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.
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