Radio's antenna

[Alon]

i read that the antenna of a radio should be a quarter of the wavlength of the recieved output wavelength, how did they come to this equation?

[Bo]

i read that the antenna of a radio should be a quarter of the wavlength of the recieved output wavelength, how did they come to this equation?

 

where did you read this? I dont think it is true ;)

 

(for example; normal FM radio is at a wavelength of 1-10 m. AM radio is at 100-1000 m) (And I don't remember the necessity of a 250m antenna to receive AM...)

 

Bo

[Qfwfq]

Indeed it isn't true that the antenna must be 1/4 lambda, but the most efficient designs take wavelength into account. The most trivial solution is the whole wavelength but a simple fraction of it is the next best thing. A coiled bit is sometimes used to simulate a longer bit.

 

The design of good, efficient antennae is a complicated topic.

[Alon]

i read it from wikipedia, on 'antenna' the third paragraph of the overview section.

[Tim_Lou]

v=f*wavelength as f varies, wavelength varies...so do they take the average wavelength or what?

[Turtle]

i read that the antenna of a radio should be a quarter of the wavlength of the recieved output wavelength, how did they come to this equation?

 

Through a combination of experiment & mathematics. Acoustic waves have the same property, e.g. the sizing of organ pipes.

http://www.physics.mcgill.ca/~guymoore/ph224/notes/lecture20.pdf

:embarass:

[Alon]

turtle in the text you've given, i don't see any mention of the antenna size proportional to the wavelength.

[Qfwfq]

That paragraph begins with "The vast majority of antennas are simple vertical rods a quarter of a wavelength long." and this is true enough, simply because it's a simple and practicle design for many purposes. It does not say that the antenna should be a quarter of the wavlength, it simply say that it very often is.

 

A quarter of the wavlength is often good enough, half or a full wavelength would be less practical especially for the lower frequencies. If the antenna isn't quite appropriate for the wavelength it radiates less of the power, or receives it less well.

[Jay-qu]

Do a test (i did it just before) I have a retractable antennae on my car - I shortened it (below one quarter) still heard the radio - then I made it full length, still worked.. length can change how much of the signal is recieved but it isnt a necissity in recieving.

[Turtle]

turtle in the text you've given, i don't see any mention of the antenna size proportional to the wavelength.

 

My link didn't mention antennas as I recall; it dealt with the principle of a standing wave in accoustics. A wave is a wave after all. If you follow the diagrams in the article they show the relationship between wavelength & length of the tube that wave is in.

Just as others pointed out that the radio still receives a discernable signal with the antenna at other than a 1/4 (or other harmonic) wave length, the sound in any tube length is audible but only creates amplified standing waves at intervals harmonic to the wave length.

[C1ay]

The longer the antenna in wavelengths, the higher the gain. Below a 1/4 wavelength the antenna efficiency drops off sharply. Quite often a 1/4 wave antenna is mounted on a ground plane though which acts like a reflector and the antenna behaves as a 1/2 wavelength antenna. Picture an antenna standing on a mirror, it appears twice as long. Sometimes a coil, aka inductor, is used at the base of the antenna to make the antenna appear longer to the receiving circuit. This is useful for low frequency antennas where the wavelength gets quite long.

[Jay-qu]

I hear it thrown around a bit - what is gain?

[C1ay]

I hear it thrown around a bit - what is gain?

Amplification in a word. The antenna itself is effectively a preamp to the input of the receiver. It is more easily visualized with parabolic antennae which collect large quantities of radiation and focus it like a magnifying glass. Each doubling of signal strength by the antenna relative to a dipole antenna represents 3db of gain.

[Jay-qu]

Ahh I get it :eek_big:

 

does it work the same way for transmitters? ie can you have a high gain transmitter?

[Qfwfq]

Certainly, a high gain is due to directionality, it obviously couldn't be adding power, and a low gain (loss) is due to inefficiency. In transmission, an ill-tuned antenna is a greater concern as the power not radiated usually goes back to the rf oscillator (or the final stage) rather than just being dissipated at the antenna. This creates a standing wave along the coaxial cable and in some cases it can even be harmful on the apparatus.

 

Do a test (i did it just before) I have a retractable antennae on my car - I shortened it (below one quarter) still heard the radio - then I made it full length, still worked.. length can change how much of the signal is recieved but it isnt a necissity in recieving.

Of course it becomes important when trying to recieve a weak signal. Try the same test in some remote place and you might find the adjustment more critical.