Helically-wound Monopoles
 

On Wednesday, March 23, 2011 4:55:40 PM UTC-6, Richard Fry wrote:
A few years ago there was some discussion on r.r.a.a. about helically-
wound, normal-mode monopoles, and the rather common expectation that
they had higher gain than a linear monopole of the same physical
height (and with other things equal).

I was always under the assumption that the helical whip was
less efficient than the same height whip loaded with a lumped coil.
I NEVER thought the helical whip had the advantage. And so far my
experience with the mobile whips seems to show that an efficient
lumped coil has the advantage.

A recent NEC-2 analysis of this topic might be of interest:
http://i62.photobucket.com/albums/h8...r_Monopole.gif

Where is the coil on the linear monopole? Seems to be a flawed
example to me. The helical example is loaded, but the linear is not?
What kind of comparison would that be to jive with the real world?
Of course, most will find that the location of the lumped coil
on a whip will effect efficiency and current distribution.
IE: the center loaded whip will generally have an advantage over
a base loaded whip.

A helical loaded whip can be a decent antenna, but I've always
had better results using the lumped coil.
Also, a helical whip needs some capacitive loading "IE: decent
length whip above the helical windings" to live up to it's
expectations. The helical whips with windings all the way to
the top doesn't do so well.

Also, the manner in which a helical whip is wound will effect
current distribution. It's best to maximize the windings near
the top rather than use a constant pitch the whole whip.
Much in the same manner that a lumped coil whip usually does
better with a center or higher mounted coil vs a base load.

If a helical winding was as efficient as a lumped coil, I
would not have converted my helical wound whips to lumped
coils. And if this were true, I would not have noticed any
difference when I did. But I did notice a difference.
So I think I will stick with the lumped coils.

I don't foresee a mad rush by the bugcatcher users to
convert to ham sticks any time soon. :/

Helically-wound Monopoles
 

Where is the coil on the linear monopole? Seems to be a flawed
example to me. The helical example is loaded, but the linear is not?
_______________

Regardless of the loss in a loading coil for the linear monopole (and
actually the helix would need one also), the salient point here is that even
though the conductor in the helix is 3.14 X longer than in the monopole, the
radiation resistance and pattern of the two forms for a given frequency is
the same for the same end-end length -- as shown in the comparison.

It is the relation of the radiation resistance to the other resistances
(mainly the matching network loss and the r-f ground terminal loss) that
determines the radiation efficiency of the complete antenna system.

Some might think that the radiation resistance of the helix in this model is
determined by the length of the conductor from which it is wound, but that
isn't the case.

Helically-wound Monopoles
 

Hello,

On 24 mar, 17:36, wrote:
On Wednesday, March 23, 2011 4:55:40 PM UTC-6, Richard Fry wrote:
A few years ago there was some discussion on r.r.a.a. about helically-
wound, normal-mode monopoles, and the rather common expectation that
they had higher gain than a linear monopole of the same physical
height (and with other things equal).

I was always under the assumption that the helical whip was
less efficient than the same height whip loaded with a lumped coil.
I NEVER thought the helical whip had the advantage. And so far my
experience with the mobile whips seems to show that an efficient
lumped coil has the advantage.



A recent NEC-2 analysis of this topic might be of interest:
http://i62.photobucket.com/albums/h8..._Linear_Monopo...

Where is the coil on the linear monopole? Seems to be a flawed
example to me.

When you put a losless inductor in series, gain and radiation pattern
doesn't change, so the simulation is useful.

The helical requries less series inductance as the capacitive
component is less (-j2080 versus -2970), so you save on the inductor
loss (for the helical case).

The helical example is loaded, but the linear is not?
What kind of comparison would that be to jive with the real world?
Of course, most will find that the location of the lumped coil
on a whip will effect efficiency and current distribution.
IE: the center loaded whip will generally have an advantage over
a base loaded whip.

A helical loaded whip can be a decent antenna, but I've always
had better results using the lumped coil.
Also, a helical whip needs some capacitive loading "IE: decent
length whip above the helical windings" to live up to it's
expectations. The helical whips with windings all the way to
the top doesn't do so well.

Also, the manner in which a helical whip is wound will effect
current distribution. It's best to maximize the windings near
the top rather than use a constant pitch the whole whip.
Much in the same manner that a lumped coil whip usually does
better with a center or higher mounted coil vs a base load.

If a helical winding was as efficient as a lumped coil, I
would not have converted my helical wound whips to lumped
coils. And if this were true, I would not have noticed any
difference when I did. But I did notice a difference.
So I think I will stick with the lumped coils.

I don't foresee a mad rush by the bugcatcher users to
convert to ham sticks any time soon. :/

For me, the disadvantage of the helical is the thickness and weight of
the construction.

Wim
PA3DJS

Helically-wound Monopoles
 

wrote
Where is the coil on the linear monopole?

Sorry, I forgot to address this question directly. Both radiators
were base fed.

As shown at the top of that comparison page, a total of 30 ohms was included
in each NEC model to account for the loss in the connection to r-f ground,
and in the loading coil. That is why the maximum gain of both models is
about -19.5 dBi, rather than 4.77 dBi.

NEC codes has disadvantages when simulating curved antennas like helix, since straight wire segments are used to model antenna geometry,which introduces a numerical error at the very beginning of the calculations, so numerical results become difficult to interpret. Curved MoM codes like AN-SOF should be used in this case.