Antenna Lengths for different Frequencies and Frequency bands

[FreshmanNewbie]

I just want to understand a few things which I couldn't get clarity from a lot of websites searched through google as I could see differing information.

My questions :

1. Do different frequency bands require different/appropriate antenna lengths?
How does a single FM antenna length works 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 88MHz to 108MHz. How will an FM antenna length will match all the frequencies from 88MHz to 108MHz? Does the antenna length scale itself within that range or how does it work?

2. Why does low frequency signal have high range but high frequency does not?

 

[KlausST]

Hi,

For example : The FM frequency band ranges from 88MHz to 108MHz.

The (geometrical) center frequency is 97,5MHz
The antenna may be calculated to this center.
The limits of the this band then are just 10.8% off the center.

With antennas there is no black and white. It will work - but with reduced efficiency - even if the frequency does not match exacly.
10.8% mismatch with a rod antenna still gives good signal.

But when you are far from the optimum frequency, then the efficiency becomes that bad that you should consider another antenna, or a combined antenna.

2. Why does low frequency signal have high range but high frequency does not?

Low frequency means long wave.
You may do an internet search for "long wave" on your own ;-).

Read chapter "Propagation" here: https://en.wikipedia.org/wiki/Longwave

Klaus

I did another internet search for you: Waves and obstacles in water. It is similar with electromagnetic waves.
https://www.youtube.com/watch?v=1bHipDSHVG4

 

[volker@muehlhaus]

2. Why does low frequency signal have high range but high frequency does not?

https://en.wikipedia.org/wiki/Radio_propagation

 

[danadakk]

Several Amateur Radio antennae handbooks over here, basics, building, etc..

https://archive.org/search.php?query=arrl antenna



Regards, Dan.

 

[FreshmanNewbie]

Thank you for the answer.

But in general, our mobile phone has FM and AM radio and other communication facilities like GPS, WiFi and bluetooth.

So, are there individual antennas for each communication frequency band? Like, suppose, if you have individual antennas for each frequency band, won't it interfere with each other?

Suppose if you have a incoming frequency signal of some frequency say 2.5GHz, won't this frequency hit the FM antenna too? Or the FM antenna ignores or it doesn't get hit by this frequency?

 

[KlausST]

Hi,

Suppose if you have a incoming frequency signal of some frequency say 2.5GHz, won't this frequency hit the FM antenna too? Or the FM antenna ignores or it doesn't get hit by this frequency?

"In the air" there all the time is a mixture of numerous frequencies. They all hit all antennas.

Go outside, open your eyes. What do you see?
Every grain of sand sends light to your eyes, every single piece of grass, everything....
But your eyes and your brain will focus on those things you want to see.
Light has a wavelength of some 100s of nanometers.

Now let´s move to waves with the wavelength of centimeters. It´s called audio, sound or noise.
Your antenna isn´t called "eyes" anymore now the antenna is called "ears".
Close your eyes and you will still hear, close your ears and you will sitll see. You can even hear and see at the same time.

Light will touch your ears...sound will touch your eyes ... but they don´t process this information.

And so does your mobile phone.

O.K. Not exactly the same way ... but let´s say analogically ;-)

Klaus

 

[betwixt]

Suppose if you have a incoming frequency signal of some frequency say 2.5GHz, won't this frequency hit the FM antenna too? Or the FM antenna ignores or it doesn't get hit by this frequency?

First things first. Don't confuse the type of transmission, FM, AM, GPS, WiFi etc. with the frequency. They are all types of modulation. Modulation is how you change the amplitude, frequency or phase to carry information, whether, data voice, video or music.

All kinds of modulation can be used at all radio frequencies, it just doesn't make sense to use some combinations or they would be inconvenient to implement. For example, a mobile phone that needed a kilometer long antenna wouldn't be very popular!

Antennas are devices for converting electromagnetic waves to electrical signals. They work best when the length of the EM wave fits nicely along the length of the antenna but that doesn't mean they only work at one particular wavelength. Wavelength is exactly that, the distance between one peak of the EM wave and the next peak, it is the reciprocal of frequency (wavelength = 1/frequency).

Although they work best at one frequency, as you move away from the optimal length the performance only gradually decreases. This means that if you want to get best results in the 88 - 108MHz band you would pick a length that was best for somewhere in the middle. It would work best at that frequency but at the band edges it would still work quite well.

You are right that all antennas pick up all frequencies, what comes out at the end is a jumble of everything they can 'hear'. It is up to the electronics in the receiver to filter out everything except the frequency you are interested in. Obviously, you try to choose an antenna that works well over the range of frequencies you are interested in receiving.

Brian.