given that the length of a half wave dipole is
calculated by 468 / freq in mhz when velocity factor
is 1, ie 468 / 1.9 is about 246 ft.

i'm sitting here wondering why folks with small
city lots don't use (468 / freq in mhz) * velocity factor,
to construct a much shorter antnenna, particularly on
80m & 160m?

as an example, rg59 coax typically has a velocity
factor of .66, so plugging to the formula,
468 / 1.9 is about 246 ft, * .66 is about 162 ft.

why couldn't 162 ft of rg59 be cut in two, attached
to a 50 ohm, have the remaining ends of the rg59 shorted
together, and hoisted into the air?

so what am i missing? there are no free lunches:-)

larry
kd5foy

larry d clark wrote:
why couldn't 162 ft of rg59 be cut in two, attached
to a 50 ohm, have the remaining ends of the rg59 shorted
together, and hoisted into the air?

The velocity factor of the inside of RG-59 is 0.66
when the currents are differential. The inside of RG-59
doesn't radiate when the currents are differential.

The outside of RG-59 is used for radiation. The velocity
factor of the outside of RG-59 is about 0.95, about the
same as ordinary insulated wire.
--
73, Cecil http://www.w5dxp.com

larry d clark wrote:

given that the length of a half wave dipole is
calculated by 468 / freq in mhz when velocity factor
is 1, ie 468 / 1.9 is about 246 ft.

i'm sitting here wondering why folks with small
city lots don't use (468 / freq in mhz) * velocity factor,
to construct a much shorter antnenna, particularly on
80m & 160m?

as an example, rg59 coax typically has a velocity
factor of .66, so plugging to the formula,
468 / 1.9 is about 246 ft, * .66 is about 162 ft.

why couldn't 162 ft of rg59 be cut in two, attached
to a 50 ohm, have the remaining ends of the rg59 shorted
together, and hoisted into the air?

so what am i missing? there are no free lunches:-)

larry
kd5foy

One wavelength in free space in feet is 984/F in feet (300/F in meters); a
half-wavelength is 492/F. 468/F assumes a vf of 0.95 of the antenna
radiating surface.
The vf of coaxial cable is dependent upon the dielectric used. 0.66 is
common for solid polyethylene. Foamed PE is typically near 0.80. As with
many RG cables, RG59 is available both ways.

In your example, the vf of the dielectric would not matter, since the
majority of the current flows on the *outside* of the shield due to
skin-effect. Thus, the formula for the length would be very close to (only
slightly shorter than) 468/F. Since I^2*R losses would be lower, bandwidth
would be better than a smaller-diameter wire.

Bryan WA7PRC

On Feb 11, 5:58 pm, larry d clark wrote:
given that the length of a half wave dipole is
calculated by 468 / freq in mhz when velocity factor
is 1, ie 468 / 1.9 is about 246 ft.

i'm sitting here wondering why folks with small
city lots don't use (468 / freq in mhz) * velocity factor,
to construct a much shorter antnenna, particularly on
80m & 160m?

as an example, rg59 coax typically has a velocity
factor of .66, so plugging to the formula,
468 / 1.9 is about 246 ft, * .66 is about 162 ft.

why couldn't 162 ft of rg59 be cut in two, attached
to a 50 ohm, have the remaining ends of the rg59 shorted
together, and hoisted into the air?

so what am i missing? there are no free lunches:-)

larry
kd5foy

To achieve resonance in a shorter antenna, you can increase either the
capacitance or the inductance--or both, of course. To increase the
capacitance, all you have to do is fill the universe with
polyethylene, or some similar low-loss dielectric. You don't have to
actually fill the whole universe with it; it would work to fill a
volume around the antenna. But to get the full effect, it should be a
pretty large volume, containing the electric field in the neighborhood
of the antenna. Not very practical. In coax, the electric field is
between the wires; in the dipole, it's also between the wires, but the
volume is very much larger. On the other hand, people have been
shortening resonant antennas for a long time by increasing the
inductance: thus, loading coils and "slinky" antennas. Similarly,
people make "slow" coax by making the center conductor a helix, and
thus make delay lines.

Cheers,
Tom

I.....verse with it; it would work to fill a
volume around the antenna. But to get the full effect, it should be a
pretty large volume, containing the electric field in the neighborhood
of the antenna. Not very practical. In coax, the electric field is
between the wires; in the dipole, it's also between the wires, but the
volume is very much larger. On the other hand, people have been
shortening resonant antennas for a long time by increasing the
inductance: thus, loading coils and "slinky" antennas.



Similarly,
people make "slow" coax by making the center conductor a helix, and
thus make delay lines.

Cheers,
Tom

ok what is a 'delay line'??

i would think that would just increase the surface area and therfore
sorta increase performance

On Feb 14, 3:21 pm, ml wrote:
I.....verse with it; it would work to fill a

volume around the antenna. But to get the full effect, it should be a
pretty large volume, containing the electric field in the neighborhood
of the antenna. Not very practical. In coax, the electric field is
between the wires; in the dipole, it's also between the wires, but the
volume is very much larger. On the other hand, people have been
shortening resonant antennas for a long time by increasing the
inductance: thus, loading coils and "slinky" antennas.

Similarly,

people make "slow" coax by making the center conductor a helix, and
thus make delay lines.

Cheers,
Tom

ok what is a 'delay line'??

i would think that would just increase the surface area and therfore
sorta increase performance

Wikipedia gives a definition of delay line; a length of transmission
line is technically a delay line, but often for longer delays, a
special line is made in which the center conductor is a wire wound
around a core (often of the same material as the dielectric between
center and outer). The winding should be done with space between the
turns, not close-wound, to give more uniform delay versus frequency.
For a uniform TEM transmission line, the delay time is the square root
of the total capacitance between the conductors times the total net
inductance of the length of the conductors: Tau=sqrt(L*C). Many E&M
texts go into how to accurately calculate the inductance and
capacitance for coaxial line with straight conductors.

In an antenna, you can increase the inductance by adding a lumped
inductance, commonly called a loading coil, or you can replace the
straight wire with a wire formed into a helix. Google "slinky
antenna". You'll find lots of info. I'm not making any claims that a
slinky antenna is either a good antenna or a poor one; it's just one
way to make a shortened dipole or monopole antenna, or even a
shortened Yagi.

Cheers,
Tom

K7ITM wrote:
On Feb 14, 3:21 pm, ml wrote:
I.....verse with it; it would work to fill a

volume around the antenna. But to get the full effect, it should be a
pretty large volume, containing the electric field in the neighborhood
of the antenna. Not very practical. In coax, the electric field is
between the wires; in the dipole, it's also between the wires, but the
volume is very much larger. On the other hand, people have been
shortening resonant antennas for a long time by increasing the
inductance: thus, loading coils and "slinky" antennas.
Similarly,

people make "slow" coax by making the center conductor a helix, and
thus make delay lines.
Cheers,
Tom
ok what is a 'delay line'??

i would think that would just increase the surface area and therfore
sorta increase performance

Wikipedia gives a definition of delay line; a length of transmission
line is technically a delay line, but often for longer delays, a
special line is made in which the center conductor is a wire wound
around a core (often of the same material as the dielectric between
center and outer). The winding should be done with space between the
turns, not close-wound, to give more uniform delay versus frequency.
For a uniform TEM transmission line, the delay time is the square root
of the total capacitance between the conductors times the total net
inductance of the length of the conductors: Tau=sqrt(L*C). Many E&M
texts go into how to accurately calculate the inductance and
capacitance for coaxial line with straight conductors.

In an antenna, you can increase the inductance by adding a lumped
inductance, commonly called a loading coil, or you can replace the
straight wire with a wire formed into a helix. Google "slinky
antenna". You'll find lots of info. I'm not making any claims that a
slinky antenna is either a good antenna or a poor one; it's just one
way to make a shortened dipole or monopole antenna, or even a
shortened Yagi.

Cheers,
Tom

In the early days of computers they used to use a length of wire as
temporary memory. At the start of a store cycle a piece of data would be
input to the wire, after a period of time the data would come out and be
placed into the computation.
Admiral Grace Hopper used to give an example of time and delay in her
speeches. She would say that one day she called down to the computer
department and asked for a micro second. They sent her 1000 feet of
wire. She then called down and asked for a nanosecond, they sent her one
foot of wire. No point to this just a good story.

Dave N

"David G. Nagel" wrote in message
...


In the early days of computers they used to use a length of wire as
temporary memory. At the start of a store cycle a piece of data would be
input to the wire, after a period of time the data would come out and be
placed into the computation.
Admiral Grace Hopper used to give an example of time and delay in her
speeches. She would say that one day she called down to the computer
department and asked for a micro second. They sent her 1000 feet of
wire. She then called down and asked for a nanosecond, they sent her one
foot of wire. No point to this just a good story.

Early color TV sets used a coaxial cable delay line for the luminance (B&W
signal) component, since the chromanance (color signal) is delayed in the
circuits that process it but they both need to arrive at the picture tube
simultaneously. Newer sets use various means besides a length of coax.

Adm. Hooper would bring her wires with her
when she was a guest on the Tonight show with Johnny Carson.

Adm Grace Murray Hooper, a pioneer in practical computing,
and the first woman admiral in the U.S. Navy.




In the early days of computers they used to use a length of wire as
temporary memory. At the start of a store cycle a piece of data would be
input to the wire, after a period of time the data would come out and be
placed into the computation.
Admiral Grace Hopper used to give an example of time and delay in her
speeches. She would say that one day she called down to the computer
department and asked for a micro second. They sent her 1000 feet of
wire. She then called down and asked for a nanosecond, they sent her one
foot of wire. No point to this just a good story.

Dave N

In the early days of computers they used to use a length of wire as
temporary memory. At the start of a store cycle a piece of data would be
input to the wire, after a period of time the data would come out and be
placed into the computation.
Admiral Grace Hopper used to give an example of time and delay in her
speeches. She would say that one day she called down to the computer
department and asked for a micro second. They sent her 1000 feet of
wire. She then called down and asked for a nanosecond, they sent her one
foot of wire. No point to this just a good story.

Dave N
The delay lines I encountered at IBM that were used for storage were
"sonic" delay lines. They were driven in a torsion mode (mechanical)
and were very reliable up to about 10 milliseconds(I think!) of data.
Above that they began to be temperamental and required constant
temperature ovens. The larger ones were used as video storage with a
screen of data in each instance.

Delay lines running at light speed (about a NS per ft) were used as
clock generators by inputting a pulse and tapping the line down stream
for a very stable clock sequence.

John Ferrell W8CCW