dipole antenna formula

[sssaaa]

hi to all
i want a dipole antenna formula for FM band.

[windoze killa]

What do you mean by formula? Do you want a half  wave or quater wave dipole?

[SM2GXN]

150/Freq(MHz)=Dipole length in meters
I would say that quarter wave dipoles are very very rare.

[windoze killa]

You are correct. They are very rare and also don't work all that well. But they are useful in confined spaces.

[sssaaa]

I want half wave dipole formula  for FM band 76-108 MHZ.

[windoze killa]

I assume when you say formula you want to know what dimensions you need to use. If this is the case then you should consider the centre frquency that you want. To make things easy we will use 100Mhz which is close enough to the centre frequency.

Now that we have 100Mhz as the frequency one full wave length at that frequency is 3 metres, so a half wave dipole will require 2 lengths of 1.5M. Most antennas for the FM band are made from 300ohm ribbon. You slice down the middle of one end 1.5M and spread them out horizontally. For it to work the best you need to make sure that it is perpendicular to the direction to the transmitting antenna.

Hope this helps a bit.

[sssaaa]

i want to make two antennas
one for 82MHZ and other for 86MHZ

[SM2GXN]

Hi sssaaa!

Did you read our posts?
windoze killa and I wrote about how to calculate the length.
The formula is in there, just calculate the antenna at center 84Mhz it will do just fine for listening no need for two antennas, 150/84=Half wave dipole length in meters, cut the length in two pieces solder a 75 ohm coax or a twin lead to the two ends depending on the transmission hang the antenna horizontal or vertical.
If you want two antennas just use the formula above and insert the frequency of interest.

[sssaaa]

yes SM2GXN i read it but i found on search many formulas
like
468/frequency

147/frequency

142.5/frequency

and you gave me 150/frequency

which one is correct?

[SM2GXN]

Hi sssaaa!

As I said for listening it is not that critical, those formulas take conductor thickness as well as speed of the radio wave into acount. One can say that as the antenna element gets thicker the total length decreases and the antenna will cover a greater bandwith. If the antenna is to be used as transmitting antenna you will have to tune the antenna by cutting to proper length and resonance.
468/Mhz= length in feet is a formula for guys living in US  ;D