[Elecraft] Random wires

[David Gilbert]

No ... it is *not* the radiation resistance.  It is the feedpoint 
resistance.  They are not the same thing and nowhere in the section of 
the ARRL Handbook that discusses folded dipoles does it call that 300 
ohm feedpoint impedance the radiation resistance.   Go read the section 
on radiation resistance (section 3-2 or thereabouts) and you will note 
that discussion refers only to a horizontal half wave antenna being 
roughly 70 ohms and does not distinguish between whether it is a single 
wire or a folded one.

Let's try another way of looking at it.  Let's say you have a single 
half wave wire and you feed it in the middle.  The radiation resistance 
is 70-some ohms and the feedpoint resistance is equal to the radiation 
resistance plus any ohmic losses.  If you then decide to feed the thing 
several feet off center the feedpoint resistance changes but you haven't 
changed the radiation resistance at all ... it's still just a half wave 
dipole.

The same thing happens if you make the dipole a folded dipole.  It's 
still just a horizontal half wave antenna, albeit with a built in 
transmission line that multiplies the original feedpoint resistance by a 
factor of four.  It is fundamentally no different than any other 
impedance transformation scheme in relationship to the radiation resistance.

Dave   AB7E




On 10/1/2013 6:38 PM, dave wrote:
>
> I don't understand the disagreement over the radiation resistance of a 
> folded dipole. ARRL Antenna Book, 19th edition, page 6-1, plainly 
> states that a folded dipole will have an impedance of approx 300 ohms. 
> This has been widely known for decades. This is for a folded dipole up 
> in the air, ran horizontally. This *is* the radiation resistance. It 
> is approx 4x the impedance (radiation resistance) of a regular, 
> unfolded, dipole.
>
> 73 de dave
> ab9ca/4