Perfect antenna size - Page 1
Forum section: electronics / rf-microwave

author: arildj78
author_rank: Regular Contributor
timestamp: January 28, 2025, 10:18:40 pm
content: My understanding of dipole antenna size is that half a wavelength is a good size. But how can you increase the connection between the antenna and the propagated wave even further? Would half a wavelenght + a few cm be better if the antenna gets proper impedance matching? How about some integer multiple of half the wavelength? This is a video that i still havent found in W2AEW's library.What happens if you need to go shorter than half a wavelength? Is a few centimeters shorter better than a large jump down to say a quarter wavelength?Arild

author: A.Z.
author_rank: Super Contributor
timestamp: January 29, 2025, 04:53:42 am
content: Never argue with idiots. They will drag you down to their level and beat you with experience

author: CaptDon
author_rank: Super Contributor
timestamp: January 29, 2025, 02:13:07 pm
content: On the receiving side of things a resonant antenna often works better. The antenna can be resonated in several ways including length. A resonant antenna can also help reject signals away from the resonant frequency knowing that those signals can sometimes lead to de-sensing if they overload the front end of the receiver. On the transmit side having a good match with low VSWR is important. An antenna with a good match usually beats one with a bad match and made to 'look good' using a match box. Two things to remember, 1. There is no substitute for 'metal in the air' it's the dream of women everywhere, bigger and longer is better. 2. Don't ignore resonance, case in point the MFJ SuperLoop. When tuned for lowest VSWR they absolutely suck, you tune a SuperLoop to peak the receiver noise and to hell with the VSWR which may be 2:1. Tuned this way the SuperLoop works very well indeed!!

author: TimFox
author_rank: Super Contributor
timestamp: January 29, 2025, 04:24:28 pm
content: For example, an antenna shorter than a resonant length such as half-wavelength can be resonated (cancelling the capacitive reactance at the feed point) with a “loading coil” inductance in series with the antenna.However, the loading coil adds a non-trivial series resistance (reducing the efficiency) but does not compensate for the lower received power due to less wire in the local RF field (V/m).

author: radiolistener
author_rank: Super Contributor
timestamp: January 29, 2025, 10:31:46 pm
content: Quote from: arildj78 on January 28, 2025, 10:18:40 pmThere are two key factors to consider:1) Antenna radiation efficiency - it is generally proportional to the antenna's physical dimensions. Since radiated power distributed across space, a larger antenna can collect and radiate more energy per oscillation cycle. However, this represents an upper limit, actual efficiency depends on design and losses.2) Antenna losses - these include resistive losses and losses in the impedance matching network. A half-wavelength dipole is naturally resonant, meaning its input impedance is mostly resistive, minimizing the need for reactive matching components. Every impedance matching circuit introduces losses, and a higher impedance transformation ratio generally results in greater power dissipation.Regarding your question, a length of half a wavelength plus a few centimeters may not provide significant advantages unless it improves impedance matching. Integer multiples of half a wavelength maintain the self-matching property but may not always be practical due to other design constraints.It is also worth mentioning that if you ignore losses in the matching network and the antenna itself, you can assume that the antenna's efficiency is directly proportional to its size. However, there is a lower size limit due to capacitive/inductive coupling effects in the reactive near field region, even if you manage to match the source to the antenna without losses. This limit can be roughly estimated as λ / (2*π). If you reduce the antenna size below this limit, you will still have to keep the space in the area of ​​this size empty, otherwise any objects that get into the antenna area at a closer distance will lead to strong losses, distortions of the radiation pattern and impedance of the antenna.Thus, λ / (2*π) can be considered as the minimum size of the antenna, even if the physical design of the antenna will take up less space, its real working space still needs at least λ / (2*π) size.

author: A.Z.
author_rank: Super Contributor
timestamp: January 30, 2025, 02:24:01 am
content: Quote from: arildj78 on January 28, 2025, 10:18:40 pmat 800 MHz " a few cm" WILL make a difference,; when it comes to antennas you should learn to reason in terms of Lambda (assuming you know and understand what "Lambda" represents)

author: A.Z.
author_rank: Super Contributor
timestamp: January 31, 2025, 07:46:20 am
content: and then... "perfect" for what ?define it

author: iMo
author_rank: Super Contributor
timestamp: January 31, 2025, 08:15:32 am
content: Quote from: arildj78 on January 28, 2025, 10:18:40 pmThe matching of an antenna and its "radiation efficiency" are two different things, imho.With the "matching" you are trying to minimize losses between the source (the transceiver's output) and the feedpoint of the antenna.With the "length of the radiator" (and its shape, material, height over the ground, etc.) you are trying to minimize the losses due to its "radiation resistance".https://en.wikipedia.org/wiki/Radiation_resistancehttps://en.wikipedia.org/wiki/Radiation_efficiencyhttps://en.wikipedia.org/wiki/Radiation_patternhttps://en.wikipedia.org/wiki/Gain_(antenna)https://en.wikipedia.org/wiki/Aperture_(antenna)So - you may literally "match an antenna of any length", but whether the antenna will work "well" at given frequency is a different question.