On Aug 29, 5:34*pm, Cecil Moore wrote:
On Aug 29, 3:04*pm, Richard Fry wrote:

The purpose and function of a loading coil used with an electrically
short antenna is to offset the capacitive reactance of the short
radiating section.

Uh Richard, (clip)

If the loading coil used to resonate an electrically short vertical
really contributed '"electrical degrees" arising from some
attribute(s) of the coil that made the short antenna system the full
electrical equivalent of an unloaded, 1/4-wave vertical, then please
explain why the loaded version does not have the radiation resistance,
and typically the radiation efficiency of the unloaded version.

RF

On Aug 29, 5:54*pm, Richard Fry wrote:
If the loading coil used to resonate an electrically short vertical
really contributed '"electrical degrees" arising from some
attribute(s) of the coil that made the short antenna system the full
electrical equivalent of an unloaded, 1/4-wave vertical, then please
explain why the loaded version does not have the radiation resistance,
and typically the radiation efficiency of the unloaded version.

The radiation resistance and radiation efficiency of a short antenna,
like any other antenna, depends upon the *physical* length of the
antenna. Nothing can be done about that fact of physics where bigger
is generally better. Short resonant antennas have a lower radiation
resistance and therefore lower efficiency. Please do not confuse
radiation resistance and antenna efficiency with feedpoint impedance
where the reflected wave must arrive 180/360 deg in phase with the
forward wave for the feedpoint impedance to be resistive and resonant.
There is simply no other possibility.

The *feedpoint impedance* of a standing-wave antenna depends upon the
*electrical* length of the antenna. If it is resistive, the reflected
wave has undergone at least a 180 degree phase shift referenced to the
forward wave. Otherwise, the feedpoint impedance would not be purely
resistive. Make no mistake, a typical loaded mobile antenna is 90
degrees long and part of that 90 degrees is furnished by the loading
coil. Note that I said "part", not *all* of the "missing" degrees.
W8JI is correct about approximately half of the phase shift between
the coil and the stinger. He is 100% wrong about the other half of the
phase shift which occurs within the coil.

Here's a question for you: If the feedpoint impedance of a loaded
standing-wave (mobile) antenna is purely resistive, how could the
reflected wave arriving at the feedpoint have undergone anything
except a 180 degree phase shift?

Why is the feedpoint impedance of a resonant short loaded antenna
usually less than that of a 1/4WL antenna? Because the radiation
resistance is lower and the I^2*R losses are lower. But all resonant
shortened monopoles are 90 degrees in electrical length. Anyone
arguing against that fact of physics is just ignorant of how standing-
wave antennas work. That includes some otherwise knowledgeable
"gurus", incapable of admitting a mortal mistake, who post to this
newsgroup.
--
73, Cecil, w5dxp.com

"Cecil Moore" wrote
...

Here's a question for you: If the feedpoint impedance of a loaded
standing-wave (mobile) antenna is purely resistive, how could the
reflected wave arriving at the feedpoint have undergone anything
except a 180 degree phase shift?

There are the two possibilities: See:
http://paws.kettering.edu/~drussell/...t/reflect.html

1.Reflection from a HARD boundary "at a fixed (hard) boundary, the
displacement remains zero and the reflected wave changes its polarity
(undergoes a 180o phase change) "

2. Reflection from a SOFT boundary " at a free (soft) boundary, the
restoring force is zero and the reflected wave has the same polarity (no
phase change) as the incident wave "

So if the feedpoint is in distance 1/4 WL from the end you have 0 or 180
degree phase shift.

Which case is in antennas?
S*

Why is the feedpoint impedance of a resonant short loaded antenna
usually less than that of a 1/4WL antenna? Because the radiation
resistance is lower and the I^2*R losses are lower. But all resonant
shortened monopoles are 90 degrees in electrical length. Anyone
arguing against that fact of physics is just ignorant of how standing-
wave antennas work. That includes some otherwise knowledgeable
"gurus", incapable of admitting a mortal mistake, who post to this
newsgroup.
--
73, Cecil, w5dxp.com

On Aug 29, 6:34*pm, Cecil Moore wrote:

The *feedpoint impedance* of a standing-wave antenna depends upon
the *electrical* length of the antenna. If it is resistive, the reflected
wave has undergone at least a 180 degree phase shift referenced to
the forward wave. Otherwise, the feedpoint impedance would not
be purely resistive. etc

However an assumption might be taken from some posts here that a short
vertical radiator loaded to resonance is the full electrical
equivalent of an unloaded, resonant vertical of about 1/4-wavelength,
while it is not. That is my point.

RF

On 8/30/2010 3:12 AM, Richard Fry wrote:
On Aug 29, 6:34 pm, Cecil wrote:

The *feedpoint impedance* of a standing-wave antenna depends upon
the *electrical* length of the antenna. If it is resistive, the reflected
wave has undergone at least a 180 degree phase shift referenced to
the forward wave. Otherwise, the feedpoint impedance would not
be purely resistive. etc

However an assumption might be taken from some posts here that a short
vertical radiator loaded to resonance is the full electrical
equivalent of an unloaded, resonant vertical of about 1/4-wavelength,
while it is not. That is my point.

RF

In the above "equivalent" is used in an interesting way. If equivalent
is used as a term of efficiency, you are absolutely correct. If you are
saying my transmitter still knows the difference between a "straight"
1/4 wave and an "equivalent loaded" antenna, the use becomes false ...
i.e., when I dip the loaded antenna with my GDO, and the antenna is
properly constructed, it tells me I have the "true" equivalent of a 1/4
wave.

Regards,
JS