Normal Mode Helix Antennas
 

Quote below

REALITY CHECK: a form of a normal-mode, helical radiator is commonly used
for FM broadcast transmitting antennas. At their design frequencies, these
elements produce the same _measured_ field strength in their directions of
peak gain as produced by a matched, linear dipole in its directions of peak
gain (over the same path). Some of these FM helical designs have _measured_
input SWR less than 1.15:1 over 15% RF bandwidth, which is most of the
88-108MHz FM band. For the purpose, this performance is far from being
inefficient and/or narrow band.

NEC2 analysis of an FM normal-mode helix shows this, of course. Examples of
it are found in the slide show (item 10) at http://rfry.org .

Reading and heeding Kraus has to take the application into account.
Generalizations lead to misunderstanding.

RF
___________________

"Richard Harrison" wrote:
Larger omnidirectional (normal-mode) helices are used by some VHF/UHF
broadcasters to provide circular polarization, but Kraus says on page
173 of his 1950 edition of "Antennas":

"For normal mode the dimensions of the helix must be small compared to
the wavelength, so that from band width and efficiency considerations
this mode is not readily applicable in practice."

There are handie-talkie suppliers and broadcasters who don`t heed Kraus.

One of Krause main interests was the Helix, and did the most, and best work
on it at the time.
The quote to me seems out of context, more of a comparison of "normal mode"
to what a helix can really do.
Helix is nice, great bandwidth, easy to make, easy matching.
The FM normal mode (shown in reference) is less than 2 turns, so it is being
used primarily to get circular pol.


"Richard Fry" wrote in message
...
Quote below

REALITY CHECK: a form of a normal-mode, helical radiator is commonly used
for FM broadcast transmitting antennas. At their design frequencies,
these
elements produce the same _measured_ field strength in their directions of
peak gain as produced by a matched, linear dipole in its directions of
peak
gain (over the same path). Some of these FM helical designs have
_measured_
input SWR less than 1.15:1 over 15% RF bandwidth, which is most of the
88-108MHz FM band. For the purpose, this performance is far from being
inefficient and/or narrow band.

NEC2 analysis of an FM normal-mode helix shows this, of course. Examples
of
it are found in the slide show (item 10) at http://rfry.org .

Reading and heeding Kraus has to take the application into account.
Generalizations lead to misunderstanding.

RF
___________________

"Richard Harrison" wrote:
Larger omnidirectional (normal-mode) helices are used by some VHF/UHF
broadcasters to provide circular polarization, but Kraus says on page
173 of his 1950 edition of "Antennas":

"For normal mode the dimensions of the helix must be small compared to
the wavelength, so that from band width and efficiency considerations
this mode is not readily applicable in practice."

There are handie-talkie suppliers and broadcasters who don`t heed Kraus.

I wrote, "At their design frequencies, these elements produce the same
_measured_ field strength in their directions of peak gain as produced by a
matched, linear dipole in its directions of peak gain (over the same
path)."

To clarify this for those who didn't read through the slide show I referred
to in the original post, the FM helix would need twice the input power as
the linear dipole to produce the same field peak strength over the same
path. The FM helix is circularly polarized; therefore its peak, net gain
per polarization plane is 1/2 that of a dipole.

Most FM broadcasters in the US are licensed to radiate a certain amount of
horizontally polarized "effective radiated power" (ERP) in the horizontal
plane. Typically they can radiate any amount of vertically polarized power
they choose, not to exceed their h-pol ERP.

The station's ERP is the product of the tx output power, the efficiencies of
their transmission line, channel combiners, filters etc, and their antenna
gain, per polarization. Using a c-pol antenna means that the broadcast
station needs twice the tx output power to produce their licensed h-plane,
h-pol ERP.

RF

Visit http://rfry.org for FM broadcast RF system papers.