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.
73 de dave
ab9ca/4
On 10/1/13 7:38 PM, Jim Brown wrote:
On 10/1/2013 3:42 PM, Fred Jensen wrote:
Isn't this why AM broadcast stations, particularly 50KW
clear-channel stations, employ base-fed half-wave verticals?
No, that's not the reason. The actual reason is that the vertical
radiation pattern is better than a shorter antenna.
They still use radial fields too I think.
Yes. A radial field under a half wave antenna reduces ground losses
(by a dB or two, depending on how bad the ground is), whether the
antenna is fed against it or not. But 180 degrees is not the only
popular height for these clear channel stations -- if you peruse the
FCC database, you'll see many with vertical heights ranging from 180
to 225 electrical degrees. Varying the height shifts the balance
between low angle radiation (for ground wave and long skip) and higher
angle (for medium distances. Making the radiator a bit taller than 180
degrees also lowers the Z at the feedpoint, making it easier to feed.
Dave is right on -- most of those posting have confused feedpoint Z
with radiation resistance. There's a nice graph in the ARRL Antenna
Book showing radiation resistance of a vertical as a function of height.
73, Jim K9YC
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