[Elecraft] Random wires

[Don Wilhelm]

Dave,

You are equating radiation resistance to feedpoint impedance, and that 
is like comparing apples and oranges, they are not the same.

73,
Don W3FPR

On 10/1/2013 9:38 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.
>
> If one were able to construct half of such a folded dipole and arrange 
> it vertically, it would have have an impedance (radiation resistance) 
> of approx 150 ohms. Again, approx 4x the radiation resistance of a 
> standard 1/4 wl monopole.
>
> Modeling with EZNEC is in agreement with the ARRL Antenna Book.
>
> So I think we can safely say that if one were able to construct a 
> vertical that is half of a folded dipole, the feedpoint Z, as well as 
> the radiation resistance, would be approx 4x that of a vertical monopole.
>
> My question is - how do you build one of those?
>
> In EZNEC it is easy. Just make two 1/4 wl elements closely spaced and 
> tie them both to MININEC ground. Place a source in the segment closest 
> to ground on one of the two. Bingo, a vertical that is half of a 
> folded dipole. Feedpoint Z is approx 150+j0 ohms.
>
> I don't think it is quite that simple in the real world.
>