11 meter dipole help???

[BOOTY MONSTER]

Coupling to earth (and things sitting on the earth, like buildings, trees, etc).

A dipole in "free space", which is about as practical as a genuine isotropic antenna, has a feedpoint impedance right around 70 ohms. Closer to the earth, at easily attainable heights - say at 30 feet or so on 11 meters - it'll be lower; somewhere closer to 50 ohms.

thank you beetle .
i thought a dipole needed sloped radials (aka starduster) to tune to 50 ohms .

 

[Needle Bender]

any dipole higher then about a 1/2 wave above average ground wont really know the diference by much. maybe an ohm or two depending on how good or bad the ground is. a 11m dipole at 30 feet will still be about 70 ohms but dropped to 10 feet will be about 60ohms. thats what i saw when i borrowed a mfj analizer. when i pulled the middle over so it made a vee at about 150 degrees the swr came down to 1.2 to 1 from 1.7 to 1 at 27 feet high

 

[W5LZ]

If by 'slopped radials' you mean making the angle between the dipole's 'legs' less than 180 degrees, then, yes, that will make it's input impedance lower. That will happen at any height, even in 'free space'. It's a very common practice.
- 'Doc

 

[wavrider]

"A dipole antennas impedance increases as its distance from the ground increases."

can someone explain how that happens ?

Dipole Antenna Radiation Patterns by WB4YJT

Long read but about halfway down is a chart showing feed point impedance vs height above ground for many of the popular frequencies.

 

[Beetle]

thank you beetle .
i thought a dipole needed sloped radials (aka starduster) to tune to 50 ohms .

I've never seen a dipole with radials. By definition, a dipole is a piece of wire half a wavelength overall, cut in the middle and fed at that point. It's a complete antenna by itself, and its characteristics are well enough known that the dipole can be used as a reproducible reference antenna.

Dipoles CAN be configured vertically, but if they are true dipoles as defined above and not "sleeve" dipoles, feeding them properly can be a challenge. The feedline must run straight away from the feedpoint at a right angle for a half wavelength or more to avoid coupling the outer shield surface.

Google up some pictures of dipoles. You shouldn't see any radials because they aren't needed. Or better yet, check a good antenna book.

 

[Needle Bender]

i think booty meant that he thought a dipole cant be both straight and 50ohms but needs an angle like the starduster or it will be higher like 70ohms

 

[Beetle]

i think booty meant that he thought a dipole cant be both straight and 50ohms but needs an angle like the starduster or it will be higher like 70ohms

Yeah - I didn't mention the inverted vee configuration, but the wires aren't "radials" there either.