> John Magliacane wrote:
>
> >
> > I believe the Normal Mode Helix, as is used in HI-VHF television
> > broadcasting, is a travelling wave antenna, rather than a
> > resonant standing wave antenna, such as a dipole.
David Woolley wrote:
> You are thinking of axial mode helices. Normal mode helices are
> the most common antennas on VHF hand-helds, and are often called
> "rubber ducks".
I suppose various antennas go by the same name, even though their
operation is considerably different.
I was referring to a Normal Mode Helix also known as a "Side Fire
Helix". It's described in the Antenna Engineering Handbook
(Johnson), and used in horizontally and circularly polarized
omni-directional applications (mostly VHF and UHF-TV broadcast).
It's a traveling wave antenna where the helix is wound around a
supporting vertical mast that serves as a reflector.
Axial mode helices are a different beast. They are typically used to
provide broadband, UNI-DIRECTIONAL, circular polarization (OSCAR
satellite communications). Invented by John Kraus, W8JK.
I always considered a "rubber duck" to be nothing more than a
continuously loaded monopole, and is considerably different in
operation from the other two helical antennas described above.
> > When Skin Effect losses become an issue in RF conductors and
> > inductors, Litz wire is typically used. I see no reason why a
>
> One has to ask why this isn't used in all the commercial
> "miracle" antennas.
Look at some of the older antennas used for FM broadcast, such as the
Collins or RCA ring dipole arrays. These are physically short,
capacitively loaded, gamma matched dipoles. Some implementations of
these antennas place a number of identical dipole rings in parallel
with one another to reduce ohmic losses and improve efficiency.
These antennas may not be made from Litz wire, but the underlying
concept of distributing RF current among closely placed parallel
conductors in an effort reduce resistive losses is still the same.
73, de John, KD2BD
Visit John on the Web at:
http://kd2bd.ham.org/
.
.
.
.
____________________________________________________________________________________
Never miss a thing. Make Yahoo your home page.
http://www.yahoo.com/r/hs
_______________________________________________
Elecraft mailing list
Post to: Elecraft@mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
