On Aug 29, 2:46*pm, K1TTT wrote:
but of course we all know that a standing wave is a figment of your
instrumentation!

Tom and Roy both measured *net* current - they did not use a
directional coupler. If they had used a directional coupler to measure
the current, they would have measured ~30 degrees shift in both the
forward current and reflected current through an 80m loading coil.

Let's talk about the net current in a 1/4WL lossless shorted stub.
Have you never looked at that equation? What is the phase-shift in the
net current from end to end in that stub? Exactly how do you
rationalize zero degrees phase-shift in the current in a stub known to
be 90 degrees in length?
--
73, Cecil, w5dxp.com

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, how does it accomplish that feat without a phase shift?
Aren't -jX and +jX, 180 degrees out of phase? Exactly how is a -jX
offset without a 180 degree phase shift?

In an electrical 1/4WL standing-wave antenna, like an 80m mobile
antenna, the phase shift between the forward wave at the feedpoint and
the incident reflected wave at the feedpoint is obviously 180 degrees.
Exactly how is that accomplished without a phase shift in the loading
coil?

There is a logical intuitive way to arrive at the phase shift through
a loading coil. Would you like to hear about it?
--
73, Cecil, w5dxp.com

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

On 8/29/2010 1:04 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. Otherwise it will not accept much power from a
transmitter or deliver much power to a receiver, due to a very high
mismatch to common types of transmission line connected to its
terminals.
. . .

Difficulty in getting power to an antenna is due to the mismatch between
the transmitter and the impedance it sees, rather than between the
transmission line and antenna.

As a simple example, consider a 75 ohm dipole connected to a transmitter
through a half wavelength of 600 ohm transmission line. The transmitter
sees 75 ohms. Most transmitters will deliver full power to a load of
that impedance and, except for line loss, all that power is delivered to
the antenna in spite of a 12:1 mismatch between the transmitter and
transmission line (assuming a 50 ohm output transmitter) and 8:1
mismatch between the transmission line and antenna. If you change the
transmission line impedance to 75 ohms, the transmitter can't tell the
difference -- it still sees 75 ohms and delivers the same amount of
power, even though the line and antenna are now perfectly matched.

Roy Lewallen, W7EL

"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 Aug 29, 10:38*pm, Roy Lewallen wrote:

Difficulty in getting power to an antenna is due to the mismatch between
the transmitter and the impedance it sees, rather than between the
transmission line and antenna.

As a simple example, consider a 75 ohm dipole connected to a transmitter
through a half wavelength of 600 ohm transmission line. /etc

Rather than using an example of a balanced antenna having reasonably
high radiation resistance and zero or low reactance at its input
terminals, let us consider a base-fed 10 foot whip at 3.8 MHz -- which
is more along the lines of this thread.

Without using a loading coil, the input Z of that whip is about 0.6 -j
1250 ohms. The SWR that this antenna input Z presents to unmatched 50
to 600 ohm transmission line ranges from 52,167:1 to 5,340:1.

Not much power will be transferred through such a match, which is the
reason for the statements in my quote which you referred to.

RF

On Aug 30, 5:12 am, Richard Fry wrote:
On Aug 29, 6:34 pm, Cecil Moore wrote:
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.

The short vertical radiator loaded to resonance *IS* the full
*electrical* length of an unloaded, resonant vertical of about 1/4WL,
which is related to the feedpoint impedance. It is NOT the full
*physical* length which is related to radiation resistance and
efficiency.

The feedpoint impedance of any electrically long 90 degree standing-
wave antenna, including resonant loaded mobile antennas, is:

Zfp = (Vfor-Vref)/(Ifor+Iref) on the antenna, not on the feedline.

The reflected voltage has undergone a 180 degree phase shift. The
reflected current has undergone a 360 degree phase shift. Part of the
phase shift occurs in the loading coil. A typical resonant mobile
antenna is *electrically* 90 degrees long. If it was less than 90
degrees long *electrically* it would exhibit capacitive reactance at
the feedpoint.

Let's discuss a base-loaded configuration which is less complicated
than a center-loaded configuration. (1) The delay through the loading
coil is part of that 90 degrees. (2) The delay through the stinger is
part of that 90 degrees. (3) The phase shift at the coil to stinger
junction is part of the 90 degrees. Tom, W8JI, assumes a lumped
inductor for calculating the phase shift at the coil to stinger
junction but a 75m bugcatcher loading coil is NOT a lumped inductor -
it is a distributed network existing in the real world with an
associated real-world delay through the coil.

The other rail of the argument assumes all of the "missing degrees"
come from the coil and none from the coil to stinger junction. Both
sides are wrong. All three phase shift components listed above exist
in a base-loaded mobile antenna.

(There are four phase shift components in a center-loaded mobile
antenna. Degrees of electrical length are actually lost at the low Z0
base section to high Z0 loading coil junction. That's why the
inductance (coil delay) has to increase for center-loaded
configurations.)

Interestingly enough, a base-loaded mobile antenna functions like the
dual-Z0 stubs covered on my web page and can be analyzed in the same
manner:

http://www.w5dxp.com/shrtstub.htm

Here is a simplified approximate representation of what a base-loaded
mobile antenna looks like electrically:

FP------Z01=5000 ohms------+------Z02=500 ohms------

The Z01 portion is the base loading coil and the Z02 portion is the
stinger. The Z0 of the loading coil can be obtained from the
inductance calculator at:

http://hamwaves.com/antennas/inductance.html
--
73, Cecil, w5dxp.com

On Aug 30, 5:44*am, Richard Fry wrote:
... let us consider a base-fed 10 foot whip at 3.8 MHz ...
Without using a loading coil, the input Z of that whip is about 0.6 -j
1250 ohms.

A 10 foot whip at 3.8 MHz is about 0.0386 wavelength or about 14
degrees. That's about -j4.0 on a Smith Chart. Can we say that -j1250/
Z0 = -j4.0? such that the Z0 characteristic impedance of the whip at
that input Z is ~312.5 ohms?
--
73, Cecil, w5dxp.com