[Elecraft] Random wires

[dave]

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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