Assume you have tons of wire, string, and hilly treed land, and want
to listen to FM 90 Mhz. Impressed with the 1/2 wave dipole nulling out
competing stations, you think, "I am the owner of tons of wire, I will
put my wire to work to pull it in even better."

What design will utilize my ton of wire at such a short wavelength?
Don't tell me to trade my ton of wire for Yagi parts, as we are on a
deserted island. Assume mom will be angry if I cut the wire more than
once or twice.

En Dan Jacobson va escriure en Sun, 04 Apr 2004 05:52:03 +0800:

Assume you have tons of wire, string, and hilly treed land, and want
to listen to FM 90 Mhz. Impressed with the 1/2 wave dipole nulling out
competing stations, you think, "I am the owner of tons of wire, I will
put my wire to work to pull it in even better."

What design will utilize my ton of wire at such a short wavelength?
Don't tell me to trade my ton of wire for Yagi parts, as we are on a
deserted island. Assume mom will be angry if I cut the wire more than
once or twice.

A nice rhombic with ladder feedline? You will also need a
resistor though. Do you have a pencil in that island?

Toni

Dan Jacobson wrote:
"What design will utilize my ton of wire at such a short wavelength
(3.33 meters)?"

Dan also indicated he would like to minimize cuts in the wire.

An obvious contender would be an axial-mode helix antenna which just
stretches out a coil of wire with no cuts at all.

The inventor, J.D. Kraus, includes chapter 8 of his 3rd edition of
"Antennas", The Helical Antenna: Axial and Other Modes, Part 2.

Kraus has an example in Fig. 8-54(a) on page 281 of an 80-turn
axial-mode (end-fire) helix with a gain of "24 dB". (This is surely dBi
as Kraus doesn`t specify dBd.)

There would also surely be a few pounds of wire left over unless the
wire is very large indeed. It might be bonded for a reflector or other
purposes.

Best regards, Richard Harrison, KB5WZI

Dan Jacobson wrote:
"I will put my wire to work to pull it in even better."

Fitst, you need a line-of-sight path to the desired station. Then, you
need an obstruction in the response to unwanted signals. You need enough
desired signal edge to capture the FM detector and to suppress noise the
undesired signals might add to the mix.

An axial mode helical antenna uses a lot of wire wound into a coil which
is pointed at the desired station, directly or indirectly. Diameter and
coil pitch are optimized for axial gain and directivity.

If a helical coil is small in terms of wavelength, its field is maximum
perpendicular to the coil axis. Such a self-resonant coil makes an
omnidirectional antenna in a plane perpendicular to the coil axis.

We have a rubber ducky as an example of an omnidirectional helix which
is inefficient, limited in bandwidth, but convenient for handie-talkies.
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.

Best regards, Richard Harrison, KB5WZI

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.