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Author Topic: Inverted V+capacity hat: unequal currents in capacity hat wires?  (Read 1273 times)
JAHAM2BE
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« on: April 30, 2013, 08:36:41 AM »

I'm considering making a wire inverted V antenna off of my balcony. (Note, if you've been following my posts, that this is different than the other aluminum-tubing vertical dipole project.) The inverted V will be shortened with loading coils and capacity hats.

For mechanical reasons the capacity hats will not be at right angles to the radiator; instead, the radiator wire will come away from the capacity hat at an angle. I originally thought this was not a problem, but modeling the antenna in 4nec2 I noticed that the currents in the capacity hat wires are no longer uniform when the radiator wire comes off at a non-right-angle from the plane of the capacity hat.

As I understand it, the capacity hat normally (when at right angles to the radiator) does not radiate because all the equal currents in the hat cancel. But if the currents are not equal, then does this mean that the capacity hat will radiate? In practice, what implications does this have? Is this a bad thing?
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WB6BYU
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« Reply #1 on: April 30, 2013, 10:28:11 AM »

Similar to the case where your radial wires are slightly unbalanced or not
exactly aligned - there will be some radiation.

Is it a problem?  Probably not - you still have RF radiated.  It may shift the
pattern or efficiency slightly, but not enough to make a difference in the
real world.  But if you wanted to you probably could figure out a way to
vary the capacity hat wire lengths to equalize the currents.

It has been my experience that many antennas are not nearly as critical
in construction and layout as the articles describing them might suggest.
Consider a bent dipole:  the traditional books all showed dipoles as being
straight, even though in practice they often had some droop.  But before
the advent of computer modeling, a straight element was the easiest to
analyze.  But a lot of us put up and used dipoles with bends in them:  they
still worked fine in spite of the fact that you couldn't find that particular
shape in a book.  Now we have the capability to model them in much more
detail and can demonstrate that they still work, even though you still won't
find one with exactly the dimensions you need in a book somewhere.

So try modeling your antenna with a standard set of capacity hat wires at
right angles to the wire, then change it to how you are proposing to build
it , and see if you see any significant difference.  Likely there will be more
differences due to the interaction with the building than due to the angle
of the capacity hats.

That's often a useful approach, especially with antennas in a limited space:
put up what you can manage and use it.
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K3VAT
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« Reply #2 on: April 30, 2013, 10:32:27 AM »

*composed the same time as the previous posting *

...
As I understand it, the capacity hat normally (when at right angles to the radiator) does not radiate because all the equal currents in the hat cancel. But if the currents are not equal, then does this mean that the capacity hat will radiate? In practice, what implications does this have? Is this a bad thing?

Capacity hats, whether employed in a monopole vertical or at the ends of a doublet do in fact radiate.  Anytime one has current flowing through the hat components you're generating a flux field and hence radiation.  The key is one needs to design and build the hat(s) so that the antenna system is balanced: currents in one side of the radiator's cap hat swing positive, the corresponding other side swinging negative (with the same magnitude absolute value).  So the net effect, radiation-wise, is that in the far field the radiation vectors are nullified.  The resultant radiation is then determined by the topological configuration of the main radiator element and by other factors such as height above ground (as for a horizontally polarized dipole).

To function properly, cap hats don't need to be at right angles to their main radiator element.  They can be angled.  Please see ON4UN LowBand DX'ing Text, section 3.6.2 of chapter Nine.  There are diagrams to help in the visualization.  The key here is that both sides of the cap hat wire have the same vertex angle.

GL, 73, Rich, K3VAT


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RFRY
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« Reply #3 on: April 30, 2013, 11:38:13 AM »

The key is one needs to design and build the hat(s) so that the antenna system is balanced: currents in one side of the radiator's cap hat swing positive, the corresponding other side swinging negative (with the same magnitude absolute value).  So the net effect, radiation-wise, is that in the far field the radiation vectors are nullified

But such a condition will not exist for the wires in a cap hat connected to a common point (e.g., the top of a monopole).  The polarity of the r-f currents on all of those cap hat wires will be ~the same, at each instant of time, because all of those wires are driven by the same source.

The cancellation of the far fields radiated by the cap hat takes place because those currents are flowing in opposite physical directions on each side of each in-line pair of wires in that cap hat.

R. Fry
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