50 ohm antenna

[walid]

i read this frequently, how I know that this antenna is 50 ohm, or how i can made a 50 ohm antenna, what the theory behind this.
If it is a transmitter antenna and the transmitter operate at 50 MHz Fm what is the optimum length of this antenna to be 50 ohm.
What the length if the freq. = 100 MHz.
Thanks.

[Alun]

For a 1/2 di-pole antenna the impedance is noramlly about 73ohms, to get any lower than this you need to make a  yaghi array, or use a special transformer called a balun to match it to your transmitter/receiver and transmition line.

Antena theory is not a simple afair, there are degree courses on this sort of thing, here is some information I've found on google:
http://en.wikipedia.org/wiki/Antenna_(radio)
http://www.borg.com/~warrend/guru.html
http://www.radio-electronics.com/info/antennas/dipole/dipole.php
http://www.chem.hawaii.edu/uham/antennas.html

[Staigen]

For a 1/2 di-pole antenna the impedance is noramlly about 73ohms

Maybee i'm wrong, but isn't this for a 1/4 di-pole? Correct me if i'm wrong!

//Staigen

[IBEW-JIW]

i am guessing the 1/2 wave dipole should be around 1.275meters.
would the type of coax used determine the impedance of the antenna?
ie.... rg59....75 ohm
....rg223....50 ohm???

[Alun]

The length depends on the frequency.

[Staigen]

Hi

For 50 Mhz the wavelength is 6 Meters and for 100 Mhz the wavelength is 3 Meters so for a halfwave dipole the length is 3 respective 1.5 meters or somewhat shorter, there is calculations for this! For a ¹/₄wave dipole(?) the length is about 1.5 Meter respective 75 Centimeters(somewhat shorter)!

//Staigen

[prateeksikka]

high frequency means less wavelength and hence lesser height.
 

[SM2GXN]

Hi!

As alun said antenna theory isn't that simple and can't be explaned in a few rows of text.
Dipoles are always 1/2 wave, increased frequency is equal to shorter antenna, if you cut the dipole to 1/4 wave length you no longer have the 73 ohms (theoretically) besides the impedance of a dipole changes with height and how it is mounted against ground. The thicker the wire/radiator is the shorter the physical length is for a given frequency and impedance will change as well, thicker wire/radiator= lower impedance and lower Q= increased bandwidth.
For the 50 ohms there is no need to build a yagi, just form the 1/2 dipole as a inverted vee (up side down V) and the impedance will decrease to aprox. 50-60 ohms but bandwith will suffer and the resonant frequency will be lowered compared to a horizontal mounted dipole.
A 1/4 wave vertical with 45 deg groundplanes will give about 50 ohms also.
Almost any antenna can be matched to 50 ohms by using different matching techniques.
If you calculate  a 1/2 wave dipole for say 100mhz the formula is 150/100Mhz which will give you 1.5 meters totally, I can guarantee that the antenna is to long meaning that the resonant frequency is lower then 100Mhz, that's because of a factor lambda/d (lambda=wave length d=diameter of the wire/radiator).
Wow it's a lot to say about this 

Bjorn

[walid]

Hi Bjorn
if i have a FM Tx transmitting at 100MHz tell me what antennat is suitable for this Tx, wire length, diameter and its impedance and tell me how you calculate it
please note that my antenna is simply a wire not dipole or yagi
thanks

[audioguru]

My little FM transmitter project has a range of more than 2km, with a piece of wire as an antenna #22AWG about 80cm long. I didn't calculate it, I just used a piece of wire that I had. The range would probably be less if the wire was shorter.

[SM2GXN]

Hi walid!

A 1/4 wave at 100 Mhz is approximately 75 centimeters but it's true length for proper resonance depends on the diameter, different ratios of length/diameter will change the resonant frequency of the antenna.
As the diameter of the wire increases the resonant frequency will decrease and the bandwidth will increase.
In most cases the antenna is shorter then the calculated length because of the l/d ratio.
As audioguru say a wire arround 80 cm will do the job but cutting the antenna shorter will "probably" make the antenna even better, can't say if there is any noticeable difference in range between 75 and 80cm

[walid]

thank you all for your responds
I know that any antennat may do the job, but i think that there are some formulas aid in proper design, that when i want to explain this to someone i depend on it. i can't say this general speaking.