Antenna Tuning for Source Impedance not equal to 50 Ohm Ask Question Asked 2 years, 10 months ago Modified 10 months ago Viewed 319 times 0 I have designed a setup for 433.92MHz wireless device (a key fob). For dipole antennas with a feed point impedance less than 50 ohms, you will need a sub-ratio impedance transformer (e.g. Such antennas typically embody conical geometry in some form, with the low frequency limit set by the base of the cone and the high frequency limit set by practal limits on the pointiness of the cone. 7947 Rating: Why is it that in many Radio Frequency systems and components, most of the time the impedance is 50 ohms? - you mean how does the, Both is true. That's impossibly fine: if it could even be manufactured with copper it would be extremely fragile. Before WWII there were literally Shacks "Radio Shacks" and I don't mean the defunct electronics stores, built right out on the main deck so as to be able to conduct the antennas to the radios. Why is an antenna's impedance always 50 ohms? Engineers, right? Well, in general, as you say, maximum power transfer rules applies when you have a given source impedance and you can only modify your load impedance. Only the wire side is what you need and can care about. Why should a receiving antenna be impedance matched to 50 ohm instead of a lower impedance? This happens when the impedance of the final amp does not match the antenna system (transmission line plus antenna). This is a measuring tool that can be used to measure the input impedance as a function of frequency. Is that a problem? By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. The reason 50 or 75 or 300 or is choosen as antenna impedances is because of practical reasons to construct particular antennas/transmission lines/amplifiers with that impedance. Antennas are impedance transformers. The Agilent 8510 Vector Network Analyzer is shown in Figure 1. if we assume that the filling material is vacuum. . The wave impedance of a coaxial cable filled with air is ~377 Ohm, but the line impedance is something with logarithm (diameters). This is quite important. Connect and share knowledge within a single location that is structured and easy to search. What's the meaning behind it? The voltage across \$Z_L\$ is (potential divider): Any coaxial cable filled with air has a wave impedance of ~377 , but this does not at all help to make the open piece of coaxial cable a good antenna. Then the feed line may be any reasonable length, and the signal may be a composition of many frequencies, or many independent signals, within the limitations of the bandwidth of the match. Does the conduit for a wall oven need to be pulled inside the cabinet? So, to clarify: the input impedance (especially the radiation resistance \$R\$) is the impedance 'seen' by the transmission line, whereas power radiated into free space depends on the free space impedance in the Poynting vector \$S = \frac{E^2}{Z_0}\$. rather than "Gaudeamus igitur, *dum iuvenes* sumus!"? Why 50 ohms? | All About Circuits Not really. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. The Problem cannot be the case since any real transmission line (i.e. Call the wizard. In such applications, power handling capability is also important, and is addressed in Section 7.4. Now, if the impedances in the transmission line and in the antenna are matched at 50, but the impedance of free space is 377, won't there be a impedance mismatch and consequently a less-than-optimal radiation from the antenna? What looks like a complex tangle of copper connections is actually a meticulously designed system that uses well-understood design rules. The old adage work smarter, not harder definitely applies to printed board design. Hence 50 ohms became a de-facto standard, and so to maximize power transfer source and load impedances need to both be 50 ohms. A monopole antenna has half the impedance of the antenna: 35 ohms. There isn't an obvious 2:15 ratio there. impedance - Why do I need to tune an antenna? - Amateur Radio Stack Transforming Reference Impedances The 50 Question: Impedance Matching in RF Design I tried to explain this in my answer. 2nd comment: this probably depends on how you define the radiation resistance. Of course the questioner were right if Zs and the caught Vs from a given RF field were independent, but they aren't except in case there's a series reactive part in antenna's Zs. (On a related note: using this resistance for the receive case is misleading, since there occurs no loss in the radation resistance. It also is not the "line impedance" nor the input impedance nor the characteristic impedance. An antenna is a transducer from a guided wave to an unguided wave. For example, \(50~\Omega\) on commonly-used 1.575 mm FR4 requires a width-to-height ratio of about 2, so the trace is about 3 mm wide. Ambitious project, for sure, but we had fun visualizing Santa and his reindeer flying across our two roofs. It is important to understand that the power is transmitted in the insulator, not the conductors. Coaxial cables and terminations. Some helpful sources and discussions I found: The input impedance of certain devices/circuits (transformers) does not neccessarily need to match their output impedance. In that section, it is shown that attenuation is minimized for characteristic impedances in the range \(\left(60~\Omega\right)/\sqrt{\epsilon_r}\) to \(\left(77~\Omega\right)/\sqrt{\epsilon_r}\), where \(\epsilon_r\) is the relative permittivity of the spacer material. True, I should have said it's an approximation. Does the transformer know balanced from unbalanced, NO it does not. Just matching impedance is not all you need. I can totally get the reasoning for a transmitting antenna to have to be matched to 50ohms (assuming the source impedance is also 50 ohms). Invocation of Polski Package Sometimes Produces Strange Hyphenation, Passing parameters from Geometry Nodes of different objects. Complete Bills of Material list the PCB along with all components soldered or adhered onto the board to make the assembly. Low-cost coaxial cables with air or low-Dk dielectric filler can shoot for 77 Ohm impedance for long cables runs, but the reason for rounding off to 75 Ohms instead of using 77 Ohms is still a mystery to me. Power applied to the feed point (in a dipole, for example) travels down to the end of the wire, and is reflected back to the feed point, where (if resonant) it will meet a voltage or current 180 degrees out of phase, thus canceling, and represented by the (so-called) standing wave. Why is it the convention? This relates directly to the feedpoint impedance, i.e. Values of \(50~\Omega\) and \(75~\Omega\) also offer some convenience when connecting RF devices to antennas. Somewhere in your house you probably have a speaker (perhaps a subwoofer) with a horn on it: that horn is there to take the very low acoustic impedance of air and transform it to something higher to better match the driver. Ifimpedance doesn't match, power is reflected from the radio and not received. That's the point though: how is the radiation resistance related to the free space impedance? because the medium inside is free space. Good answer. For other antennas, such as a printed antenna, it may be difficult to design the antenna to perfectly hit a 50 Ohm target impedance . is not a correct statement. After reading the free "RF Design" magazine for many years, that after repeatedly/thoroughly reading "The Amateur Radio Handbook" and admiring those EIMAC 4CX-1000A triodes shown in the back, AND examining the primarily narrow-band behavior of RF circuits (antennas are NARROW BAND, maximum-power-transfer matching PI networks are NARROW BAND), I sat back and considered the impedance of 10pF of Cin (Cbe + Cob with Miller Effect) at 1,000 MHz. However, having a look at the fields inside the cable, we find that the electric field has only the radial component (exact values are irrelevant in this context) E and H or into Magnetic Field in the case of the Magnetic Loop antenna. Although I love the crossword puzzles where you complete the words from a given clue, I think puzzles that require you to put the pieces together can be the most challenging, especially when it is a group activity. The radiation resistance, \$\small R_r\$, of a half-wave dipole is \$\small 73\Omega\$. Regulations regarding taking off across the runway, Node classification with random labels for GNNs, How to add a local CA authority on an air-gapped host of Debian, Equality of expected value using Fubini's theorem. "My question to this is: how does a single wire, (1/4 or 1/2 wavelength long) convert form 50 to 377?" Why is it "Gaudeamus igitur, *iuvenes dum* sumus!" Dissolve neighboring polygons or group neighboring polygons in QGIS. You can find an example for such a calculation in the Wikipedia acticle about "Dipole antenna", in paragraph Short Dipole. This means that if a sinusoidal voltage is applied at the antenna terminals with an amplitude of 1 Volt, then the current will have an amplitude of 1/50 = 0.02 Amps. Free space can be regarded as a further element, since a transmitting antenna eventually should radiate all power from the transmission line into it. Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. All kinds of techniques can raise or lower the impedance of an antenna without having a substantial impact on effective aperture, which is what really dictates receive power. Why is 50 often chosen as the input impedance of antennas, whereas the free space impedance is 377? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. It is horrible! Nearly all practical antennas are some sort of 1/2 wave dipole. Besides 377 ohms is theoretical freespace and just like isotropic Virginia It Simply Does Not Exist! @ChrisStratton Ah, I completely forgot about the dummy load, right. Sorry if the answer looks stupid, I'm just a "BEGINNER" :). V_{Z_L} = V_s \frac{Z_L}{Z_L+Z_S} This energy can propagate in two directions. When we talk about S-parameters, impedance matching, transmission lines, and other fundamental concepts in RF/high-speed PCB design, the concept of 50 Ohm impedance comes up over and over. However, what if you are only using your antenna to receive? In the case of the 1/4 wave the other half of the antennas is usually the car or some other ground plane. The free-space impedance has no relation to any antenna or material configuration. Thanks for the answer! One point to understand about the above graph is that dielectric dispersion is generally not included and will affect the results at higher frequencies. A third commonly-encountered antenna is the folded half-wave dipole. However, your received signal is also much smaller because the transformation ratio has dropped even lower. The 50 Ohm is chosen as an input not as an output impedance, if we want to transmit or receive the maximum power between the coaxial line and the antenna we have to match their impedance. To learn more, see our tips on writing great answers. rev2023.6.2.43474. Use it off-centre and the reactance can go positive (inductive) or negative (capacitive). It is only by accident that the unit of both terms is the same (i,e., Ohms). I often see mention of 1/4 wave antennas mentioned, truth is there really is no such thing. A folded half-wave dipole has an impedance of about \(300~\Omega\) and is balanced (not single-ended); thus, there is a market for balanced transmission line having \(Z_0=300~\Omega\). One would think that 75 Ohms is a nice rounded number thats easy to remember, while an external article on Microwaves 101 claims this was an intentional design. You've got this a bit backwards - for a lot of set-ups, the antenna is somewhat distant to the transceiver and are connected via coaxial cable. for the matching of inset fed microstrip patch antennas). Antenna Impedance - an overview | ScienceDirect Topics 101 2 I don't think I would risk connecting a transmitter to that telescoping antenna. First story of aliens pretending to be humans especially a "human" family (like Coneheads) that is trying to fit in, maybe for a long time? Loosely, it is a measure of the capacity of a volume of the medium to store energy in electro-magnetic form. But when you design the output impedance of a 'source', you try to choose the best impedance that give you the maximum voltage (zero output resistance) or current (infinite output resistance). As does the Off Center Fed Dipole. Where Glossing attempts to shorten the overall route length and reduce the number of corners, Retrace re-applies the preferred width and clearance requirements to an existing route, Get Altium Designer for free for 2 weeks, Power Analyzer by Keysight Technologies Now available in Altium Designer. Why is 50 often chosen as the input impedance of antennas, whereas Taken in isolation, selecting 50 Ohm impedance would seem totally arbitrary: why not 10 Ohms or 100 Ohms? However, it is not practical or efficient to manufacture and sell products for every possible impedance in this range. Ive never seen this discussed in terms of what happens on a PCB except from one expert reference, but the answer for PCBs relates back to the internal structure and electrical characteristics of common logic circuits. A simple antenna like a dipole is operated at resonance. It describes the ratio of electric and magnetic fields in a propagating plane wave, which is approximatly obtained in an infinite distance to a radiating antenna. If you chose 377 Ohm as the input impedance of the antenna to match it to the air impedance you will lose the power transmission between the coaxial line and the antenna. These ideas apply equally well to coaxial lines, but it is easier to visualize in a twin lead. Why is the cable impedance of 50 ohm cable is considered to be real in Is that more or less correct? Learn more about Stack Overflow the company, and our products. An antenna serves the same function, but for electric waves. monopole) has a complex impedance characteristic versus frequency: -. - Electrical Engineering Stack Exchange Why is 50 often chosen as the input impedance of antennas, whereas the free space impedance is 377 ? In particular, it is a very popular value for the characteristic impedance of transmission line, and is commonly specified as the port impedance for signal sources, amplifiers, filters, antennas, and other RF components.