Why a longer antenna is able to receiving smaller bands? - Page 1
Forum section: electronics / rf-microwave

author: prabhatkarpe4
author_rank: Contributor
timestamp: September 28, 2024, 11:08:45 am
content: A resonant antenna is one that will be able to receive the frequency as per its 1/4 wave length. So, a 75 cm whip antenna will receive 100 MHz properly because (300/100)/4 = 0.75 meters or 75 cm.Similarly, to receive 120 MHz (Air band), a (300/120)/4 = 0.62 meters or 2 feet long whip is required. But I have noticed that on my radio receiver, I am not clearly able to receive 120 MHz using 60 cm whip but I am able to hear the broadcasts between 160 - 200 MHz. Any signal above 160 MHz would need a smaller antenna, but how come the longer antenna was able to receive those higher frequencies?Does that mean, a longer antenna is always better and is able to receive any higher frequencies starting from its resonant frequency? Which also means that a 60 cm whip would be able to receive any frequencies from 120 MHz to 300 MHz?

author: A.Z.
author_rank: Super Contributor
timestamp: September 28, 2024, 02:13:34 pm
content: https://ia800501.us.archive.org/30/items/AntennaTheoryAnalysisAndDesign3rdEd/Antenna%2520Theory%2520Analysis%2520and%2520Design%25203rd%2520ed.pdfgoogle url ishttps://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://ia800501.us.archive.org/30/items/AntennaTheoryAnalysisAndDesign3rdEd/Antenna%2520Theory%2520Analysis%2520and%2520Design%25203rd%2520ed.pdf&ved=2ahUKEwiNtOC26OWIAxWPhv0HHRZ6A1UQFnoECDcQAQ&usg=AOvVaw257AIHJRsq-6V1wHp18Fgc

author: Sorama
author_rank: Frequent Contributor
timestamp: September 28, 2024, 02:25:01 pm
content: This link is broken?

author: Andy Chee
author_rank: Super Contributor
timestamp: September 28, 2024, 02:43:27 pm
content: Quote from: prabhatkarpe4 on September 28, 2024, 11:08:45 amThe above statement makes an incorrect assumption, that an antenna must be resonant in order to function.What you need to understand is that antennas do NOT need to be resonant, especially for receive.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 02:52:10 pm
content: An arbitrary length receiving antenna at a single frequency looks like a voltage generator (at that frequency) in series with a non-resonant impedance (radiation resistance plus reactance).A resonant antenna has zero (or small) reactance, and is easier to couple to a receiver for good signal at the input.Far from resonance, the reactance is large and you lose a lot of signal when connected to a resistive load.However, it is possible to tune out the reactance at that frequency with an appropriate circuit (short antennas are capacitive, so a series inductor can cancel that reactance).

author: tggzzz
author_rank: Super Contributor
timestamp: September 28, 2024, 02:52:50 pm
content: Click on the "see an archived version of this page" link.It is what the OP needs, whether or not they know it. For example, p239 is about one of his othermanyquestions about antennas (the Wheeler method).Currently he has made 21 posts,of which the majority (11) are starting a topic. Plus he has apparently ignored responses in 5 of his threads; that's impolite.https://www.eevblog.com/forum/profile/?area=showposts;u=1017709

author: A.Z.
author_rank: Super Contributor
timestamp: September 28, 2024, 03:37:51 pm
content: Quote from: Sorama on September 28, 2024, 02:25:01 pmcorrected the post, should work now, anyone it's just one of the many available books useful to start getting a grip on antennas... assuming one WANTS to, by the way

author: radiolistener
author_rank: Super Contributor
timestamp: September 28, 2024, 06:49:07 pm
content: A resonant antenna is most efficient at its resonant frequency because resonance amplifies the signal at the given frequency and attenuates signals at other frequencies. In the case of your 60 cm antenna, it is designed for frequencies around 120 MHz, but it can also receive higher frequencies, such as 160–200 MHz, due to harmonics and broadband characteristics.However, resonance does not always guarantee optimal performance at all frequencies. Antennas longer than the resonant wavelength can receive higher frequencies, but not necessarily with greater efficiency. Reception efficiency depends not only on the length of the antenna but also on other factors such as its geometry, impedance matching, directivity, and orientation in space.Thus, while a larger antenna can be effective, its effectiveness at specific frequencies is not always greater than that of a smaller antenna. For optimal reception, it’s important to consider not just the length, but also the other parameters of the antenna.