Dipole Questions 1

[m0ngoose]

Hi guys,

Just started reading my new ARRL antenna book that came in... I have a few questions in reading Chapter 2 (fundamentals)

- when we say a 432-MHz half-wave dipole (for example):
- according to v = f*lambda: lambda = 3*10^8 / 432 * 10^6, correct?
- so is the length of the entire dipole antenna (not just one side) half of the lambda found above?
- what if I change the length of the antenna? that changes the resonant frequency to no longer be 432-MHz, right? therefore it is no longer a "432-MHz half-wave dipole antenna"?

- what does half-wave, full-wave, 3/2 wave... resonance mean? i.e., "I'm transmitting at the half-wave resonant frequency"?

- Let's say I measure the radiating peak power for a dipole antenna and wanted to find out the directivity.. I should be able to use Directivity = Peak Power measured from dipole / Average Power density.... what average power density is this? is that for an ideal isotropic antenna? If so, how do I know what value to use... is there a table or chart somewhere that has it?

Thanks in advance

 

[Higgler]

dipole= half lambda.
Change length, change frequency true.
resonance means the current flows back and forth and radiation becomes efficient at the resonance point, typically just short of half wavelength.
Just use directivity on half wave dipole = 2.1 dB and save the effort.

Buy some other books on antennas, or check the web. Balanis is a famous author with alot of information on antennas.

It's good you're learning, but to slightly forewarn you; this website is typically job related questions or Ham Radio people, so prepare for info related to that.

 

[m0ngoose]

thanks for the response but I'm still not clear on:

- what does half-wave, full-wave, 3/2 wave... resonance mean? i.e., "I'm transmitting at the half-wave resonant frequency"?

- I used the dipole antenna as an example, so although its good to know that 2.1dB figure, I wanted to know in general if I wanted to find directivity how I would know the P(average) number? I also didn't know directivity was expressed in decibels, but I'll do some research on it

thank you

 

[Higgler]

directivity and gain is dBi(?), dBiL (Linear) or dBiC (Circular), my error on the dB, needed something after it.

I suggest Google searches for most of your info. Also, it can help alot if you click on images in the Google search so that you can see the antenna or plot you are looking for.

Here is a good article on half wave dipoles.

http://www.cvel.clemson.edu/emc/tutorials/Radiation/EM_Radiation.html

Antennas have "real" and "imaginary" parts of its' resistance (impedance is the official name). The Imaginary part is energy stored on the antenna that doesn't actually radiate. At half wave resonance, that storage is zero in this link (near the bottom of the link) and hence it's the most efficient at that point.

 

[fmradio]

Previously...
"- what does half-wave, full-wave, 3/2 wave... resonance mean?"
______________

"Resonance" of a radiator simply means that the impedance of that radiator has no reactance at its input terminals, at the operating frequency.

For example, a self-resonant, center-fed dipole a bit shorter than 1/2-wavelength end-to-end has an input impedance in free space of about 70 +j 0 ohms. Its radiation resistance is ~70 ohms, and its reactance (the "j" term) is zero ohms.

However, self-resonance of a radiator is not required in order for it to efficiently radiate nearly all of the r-f power available from the transmitter.

All THIS requires is that the reactance term of the antenna input impedance is offset by a network at the antenna input -- which also matches the "real" term there to the characteristic impedance of the transmission line in use, and that other antenna system losses are kept low.