when does a vertical become vertical dipole?

when does a vertical become vertical dipole?

Post by greg mushia » Sat, 16 Jun 2007 03:33:53

Been out of ham radio since the '60's (raising a family etc) and am
slowly coming back (though given the noise I see in r.r.a.misc I
wonder if I really want to  :-(    ).  

My question is one of trying to understand propagation - with a ground
mounted vertical, if I remember correctly, one treats it as if it's a
half wave dipole, where one leg is above ground and the other is
virtual and is a "ground reflection", hence the importance of good
ground/radials etc.  I assume if I had a balloon up several thousands
of feet and dropped a half wave dipole (still center fed, but hung
from one end), then it would behave like a ground based horizontal
dipole, other than being polarized vertically (ie, no virtual / ground
reflection for the lower half). The question is: when does one make
the transition btwn the two modes, how far off the ground (in terms of
wavelengths)? If a vertical dipole is 10 wavelengths above the ground,
does one still have to consider the ground as part of the antenna? how
about 2 wavelengths? If someone could walk me through a thought
experiment on this I'd appreciate it.

thanks,
greg

ps. yes I've Googled this and found nothing, likewise have and read
ARRL big antenna book + several of their long wire books, likewise, no
mention.

 
 
 

when does a vertical become vertical dipole?

Post by Yuri Blanarovic » Sat, 16 Jun 2007 03:51:51



Quote:> Been out of ham radio since the '60's (raising a family etc) and am
> slowly coming back (though given the noise I see in r.r.a.misc I
> wonder if I really want to  :-(    ).

> My question is one of trying to understand propagation - with a ground
> mounted vertical, if I remember correctly, one treats it as if it's a
> half wave dipole, where one leg is above ground and the other is
> virtual and is a "ground reflection", hence the importance of good
> ground/radials etc.  I assume if I had a balloon up several thousands
> of feet and dropped a half wave dipole (still center fed, but hung
> from one end), then it would behave like a ground based horizontal
> dipole, other than being polarized vertically (ie, no virtual / ground
> reflection for the lower half). The question is: when does one make
> the transition btwn the two modes, how far off the ground (in terms of
> wavelengths)? If a vertical dipole is 10 wavelengths above the ground,
> does one still have to consider the ground as part of the antenna? how
> about 2 wavelengths? If someone could walk me through a thought
> experiment on this I'd appreciate it.

> thanks,
> greg

> ps. yes I've Googled this and found nothing, likewise have and read
> ARRL big antenna book + several of their long wire books, likewise, no
> mention.

The main difference is the interaction between the antenna and the ground in
forming the lobes. Ground mounted vertical will have one fat lobe, as you
start increasing the height the lobes will start splitting, more of them,
with different magnitudes. Also "looking" out for the ground for reflections
further out.
Using demo version of EZNEC and modeling it will demonstrate the effect.

73 Yuri, www.K3BU.us

 
 
 

when does a vertical become vertical dipole?

Post by n.. » Sat, 16 Jun 2007 04:50:44


 If a vertical dipole is 10 wavelengths above the ground,

Quote:> does one still have to consider the ground as part of the antenna? how
> about 2 wavelengths? If someone could walk me through a thought
> experiment on this I'd appreciate it.

Any vertical that is complete, and is clear of the ground qualifies.
Even a 1/4 wave ground plane with radials.
No properly built elevated verticals rely on the ground to complete
the
antenna.
Of course, you can still be effected by ground loss though, if you
are low to the ground, or don't have enough radials for the height
in wavelength you are at.
But a horizontal dipole can suffer the same losses, usually to a
lesser degree. The extra lobes are mainly a function of height above
ground more than anything.
You can have a monopole at 5 ft off the ground, and if it has it's
own elevated radial set, it's a complete antenna and does not
rely on ground to provide the lower half of the antenna.
Of course at that height, you need a lot of radials to avoid the
ground losses, but that's not quite the same as using the ground
itself as the lower half of the antenna, with no elevated radials.
MK
 
 
 

when does a vertical become vertical dipole?

Post by n.. » Sat, 16 Jun 2007 11:55:29


Quote:

> Wrong......

> There was a study in Australia where the earth's influence on a
> horizontal
> antenna showed where the earths influence disappeared rapidly. That
> cannot be
> said of a short vertical. If it was of full length ,as in dipole, I
> suspect
> the earth's influence would be the same as a horizontal antenna.
> Note: horizontal and vertical referes to polarization not to physical
>  orientation of radiators

Well sure, but you can see noticable ground loss with really low
dipoles.
And the vertical examples I've talked about so far, are full size.
When I talk about dipoles suffering from ground loss, I'm talking
about
really low ones.. IE: a 160m dipole at 16 ft usually shows noticable
ground loss vs one at 125 ft, which would be a quarter wave up. Using
close up/down NVIS paths would be the best way to fairly compare..
Running a 160m dipole at 16 ft is like running a 10m dipole at 1 ft
off
the ground as far as height/wavelength... But, some people, due to
restrictions, end up running 160m dipoles at 15-20 ft up and live
with it.. Just cuz they make contacts doesn't mean there are no
ground losses lurking in the program.
MK