Electronics > RF, Microwave, Ham Radio

Dipole Antenna Length

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Fried Chicken:
I am getting mixed reports on the optimum length of a dipole antenna.

My understanding is the two dipoles consist of 1. the ground/shielding, 2. the other/positive.
I've also heard of a rule, 468/frequency in MHz to give an approximation of the length of the wavelength in feet.  I've heard you further divide that by two to get each element length: each element is then 1/2 the wavelength.

Now I found a resource suggesting the length of the total dipole should be half the wavelength, each element a quarter of the wavelength:



For my reference frequency let's do 117MHz, so 468/117MHz gives a wavelength of 4 feet.

If I follow the Antenna Dipole calculator in SDRAngel I get the following:


So each element is 2 feet, or half the wavelength.  If I follow the youtube video's recommendation, each element of the dipole should be 1 foot, or 1/4 the wavelength.

I'm at a loss and am on the verge of breaking out the university physics textbook.  What works best here?

A.Z.:
in meters it's easier

Lambda=300/MHz

and then apply a correction factor to deal with the conductor speed and the end effect, assuming copper you will then have

Lambda=Lambda*0.965

at this point, for a dipole your total length will be

TotLen=Lambda/2

and each arm length will then be

ArmLen=TotLen/2

all the above in ideal conditions, you will usually cut the arms longer and then, trim them equally to resonance

RFDx:
Stick to the calculator in SDRangel.
Length (not wavelength) of a lambda/2 dipole antenna in feet is ~468/frequency, with frequency in MHz. For 117MHz both arms of the dipole would have 2 feet, 4 feet in total for the whole antenna.

A.Z.:
is it better learning the calcs or using a calc w/o knowing how it works ?

Oh and then Lambda=Wavelength in case you weren''t aware; what's next the dipole gain ? Well, at 0.6 Lambda height it's more than 7 dBi, now maybe you'll need to search for dBi :P

radiolistener:

--- Quote from: Fried Chicken on April 11, 2024, 06:18:32 pm ---I am getting mixed reports on the optimum length of a dipole antenna.

--- End quote ---

Half wavelength dipole resonant frequency is very close to a half wavelength. It should be trivial from antenna name.  :)
It is not exactly half wavelength but very close.

Actual frequency may vary depends on the wire thickness, presence of isolation material around wire (include air humidity, etc) and height above ground.

You can precisely tune your dipole with antenna analyzer by cutting wires step-by-step.

Also note that antenna feeder may be a part of your antenna, especially if you're don't use balun to match half wavelength dipole with coax cable and don't use RF-chokes to cut-off common mode currents. In that case coax cable length and geometry may significantly affect your antenna resonant frequency.

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