Transmission antenna

arvik

Hey guys, professor at college just completed teaching amplitude modulation and other stuff.Well while explaining need for modulation he said that height of transmitting antenna should be a multiple of "one fourth the wavelength" and so it is required to modulate the signal to higher carrier frequency.
Can somebody explain just briefly how we get the relation between height of antenna and wavelength?I tried searching for it on the net but couldnt find any xplanation in regards to it.
thanks.

NoTime

It depends on the type of antenna and you can compensate for breaking the rule somewhat.
IIRC the rule applies mostly to horizontal dipole where the ground can work as an passive element to the driven element changing the impedance and radiation pattern.
An end feed vertical has the opposite constraint.
The end needs to be at the ground or you need to construct a ground plane.

A good source for antenna design is the ARRL Antenna Handbook.
(ARRL = American Radio Relay League)

berkeman

I think the prof was talking about the length of the radiating elements, not the height above the ground plane. Both affect the radiation pattern, but the quarter wavelength refers to the resonant length of each half of a dipole antenna, or the single element of a monopole antenna. This wikipedia page is a good intro to antennas, with plenty of helpful links:

http://en.wikipedia.org/wiki/Antenna_%28radio%29

And here is a recent PF thread where we talked about antennas a fair bit:

http://www.physicsforums.com/showthread.php?t=210178


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NoTime

Here is some height above the ground stuff.
Starts on page 5.
http://www2.arrl.org/FandES/field/re...al/antplnr.pdf

mgb_phys

In the tradiational PF thread going off at a tangent mode.

The wiki article says that antenna / aerial means a rigid vs stiff wire but mean the same in British English.
I always used antenna for a single element and aerial for the entire system, mutliple dipoles, reflectors etc. Is this common usage?

NoTime

Welcome to short attention span theater.

Don't know.
Personally, I'm inclined to think of aerial as archaic usage, at least in the US.
For those times when I need to be more specific than antenna I name the type.

berkeman

At least among US HAMS (and the EEs that I work with), antenna refers to the whole thing, and we use the term "element" when referring to a piece of the antenna.

zeitghost

Mmmmmmmmmmm... love the concept of getting a 198kHz antenna a quarter wavelength up...

berkeman

Yep, as you get to lower frequencies, the size of an efficient antenna can get to be pretty challenging!