Radiation resistance...

[Externet]

Hi all.
Never properly grabbed the concept of antenna radiation resistance and how is it calculated...

Is the formula
Rr = 1580 (He ÷ wavelength)²
valid ?

Rr = radiation resistance,
He = effective height, expressed in fractions of a wavelength, of the distributed common-mode current.

Some clarification of 'distributed common-mode current' please ?

And; is a lower or a higher value of radiation resistance preferred ?

 

[redcrystalline]

hi
as i know resistance=v/i
and based in this formula radiation resistance show us radiated power and it can differ on different antenna,
usually designers dont work with this parameter, they usually work with efficiency and input impedance efficiency show the ratio of radiated power to total power and in ideal case it is 1 that mean all the power is radiated and the input impedance is pure real.
usually we want input impedance of 50 ohm.
based on this it dosent matter lower or higher value
regards

 

[FvM]

Is the formula
Rr = 1580 (He ÷ wavelength)²
valid ?

Nice guessing game. A more reasonable question could be "do you know for which antenna type this radiation resistance formula is valid"? Because each antenna type has a different radiation resistance.

My guess is, you are talking about an electrically short monopole. As far as I know the expression is exactly valid for an inifinitesimal monopole, you find the derivation in antenna theory text books like Balanis. Personally I'm not exceedingly motivated to do the calculation. On a more intuitive theory level, I find it important to understand what radiation resistance is.

An electrically short antenna has a mostly reactant input impedance with a small real component which is ideally only the radiation resistance (= no internal antenna losses). In this case you'll want of course to maximize the radiation resistance.

 

[Externet]

Thank you, FvM.
I can assure am not playing any guess game. This concept has eluded my understanding.

OK, so for a plain 40 metres wavelenght halfwave horizontal dipole, at 5 metres over the ground, presenting 1.3 SWR on 50Ω is there another formula to calculate its radiation resistance ?

Does "maximizing the radiation resistance" means a higher number is desirable ?

 

[FvM]

The radiation resistance of your example antenna can be surely not described with a formula for an electrically small antenna, because a halfwave antenna isn't electrically small. Similarly increasing the radiation resistance isn't a specific problem in this case.