Antenna length

[polishdude20]

SO how do i go about calculating the best antenna length for my fm transmitter? what is the formula?

 

[Papabravo]

A dipole should be approximately one half wavelength. You do know the relationship for frequency, wavelength, and the speed of light right? Anyway the approximation gets you close but for best performance you can use an antenna analyzer to adjust the length for minimum return loss.

 

[polishdude20]

A dipole should be approximately one half wavelength. You do know the relationship for frequency, wavelength, and the speed of light right? Anyway the approximation gets you close but for best performance you can use an antenna analyzer to adjust the length for minimum return loss.

ok im reallllyyy newb to this so plz explain more

 

[Papabravo]

frequency * wavelength = speed of light

Speed of light is ≈ 3 * 10^8 meters/second

 

[polishdude20]

frequency * wavelength = speed of light

Speed of light is ≈ 3 * 10^8 meters/second

lol your signature is the same quote we have on our electronics room wall oh and thanks for the info so that would be 3 * 10^8 / Frequency (101.1) = wavelength ?

 

[polishdude20]

and BTW it is a 9 volt small fm transmitter i made so its not powerful

 

[polishdude20]

but then what is it in units? meters ? cm? mm? cause i get a number like 2000

 

[mneary]

For a frequency of 101.1 Hz , your wavelength is approximately 3,000,000 meters! (half wave antenna a little more than a thousand miles). But this is not a common transmitter frequency.

If you choose a more normal frequency such as 101.1 MHz, your wavelength will be a more reasonable 3 meters, and the half wave antenna about 1.5 meters.

[edited: I originally said 3,000 meters]

 

[ke5frf]

For what purpose and band is this transmitter?
I ask because dipoles are typically horizontal polarity antennas and "typical" FM applications use vertical polarity...More out of custom than anything else, but vertical polarity DOES allow for omnidirectional coverage which is desireable with mobile or portable communications (which FM typically is)

 

[Papabravo]

lol your signature is the same quote we have on our electronics room wall oh and thanks for the info so that would be 3 * 10^8 / Frequency (101.1) = wavelength ?

The signature is well known and ubiquitous.

Frequency in Hertz(Hz.) has units of (sec)^-1 and wavelength has units of meters so if you multiply them together you have units of velocity or meters/second. This is true for any similar formula and you should never have to ask what the units are because the answer is unique. Of course this presumes you know what units things are measured in.

 

[audioguru]

The FM antenna on my car radio is about 80cm tall.
The antenna on my FM transmitter is also 80cm long. I hang it vertically.
The rabbit-ears dipole antenna on my home stereo is abot 160cm from tip to tip.