Radiation Resistance of an off-centre fed dipole

[alrubaie]

Hi,

I'm trying to find the input impedance of an OCF dipole and I need a sanity check by one of the more experienced guys here. Now, I have assumed that the voltage and current distribution along the line is always the same, no matter where you feed the antenna at [1]. The radiation resistance doesn't seem to be affected, I was looking at the notes in [2]. So that leaves us with the sin^2 (\beta l/2) term in equations 9.37-9.38, controlling the input impedance.

Am I making sense?

Thanks,
Hasan[1] http://www.radioelectronicschool.net/files/downloads/ocfdipole.pdf
[2] http://www.ece.mcmaster.ca/faculty/nikolova/antenna_dload/current_lectures/L09_Dipole.pdf

 

[Averagesupernova]

I don't see how you can say that the radiation resistance does not change along an antenna. Radiation resistance IS the ratio of voltage and current. The few minutes I took to look at link #1 told me (I looked at it very briefly and incompletely) shows this. The link talked about feeding an antenna anywhere as to avoid the high voltage points. Pretty much says in black and white that the voltage and current distribution along the line is not the same.

 

[litup]

I got this from an antenna link:
Now, what happens if we feed the dipole off-center? The feedpoint impedance changes, but
the radiation resistance does NOT, because the latter is defined at a point of maximum current.
Another example is a folded dipole: the input impedance may be 300 ohms, but the actual
radiation resistance of the antenna is that of both wires in parallel, which is 75 ohms.

So, while the resistive component of the feedpoint impedance SOMETIMES is due to the radiation
resistance, it isn't always. If you limit your analysis to dipoles not more than 1/2 wavelength long
that are series-fed in the center, when the other losses are low, it's a good start. But once the
dipole is any longer than 1/2 wavelength, maximum current is no longer in the center.

http://www.eham.net/ehamforum/smf/index.php?topic=73648.0;wap2

 

[alrubaie]

The V/I ratio is definitely different at different points giving us different input impedances. As for the radiation impedance, it seems to me that it's analogous to the characteristic impedance of a transmission line which is geometry specific. I might be mistaken.

I don't see how you can say that the radiation resistance does not change along an antenna. Radiation resistance IS the ratio of voltage and current. The few minutes I took to look at link #1 told me (I looked at it very briefly and incompletely) shows this. The link talked about feeding an antenna anywhere as to avoid the high voltage points. Pretty much says in black and white that the voltage and current distribution along the line is not the same.

 

[alrubaie]

Nice, so this confirms the radiation impedance part. Problem is my analytical solution is not matching the simulation. I'll play with it and see if I can get them to match.

I got this from an antenna link:
Now, what happens if we feed the dipole off-center? The feedpoint impedance changes, but
the radiation resistance does NOT, because the latter is defined at a point of maximum current.
Another example is a folded dipole: the input impedance may be 300 ohms, but the actual
radiation resistance of the antenna is that of both wires in parallel, which is 75 ohms.

So, while the resistive component of the feedpoint impedance SOMETIMES is due to the radiation
resistance, it isn't always. If you limit your analysis to dipoles not more than 1/2 wavelength long
that are series-fed in the center, when the other losses are low, it's a good start. But once the
dipole is any longer than 1/2 wavelength, maximum current is no longer in the center.

http://www.eham.net/ehamforum/smf/index.php?topic=73648.0;wap2

 

[Antiphon]

The radiation resistance does not depend on the feed point directly; it depends on the current distribution only.

However, the current distribution will depend on the feed point.

 

[alrubaie]

Yes, if you look at the applet in here, the end-fed antenna and center-fed antenna have different current distributions. Then the radiation resistance will change , now I have to rewrite the current distribution equation (no longer sinusoidal). Doesn't look like a trivial task! I am wondering if someone else had done it already.

The radiation resistance does not depend on the feed point directly; it depends on the current distribution only.

However, the current distribution will depend on the feed point.

 

[alrubaie]

Inaccurate; I take it back.

(no longer sinusoidal).