resistance -- mainly a larger antenna, so that resonant Q is lower, i.e. less reactive power is flowing, wasting less power on circulating currents.Likely, the improvement is small; I think I'd be surprised if that thing has more than 3dB insertion loss? But I'm honestly not too familiar with typical specs of those sorts of antennas.#2 can be done by using multiple elements: a Yagi isn't hard to make at this frequency, for example: a few bits of straight wire, cut to length, spaced out on a frame (can also be wire, so the whole thing can be crimped or soldered together), and a feedline connected to the driven element. Mind that this only increases ERP along the antenna axis (main lobe, or any side lobes that also happen to be above isotropic/dipole level), and significantly reduces it elsewhere (that's where all the ERP came from, collecting it into a narrower beam). So this only works if RX/TX can be pointed at each other reliably.And increasing ERP may not be acceptable, because bands are limited in various ways, ERP being one of them. (I don't recall what the rules are, specifically for 432MHz.)Also, with linear polarization, RX/TX need to be oriented in the same plane, give or take a modest margin; I mean, it's only down by half at 45 degrees tilt, it's not super bad, but it goes to zero at 90° so you at least have to avoid the null. Circular polarization tends to help with this, but nothing can be perfectly omnidirectional, there's always a null somewhere (ultimately because you can't comb a sphere).Don't forget the elephant outside the room: reflection or absorption in the environment. It might do to simply move the RX/TX by some 17cm to get away from an unlucky multipath null. Expect poorer performance on rainy days. If these can wobble around a lot, add windy days to that too; or if they're freely moving, expect intermittent reception and plan accordingly.Finally, note that, anything you do, assumes it is still well-matched to the RX/TX. Any SWR / reflection / insertion loss in the feed will have a marked impact on reception. You can't simply throw wires at a PCB and expect anything to work, it has to present the correct impedance (whatever that may be: it's often 50 ohms resistive, but this is not necessary, and it could be anything). The best way to measure this is with a standard setup (that adequately mimics the environment of the radio), and a network analyzer. Presumably the module/chip datasheet will tell you what impedance it expects. Common mistakes include, using the PCB as an antenna (hence the caution at the top!), i.e. there's common mode on the PCB so it's not a trivial matter to connect a coax cable up to it without disturbing the environment. This is the primary reason why balanced antennas are preferred: they are easier to work with, you don't need to make precautions or work around common mode (feedline currents). And a primary failing of amateur (and commercial!) antennas, where their reception depends on feedline length and position, not to mention the feedline current into the shack can cause all sorts of mayhem (in extreme cases, RF burns off the chassis?). Not all of these issues are going to be relevant to this little module, sure, but it's all the same underlying problem.So, to put that another way: a properly balanced antenna, on a feedline off the module, hopefully using a proper connector on board, gives the most predictable results. This is not a requirement for good reception -- but without that predictability, how long can you guess in the dark, y'know? Is it element length that needs trimming? TX impedance matching? Does it vary with wiring? It's a big parameter space, and relatively easy to work with, given suitable tools and methods (e.g. NanoVNA is quite affordable), but without, there's just so many directions to wander.Tim

author: Jester
author_rank: Frequent Contributor
timestamp: February 02, 2022, 06:15:16 pm
content: Quote from: T3sl4co1l on February 02, 2022, 05:37:39 pmTim,Thanks for responding, fortunately this board does have an antenna connector so I have ordered an antenna with a short cable and mating connector. I’m looking forward to see how that might help.The supplied spiral antenna is actually coated in something resembling silver, looks more like nickel.I have a few short antennas of unknown frequency range laying around and I tried each one as a receive antenna on my Siglent SA to see the effect on signal level while activating the transmitter. One of them is likely more of a WiFi type, as the signal is down quite a bit with that one, another came as part of a LINX demo kit for I think lowish 4xx MHz devices and it worked the best of what I have.When I receive the actual 433MHz antenna I will do a comparison vs. the spiral.I’m having fun, as usual always interesting.Thanks,J

author: Jester
author_rank: Frequent Contributor
timestamp: February 08, 2022, 03:09:33 pm
content: Update…..With a proper 433MHz 1/4 wave monopole on one side and a 418Mhz 1/4 wave monopole at the other, with a tiny 2” square perforated board as a counterpoise, so effectively no ground plane, I was able to up the distance to 700m. The antenna is designed to be referenced to a 10cm X 10cm ground plane.I had to ship the device for Beta evaluation so I need to assemble another pair to do more testing. Next up will be an actual Center fed dipole because the device will be quite small and the final board will likely be too small to be an effective counterpoise.I’m looking for a screw on dipole with the counterpoise integrated into the lower portion of the whip. So if anyone knows where I should be looking, please let me know. Something like image below.Thanks.

author: David Hess
author_rank: Super Contributor
timestamp: February 08, 2022, 05:12:58 pm
content: Shorter antennas have a smaller aperture so a loaded 1/4-wave antenna always has lower gain than an unloaded (straight) 1/4-wave antenna.This is is also the main reason higher operating frequencies result in lower range. Path loss in free space does not increase at higher frequencies, but if the antennas are scaled in size for operating at a higher frequency, then their aperture is smaller.The best way to increase range is to use a directional antenna. This not only increases the signal strength, but also rejects interfering signals and noise outside of its radiation pattern.

author: geggi1
author_rank: Frequent Contributor
timestamp: February 09, 2022, 07:47:15 am
content: If the antenna is a shortened resonant antenna the antenna will have lower gain than a 1/4 wave.For 433 frequencies a 1/4wave is just about 17cm.In case you have some distance between the two sites a small directional antenna will do alot.