antenna-theory.com

[admin]

I've added a page describing Spiral Antennas, their properties, design and implementation here:

http://www.antenna-theory.com/antennas/travelling/spiral.php

I've also added some information in audio visual format here at youtube:
http://www.youtube.com/watch?v=JaJkZgiyTas

The information on the two pages is somewhat overlapping; somewhat complimentary.

Basically, the spiral antenna is one of the most fascinating antennas due to its wideband properties and circular polarization. Putting it on the website has been sorely missed up until now.

[admin]

Spiral Antennas are also useful in describing the infinite balun, wherein you use the feeding coaxial cable as a part of the antenna itself. It is described on this page:
http://www.antenna-theory.com/definitions/infinite.php

[tensor20]

Hello,

the spiral antenna is a traveling wave antenna. Thanks for the posts. I will study them in detail.

I have a trivial question: in the short dipole (L<<lambda)
the current distribution on the wire is triangular. That means that the current is flowing in the same direction at every instant of time but it is locally different in strength. At exactly half a cycle in time the current is everywhere zero. After half a cycle, the current change direction.....

Is this current distribution representing a current standing wave or is it a traveling current?

The sign of the current represents its direction which induces me to thing that it is flowing....but then it cannot be a standing wave if it is flowing....

Thanks
tensor20

[bigSteve]

Current flows back and forth on the dipole. The charge is stored on the ends and changes in magnitude sign back and forth between the two ends. The flow of charge is current.

A standing current wave still involves current flow. To understand this, note that a standing wave is caused by two sinusoids traveling in opposite directions to each other. So yes, charge must be moving if there is current, and if the current is changing then charge is moving. Hence, there is current flow.

[florescent]

Dear Admin

There is another type of spiral antennas as the link below

http://commons.wikimedia.org/wiki/File:Archimedean_spiral.svg

For this antenna design, one of the polarity locates at the end of the outer winding. The other polarity locates in the middle (it can be extended to the other side of the planar)

Big difference between this antenna with what you introduce is the current flow between two adjacent turns; same direction for all turns for this type of antennas instead of opposite direction against adjacent turns for the introduced antenna type. This difference will make different electro-magnetic fields.

Would you explain the difference in theory?

Thank you