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

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

On 8/30/2010 3:44 AM, Richard Fry wrote:
On Aug 29, 10:38 pm, Roy 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

Power will indeed be transferred through such a match. Using your
antenna as an example, suppose that a transmitter with output Z of 50
ohms is connected to a tuner that transforms its output impedance to 0.6
+ j1250 ohms. Connect the output of the tuner to a half wavelength 600
ohm transmission line to the antenna. The transmitter will see 50 + j0
ohms, the antenna will see an impedance of 0.6 + j1250 ohms, and full
power will be transferred. Power transfer has nothing to do with the SWR
on the line or the match between the line and antenna.

In practice, the line loss will increase some due to the very high SWR,
but the loss increase won't be much if the matched line loss is low.

I chose a half wavelength for simplicity, but it's not necessary. Other
lengths of line will transform the antenna impedance to different
values. All that's necessary is to readjust the tuner accordingly to
match the different impedance.

Amateurs have successfully been using this method to feed nonresonant
and multi-band antennas for decades.

Roy Lewallen, W7EL

On Aug 30, 3:19*pm, Roy Lewallen wrote:
Using your antenna as an example, suppose that a transmitter
with output Z of 50 ohms is connected to a tuner that transforms
its output impedance to 0.6 + j1250 ohms. /etc

Note that my post stated "UNMATCHED."

RF

On 8/30/2010 3:19 PM, Richard Fry wrote:
On Aug 30, 3:19 pm, Roy wrote:
Using your antenna as an example, suppose that a transmitter
with output Z of 50 ohms is connected to a tuner that transforms
its output impedance to 0.6 + j1250 ohms. /etc

Note that my post stated "UNMATCHED."

RF

I did. You stated:

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.

In my example, the antenna is not matched to the transmission line. Nor,
for that matter, is the transmitter matched to the transmission line. My
point is that power transfer doesn't depend on either of these points
being matched.

Roy Lewallen, W7EL

On Aug 30, 5:52*pm, Roy Lewallen wrote:

In my example, the antenna is not matched to the transmission line. Nor,
for that matter, is the transmitter matched to the transmission line. My
point is that power transfer doesn't depend on either of these points
being matched.

Roy:

My post showing very high input SWR at the base of an unloaded, base-
driven, 10 foot vertical on 3.8 MHz described an UNMATCHED system
resulting from its connection to transmission lines of typical
impedance values. It did not include matching networks, whether
located at the base of the vertical radiator, the output connector of
the transmitter, or wherever.

Then you posted, "Using your antenna as an example, suppose that a
transmitter with output Z of 50 ohms is connected to a tuner that
transforms its output impedance to 0.6 + j1250 ohms. ... The
transmitter will see 50 + j0 ohms, the antenna will see an impedance
of 0.6 + j1250 ohms, and full power will be transferred."

That configuration you posted is a MATCHED system, and its performance
does not disprove the accuracy of my post.

RF

On 8/31/2010 3:09 PM, Richard Fry wrote:
On Aug 30, 5:52 pm, Roy wrote:

In my example, the antenna is not matched to the transmission line. Nor,
for that matter, is the transmitter matched to the transmission line. My
point is that power transfer doesn't depend on either of these points
being matched.

Roy:

My post showing very high input SWR at the base of an unloaded, base-
driven, 10 foot vertical on 3.8 MHz described an UNMATCHED system
resulting from its connection to transmission lines of typical
impedance values. It did not include matching networks, whether
located at the base of the vertical radiator, the output connector of
the transmitter, or wherever.

Then you posted, "Using your antenna as an example, suppose that a
transmitter with output Z of 50 ohms is connected to a tuner that
transforms its output impedance to 0.6 + j1250 ohms. ... The
transmitter will see 50 + j0 ohms, the antenna will see an impedance
of 0.6 + j1250 ohms, and full power will be transferred."

That configuration you posted is a MATCHED system, and its performance
does not disprove the accuracy of my post.

RF

So you're saying that the mismatch between the impedance of an antenna
and the transmission line connected to it doesn't inhibit power flow
when there's a matching network anywhere in the system. But it does
interfere with power flow when there's no matching network?

What if the antenna is 50 ohms and the transmission line is a half
wavelength of 600 ohms, for a 12:1 mismatch? There's no matching
network. The transmitter sees 50 ohms. The antenna sees 50 ohms. What
will interfere with the power flow?

Roy Lewallen, W7EL

Roy,

The mention of reactance means we are talking in the frequency domain,
and a steady state solution is being discussed.


Roy Lewallen wrote in
:

....
Power will indeed be transferred through such a match. Using your
antenna as an example, suppose that a transmitter with output Z of 50
ohms is connected to a tuner that transforms its output impedance to
0.6 + j1250 ohms. Connect the output of the tuner to a half wavelength

Does the use of "output impedance" here mean that he transmitter can be
validly represented by a Thevenin equivalent circuit, and that "output
impedance" is the Thevenin equivalent source impedance.

Without getting into that arguable postion and reinforcing the notion
that a transmitter rated for a nominal 50 ohm load has a source impedance
of 50+j0, you could say:

.... Using your antenna as an example, suppose that a transmitter designed
to operate into a load Z of 50+j0 ohms is connected to a tuner that
transforms the antenna (0.5-j1250) to its preferred load impedance (50
+j0). Connect ...


600 ohm transmission line to the antenna. The transmitter will see 50
+ j0 ohms, the antenna will see an impedance of 0.6 + j1250 ohms, and
full power will be transferred. Power transfer has nothing to do with
the SWR on the line or the match between the line and antenna.

In practice, the line loss will increase some due to the very high
SWR, but the loss increase won't be much if the matched line loss is
low.

I chose a half wavelength for simplicity, but it's not necessary.
Other lengths of line will transform the antenna impedance to
different values. All that's necessary is to readjust the tuner
accordingly to match the different impedance.

And:

All that's necessary is to readjust the tuner accordingly to deliver the
transmitter its rated load impedance.


Amateurs have successfully been using this method to feed nonresonant
and multi-band antennas for decades.

Roy Lewallen, W7EL


As you will have noted, some band the term 'match' around with abandon,
and it means different things in different contexts, and to different
readers.

Take a transmitter designed for a 50+j0 load, connected by an electrical
half wave of 70 ohm coax to a 50 ohm load. Is it 'matched'? Well, from
the information, it is correctly loaded, it is designed for a 50+j0 load,
and it has a 50+j0 (approximately) load. We don't actually know the
source impedance, and even if we did, it this case, whether the
transmitter is 'matched' to the line, and whether the line is 'matched'
to the load is unimportant.

'Output impedance' is another term that is used differently, some use it
to mean the equivalent source impedance, some to mean the required load,
and some insist the foregoing are naturally the same, or will be if the
transmitter is 'matched' for maximum power output.

Owen