Antenna - Air impedance matching? - Page 1
Forum section: electronics / beginners

author: fonograph
author_rank: Frequent Contributor
timestamp: July 30, 2018, 01:40:35 am
content: I have read conflicting information,one is that matching impedance of antenna to air is not needed,that full power transfer will happen no matter what the impedance antenna is,becose the reflection is in phase with the outgoing signal.Second opinion is that matching is needed,which one is true?I does not make sense to me that at RF frequencies,we must always impedance match everything but somehow large impedance discontinuity when transitioning to air is somehow not causing any issues.Futhermore,even if the antenna - air impedance mismatch doesnt affect frequency domain or power,wont the "in-phase reflections" cause time domain smearing of somethe sorts? For example in case of pulse position modulation that depends on short pulses in time domain,would it not cause some ringing or some time domain pulse shape degradation?

author: tautech
author_rank: Super Contributor
timestamp: July 30, 2018, 01:48:11 am
content: Have been down this road and I'm certainly no expert even still finding my way and still somewhat lost at times.Have a little read of something I'm working on again now I've got the right tools.......and some small understanding.https://www.eevblog.com/forum/testgear/siglent-sva1015x-1-5ghz-spectrum-vector-network-analyzer-(coming)/msg1686137/#msg1686137

author: tautech
author_rank: Super Contributor
timestamp: July 30, 2018, 03:03:42 am
content: Simply for antenna construction you must satisfy 3 main requirements.Resonance frequency, where the antenna emits the most energy at the desired frequency.Impedance, so the antenna matches the feedline and its source impedance at the desired frequency.50 Ohms is normal.SWR so the antenna is the most efficient at the desired frequency with the energy available.Easy, no it isn't ! Hard, not terribly with the right gear and/or some experimentation.

author: D-Jack
author_rank: Contributor
timestamp: July 30, 2018, 03:06:42 am
content: Quote from: fonograph on July 30, 2018, 01:40:35 amAn impedance mismatch when designing an RF circuit will affect the power and range of your signal. If it is bad enough, it will not transmit at all. The majority of circuits will "work" without tuning (as long you follow the basic layout recommendations) but instead of having a 10 meter range, you will have 1 meter. This can deeply impact your product since you will either have to increase output power increasing power consumption or you will have a product with poor signal performance.Some people may say the a tuning process for antenna-air is not need because they often assume the circuit will be enclosed somehow, so in that case the tuning must take into account the enclosure.This is an excellent application note that will walk you through the various steps of designing and tuning an RF circuit:http://www.cypress.com/file/136236/download

author: GregDunn
author_rank: Frequent Contributor
timestamp: July 30, 2018, 06:09:21 am
content: Not sure what is meant by matching the antenna to the air? What you want is for the native impedance of an antenna to approximately match the impedance of free space (about 377 ohms). It's most important if you're trying to deliver prodigious amounts of power; if the impedance is reasonably close you'll get some power transfer regardless, but if lots of power is being sent, much of it may come back down the line to your transmitter and that's generally not good.Most antenna cookbooks take 300 ohms as a starting point, and unless you're trying to couple directly to a receiving antenna rather than actually broadcast or transmit RF, you'll want to use an antenna element designed for that approximate impedance. There are lots of special cases, so if you're trying to design a VLF high power system or a microwave point-to-point link, it's best to consult the literature. But in general, even these types of systems take the free space impedance into account at some point in the design.

author: fonograph
author_rank: Frequent Contributor
timestamp: July 30, 2018, 06:42:12 am
content: Quote from: GregDunn on July 30, 2018, 06:09:21 amBy "matching antenna to air" I mean making the antenna have 377 ohm output impedance.Free space and air both have 377 ohm impedance,free space and air are in this case synonymsQuote from: GregDunn on July 30, 2018, 06:09:21 amThis is completly opposite of what I read on other forum,user there wrote that matching antenna to air is useless waste of time,that it makes no difference becose the energy that is reflected back is "in phase" so it doesnt negatively affect power transfer,its sounds like bull schmitt to me so that is why I asked here.

author: tautech
author_rank: Super Contributor
timestamp: July 30, 2018, 07:22:10 am
content: Quote from: fonograph on July 30, 2018, 06:42:12 amThat may be so but in practice the reciprocity of an antenna makes for close to equivalent performance whether transmitting or receiving. So sweeping an antenna with the desired frequency and optimizing it for such is all that needs be undertaken without needing to factor in 'air impedance'.The tools you use to do this don't need such an input as the tuning of an antenna reflects this in the measurement results.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 30, 2018, 08:18:46 am
content: Quote from: fonograph on July 30, 2018, 01:40:35 amYou can confidently discard that information source of yours, it's bollocks.1. Full power transfer will always happen only when the impedance of the source and sink are matched. There are no exceptions to this rule.2. The sign of the reflection depends of the difference between the source and sink impedances. Reflection can be either in phase, anti-phase, or zero, depending on the two impedances.

author: fonograph
author_rank: Frequent Contributor
timestamp: July 30, 2018, 09:28:11 am
content: Quote from: RoGeorge on July 30, 2018, 08:18:46 amThis pretty much concludes this thead,I instantly suspected its "bollocks",thank you

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 30, 2018, 01:41:21 pm
content: To a certain extent, yes.To a useful extent, probably not.I prefer to think of a wideband antenna as the most general form. Consider an infinite horn antenna, which therefore couples all frequencies DC-light into a similarly ideal transmission line. This has no resonance (a Q of zero), and an impedance defined by the angle of the horn.The horn can be arranged with many symmetries: when linearly self-similar, it is a regular wedge-shaped horn as such, with a unidirectional radiation pattern. When revolved around an axis, it is a conical dipole, with a dipole radiation pattern (also symmetrical around the axis). When drawn in a plane, it is a planar horn, with a wide beam perpendicular to the plane and a narrow beam along the horn's mouth axis. When a planar horn is twisted around an axis, forming a spiral, it has a wide radiation pattern with circular polarization (the others listed so far have been linear polarized).The exact shape of the horn doesn't matter to the frequency response; that controls polarization and beam pattern.That leaves the impedance, which depends on the ratio of space occupied by the antenna, to free space. More or less. Consider a planar spiral antenna: there is positive space occupied by the antenna elements, and negative space inbetween them. If these spaces are equal (in both size and shape), then the antenna is self-dual, and the impedance is half that of free space.In that sense, you could say such an antenna is matched to free space. I don't think the match is meaningful, though: you get the impedance transformation for free. (Consider this speculation on my part, until an E&M expert gets in here to put some proof behind this. I personally never got that deep into fields -- admittedly this is probably a masters level subject, so that is understandable!)So then, what is a practical antenna, if not a perfectly general, ideal, everything-band antenna? A real antenna is one that has limited bandwidth, and therefore finite overall size (lower frequency limit), and finite minimum feature size (practical upper frequency limit).By restricting the size, whole bands are cut off from reception. The antenna is itself a filter! As a filter, there are further opportunities for impedance matching. Example: you can wire up a dipole directly, and get so-and-so feedpoint impedance. Or you can short the would-be feedpoint, and connect off-center with a gamma match. Or you can add capacitors and inductors to transform the impedance (at a frequency) to anything else.So, TLDR: the antenna is a filter, and consequently, a matching network. You can get pretty much any number you want, at the feedpoint. Very wideband antennas approach an ideal infinite antenna, whose impedance is defined by geometric ratios, and much more restrictive (without adding a transformer proper).Tim