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How does a single FM antenna length work in the entire FM frequency band range, when there are theories which state that the antenna size should always be close to wavelength/4 for proper reception & transmission?
For example: The FM frequency band ranges from 88 MHz to 108 MHz. How will an FM antenna length will match all the frequencies from 88 MHz to 108 MHz? Does the antenna length scale itself within that range or how does it work?
Design goals for a receiving antenna and a transmitting antenna can be very different. For transmitting antennas, the goals are generally efficient power transfer, directivity (a.k.a. "gain"), and efficiency. Directivity has to do with the radiation pattern of the antenna (omnidirectional or aimed in a certain direction); efficiency generally has to do with the size of the antenna relative to the wavelength of the transmitted signal.
For efficient power transfer, the impedance of the transmission line and the load have to be matched to the output impedance of the transmitter. In ham radio terms, the SWR should be as close to 1:1 as possible. If the load (the transmission line and antenna) is not perfectly matched to the source (the transmitter), then reflected power makes the output current and voltage rise. Transmitters can often tolerate a certain degree of mismatch, but beyond that the output power must be reduced, or the transmitter may be damaged.
The impedance of an antenna varies with frequency. Transmitting antennas are typically tuned so that the mismatch is lowest at the center of the frequency band of interest. The changing impedance of the antenna, combined with the degree of mismatch that the transmitter will tolerate, gives a range of frequencies that the transmitter, feed line, and antenna can be used to transmit at full power.
For receive-only antennas, efficient power transfer is not necessarily a concern. Impedance bridging, where the load impedance (of the receiver) is chosen to be much larger than the source impedance (of the antenna and transmission line), may be enough. Impedance bridging maximizes the signal voltage rather than the signal power. Maximum power transfer is generally unnecessary in a broadcast receiver because amplifier circuits can usually be designed to have as much gain as necessary. (The limiting factor in how much gain can be used generally has to do with noise generated by the receiver itself, which is usually not a concern for a broadcast receiver.) In an impedance bridging situation, as used by a typical FM broadcast antenna and receiver, small changes in the impedance of the antenna don't matter very much, because the input impedance of the receiver is much larger than that of the antenna and feed line anyway. So a receive-only antenna can have a much broader bandwidth than a transmitting antenna, which needs to be concerned with efficient power transfer.
FM antennas (the sort that come with your receiver) are trimmed for best performance in the middle of the FM band. Because that band is not very wide, and the signals they receive are generally not weak, they work adequately well at the low and high ends of the band.
For picking up weak signals, you'll use a broadbanded FM antenna on a mast, where there are enough antenna elements in the design to allow adequate operation all the way across the band.
It appears that only reception on the 88 -108 MHz FM is being referred to.
With high power FM transmitters located in the heart of a city, a 75 cm length of wire would suffice to receive stations across the band. In the case of a receiver with a telescopic antenna, it's length could be adjusted, if required, to receive weaker signals.
In fringe areas, a broadband log-periodic antenna, covering 88-108 MHz, would be the right solution.
The answer to your question would be that reception across the 88-108 MHz FM band would be possible using a single log-periodic antenna specifically designed for the FM band and having a bandwidth of 20 MHz.
More details on log-periodic antennas at
Bandwidth of an antenna is primarily dependent on the design of the antenna and the electrical radius of the active element(s).
With FM radio stations, the high power 1 - 100+ kilowatt transmitters, allows a lot of loss with respect to the receive antennas: any approximately eighth wavelength wire or longer will adequately receive any station in range(20 to 150 miles depending on terrain and antenna gain or loss).
Most car antennas are given a length the corresponds close to the center of the FM band. The power output of the stations make up the difference.
In the AM band the output power tends to be lower overall( .01 - 5KW+), since the band will travel longer distances. AM Reception is done in car radios through the use of loading coils applied to the FM antenna. The efficiency of the antenna + loading coils is about 5 to 60 percent of the power available in the airwaves. Reception depends on propagation factors.
Bluetooth is a whole different story, it operates on the 2.4 GHZ spectrum with a power in the milliwatt power range. To minimize interference with WiFi occupying the same bands, it produces a signal that hops through all of the allocated frequencies in such a way to cause the minimal amount of disruption to the WiFi channels it conflicts with.
Now back to antenna bandwidth:
There are very wide designs based on angles like the rhombic design (high gain, highly directional, 1+ decade of 3db bandwidth)
The length specified wideband: (each band must be represented as a length in the elements of the design)
log-periodic(Medium gain and directional)
fan dipole
I would be failing to represent all widebands, I am sure I skipped many, if I did not include the End-Fed-Half-Wave(EFWH). The high impedance of an EFWH makes it a perfect longwire antenna for evenly divisible frequencies.
Back to the electrical radius(where surface area dominates):
The bowtie UHF, similar in concept to VHF rabbit ears: Four, or two, short sticks emanating from a common point and spaced appropriately. The bounding frequencies of this design are dependent on the length and size of the elements.
For a dipole, the bandwidth is most dependent on the size of the conductor, but may be extended by introducing a second(or more wires) of the same length using (relative to wavelength) small spacers.
For microwave frequencies like 2.4GHz when space is a consideration, microstrip fractals tend to replace conventional antennas. The fractals offer a wider bandwidth and more electrical length, therefore more bandwidth for a smaller design.
Maximizing surface area leads to a higher bandwidth antenna.
