dipole center space?
 

hi


i was wondering , currently my dipole is fed via a 5ft section of 450ohm
ladderline. it goes up a pvc mast that i have 2 eye bolts on the top
of ...

i've spaced the eyebots such that they are about the same space as the
latter line conductors , and my dipole wires attach to the eye bolts


in different dipole constructions i've often noted that this center
spacing is often at different gaps, typically larger(coiencidentally ).

so if m spacing was say larger (now i have other than 450 ohm feed i'd
guess

what happens

so my ladder is 450 ohm if my gap is say 2"(think thats 600ohm?) how
does the ohms add"" up

and does this typically increase /decrease loss (antenna/and feedline)

does it alter resonance

does the voltage/current nodes move out?


i got mixed up

thanks

yes, the spread in the center insulator does affect the resonance and
current distribution and match and all that stuff... but for typical spacing
changes on hf the effect is small enough that you can normally ignore it.
might as well worry about the effect of capacitance between the larger
conductor of the bolts vs the wire if you are going to worry about the small
spacing change.

"ml" wrote in message
...
hi


i was wondering , currently my dipole is fed via a 5ft section of 450ohm
ladderline. it goes up a pvc mast that i have 2 eye bolts on the top
of ...

i've spaced the eyebots such that they are about the same space as the
latter line conductors , and my dipole wires attach to the eye bolts


in different dipole constructions i've often noted that this center
spacing is often at different gaps, typically larger(coiencidentally ).

so if m spacing was say larger (now i have other than 450 ohm feed i'd
guess

what happens

so my ladder is 450 ohm if my gap is say 2"(think thats 600ohm?) how
does the ohms add"" up

and does this typically increase /decrease loss (antenna/and feedline)

does it alter resonance

does the voltage/current nodes move out?


i got mixed up

thanks

It's the overall end-to-end length that matters most--along with
elevation above ground and some other effects. If the center insulator
is one of those ceramic "eggs" so the two wires form intertwined loops,
you might end up with a couple pF extra capacitance, which is over 2000
ohms of capacitive reactance at 30MHz and even less effect at lower
frequency. You're really unlikely to notice that in a dipole near
half-wave resonance (low feedpoint impedance), though for full-wave
resonance with small wire diameters (high feedpoint impedance) you
_might_ be able to detect the effect. But if you're feeding with 450
ohm line, you presumably are using a tuner, and why on earth would you
be worried about it?

Cheers,
Tom

On 31 May 2005 11:23:36 -0700, "K7ITM" wrote:

It's the overall end-to-end length that matters most

Hi Tom,

Well, I guess it needed to be said.

What is missing here, that is about this insulator business, is that
(if it isn't already obvious) there is wire coming from the
transmission line that has been split out to make the connections at
this insulator. This wire, too - as short as it may be, is
transmitting as much (or as little) as ANY similar length of ANY
section of the purported antenna.

To put it in another perspective (grossly exaggerated for the purpose
of illustration, mind you); let's take a power line insulator that is
three feet long. Now, does anyone imagine that this antenna is
shorter than another antenna by that same three feet simply because
the end-to-end length of both is the same? Perhaps if you haven't
connected your transmission line to it to drive it - but then what
purpose is your antenna?

As Reg might offer, I am merely "stating the bleeding obvious."

73's
Richard Clark, KB7QHC