First resonate a short antenna, then impedance match it ? - Page 1
Forum section: electronics / rf-microwave

author: kellogs
author_rank: Regular Contributor
timestamp: February 25, 2025, 10:46:08 am
content: Or incorporate the reactive resonator component (one capacitor or one inductor) into the impedance matching network, any difference besides lower component count ?

author: A.Z.
author_rank: Super Contributor
timestamp: February 25, 2025, 08:40:46 pm
content: Quote from: kellogs on February 25, 2025, 10:46:08 amsome modeling software like ezNEC or 4NEC2 may help you, start by modeling your antenna and get its complex impedance value, with that in hand you may then reason about impedance matching or even simulate it

author: kellogs
author_rank: Regular Contributor
timestamp: February 25, 2025, 09:06:36 pm
content: Done the antenna simulation, came up 2.5 -j467 ohm. For better bandwidth i am going to load by 6.6 ohm, so that will make it 9.1 -j467 ohm.Not sure how close this impedance will be when installed in its operating place, plus manufacture tolerances will come into play... but this is what I have to work with.

author: antenna
author_rank: Frequent Contributor
timestamp: February 25, 2025, 10:52:09 pm
content: Quote from: kellogs on February 25, 2025, 10:46:08 amI've been seeing this type of question pop up a lot on the various forums. When you tune an antenna to resonance, you may be getting rid of the reactance at the feed point, but the resistive component will still need to be moved. To match it, you have to move that impedance value off the purely resistive axis anyhow, placing it back into the reactive realm, for another reactive component to bring it to 50ohm (or whatever your system impedance is). There is no significant benefit to tuning an antenna to resonance before matching it unless you are able to make a big enough change to significantly impact the radiation resistance or beneficially change the pattern.From your modeled impedance value, I'd guess your antenna is around an eight of a wavelength long. That antenna will not perform well, and in the case of a ground plane antenna, radiation at the horizon will be greatly reduced compared to a quarter wavelength GP.Why are you choosing to go with such a short antenna if tuning it to resonance (making it a quarter wavelength) is an available option? You would have much better radiation resistance (more power out), and you'd have a better pattern, if you made it longer. Very few situations benefit from a physically short antenna, one being mobile VHF from a low valley to a repeater on a high hill, but those cases are few and far between.As for component count, incorporating it into the network doesn't necessarily mean a third reactive element is needed, it could simply mean changing the value of one or both in the matching network. A T-match, for example, is just two L matches back to back with the parallel two in the middle combined. PI match is the same, the two series elements are combined into one. Thats the beauty of impedance matching with a smith chart, you are not confined to over-simplified formulas based on matching pure resistances.Edit: there are some bandwidth considerations as well, like making smaller impedance changes utilizing more networks to do the same overall transformation, but this can come at the cost of more loss. And if the antenna is noodle thin and can't handle the added BW, then whats the point.

author: Sassy Taste
author_rank: Contributor
timestamp: February 27, 2025, 09:06:46 pm
content: Is it better to use lower impedance on speakers then? For example, cars basically have 4 ohms impedances to the speakers. Will using 2 ohms acquire more necessary frequency response from the antenna? Probably not. OP AMPs have high input impedance, and low output impedance. I always thought just using copper long wires cable rods connected to the radio part of the circuit will give the necessary frequency. They added this as a built in module. Resonator.