Slow wave antenna design for top band
 

Denny wrote:
Dunno... Interesting problem...

Not really - a helix can slow down a wave by
a factor of 50 or more. Approximately a foot and
a half of 75m Texas Bugcatcher stock will be self-
resonant at 4 MHz. That's 90 degrees of antenna
stuffed into about a foot and a half of coil.
90 degrees at 4 MHz is 61.5 feet. The VF of that
coil is 1.5/61.5 = 0.024. Actually, pretty simple.
--
73, Cecil http://www.w5dxp.com

Slow wave antenna design for top band
 

On 25 Oct, 14:47, Cecil Moore wrote:
Denny wrote:
Dunno... Interesting problem...

Not really - a helix can slow down a wave by
a factor of 50 or more. Approximately a foot and
a half of 75m Texas Bugcatcher stock will be self-
resonant at 4 MHz. That's 90 degrees of antenna
stuffed into about a foot and a half of coil.
90 degrees at 4 MHz is 61.5 feet. The VF of that
coil is 1.5/61.5 = 0.024. Actually, pretty simple.
--
73, Cecil http://www.w5dxp.com

Yes Cecil I see where you are coming from
but and it is a big but the turns must be
exposed to radiate and it is that that one
must not lose sight upon
Art

Slow wave antenna design for top band
 

"art" wrote in message
ups.com...
On 25 Oct, 14:47, Cecil Moore wrote:
Denny wrote:
Dunno... Interesting problem...

Not really - a helix can slow down a wave by
a factor of 50 or more. Approximately a foot and
a half of 75m Texas Bugcatcher stock will be self-
resonant at 4 MHz. That's 90 degrees of antenna
stuffed into about a foot and a half of coil.
90 degrees at 4 MHz is 61.5 feet. The VF of that
coil is 1.5/61.5 = 0.024. Actually, pretty simple.
--
73, Cecil http://www.w5dxp.com

Yes Cecil I see where you are coming from
but and it is a big but the turns must be
exposed to radiate and it is that that one
must not lose sight upon
Art

try a ctha that may do what you want.

Slow wave antenna design for top band
 

art wrote:
Yes Cecil I see where you are coming from
but and it is a big but the turns must be
exposed to radiate and it is that that one
must not lose sight upon

I can only wish that my 75m Bugcatcher coil
had all of its losses going into radiation. :-)
--
73, Cecil http://www.w5dxp.com

Slow wave antenna design for top band
 

"Cecil Moore"
Approximately a foot and a half of 75m Texas Bugcatcher
stock will be self-resonant at 4 MHz. That's 90 degrees of
antenna stuffed into about a foot and a half of coil.
__________

But that self-resonant coil is not the electrical equivalent of a 90-degree
linear antenna. A coil by itself is not a very efficient producer of EM
radiation, whether self-resonant or not.

RF

Slow wave antenna design for top band
 

Richard Fry wrote:
"Cecil Moore"
Approximately a foot and a half of 75m Texas Bugcatcher
stock will be self-resonant at 4 MHz. That's 90 degrees of
antenna stuffed into about a foot and a half of coil.

But that self-resonant coil is not the electrical equivalent of a
90-degree linear antenna. A coil by itself is not a very efficient
producer of EM radiation, whether self-resonant or not.

I'm not saying anything about radiation efficiency
just commenting on the phase relationship between
the forward wave and reflected wave.

At the first self-resonant frequency, the feedpoint
impedance is purely resistive indicating that the coil
is indeed electrically 90 degrees long.
--
73, Cecil http://www.w5dxp.com

Slow wave antenna design for top band
 

"Cecil Moore"
At the first self-resonant frequency, the feedpoint
impedance is purely resistive indicating that the coil
is indeed electrically 90 degrees long.
___________

But that doesn't make the coil an "antenna," as you described it
for that condition.

RF

Slow wave antenna design for top band
 

Richard Fry wrote:
"Cecil Moore"
At the first self-resonant frequency, the feedpoint
impedance is purely resistive indicating that the coil
is indeed electrically 90 degrees long.

But that doesn't make the coil an "antenna," as you described it
for that condition.

We know that helical antennas exist and radiate. The
form factor of a self-resonant 75m bugcatcher coil is
helical and will radiate (even though very inefficiently)
and contacts are more than likely possible. What is your
definition of "antenna"? A self-resonant 75m bugcatcher
coil is not all that different from an Isotron "antenna".
--
73, Cecil http://www.w5dxp.com

Slow wave antenna design for top band
 

"Cecil Moore"
We know that helical antennas exist and radiate. The
form factor of a self-resonant 75m bugcatcher coil is
helical and will radiate (even though very inefficiently)
and contacts are more than likely possible.
What is your definition of "antenna"?
___________

The sole purpose of a helical (or any other) antenna is to radiate and/or
receive EM waves with pattern, polarization and other characteristics
suitable for the application.

A bugcatcher coil is not designed to be an antenna, and radiates only
incidentally.

RF

Slow wave antenna design for top band
 

Richard Fry wrote:
A bugcatcher coil is not designed to be an antenna, and radiates only
incidentally.

Maybe you missed what I am saying. Take a 75m bugcatcher
coil and add turns to it until it is self-resonant at
4 MHz. You will have a very inefficient helical antenna.
Radiation from such a helical antenna would be purposeful,
not incidental. Heck, 75m hamsticks are less than 1%
efficient and they still call them "antennas".
--
73, Cecil http://www.w5dxp.com