Whip antennas with coils
 

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

Whip antennas with coils
 

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

Whip antennas with coils
 

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

Whip antennas with coils
 

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

Whip antennas with coils
 

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

Whip antennas with coils
 

Dear Group:

I add a vote for avoiding the arguable position about the "output Z" of
a transmitter (note, transmitter) and instead using words similar to those
used by Mr. Owen Duffy: a transmitter designed to operate into a load Z of
50+j0 ohms

Conclusions reached with a minimum of needed assumptions are preferred.
Roy's observations do not require a particular output Z and are true.

A signal generator with an emitter-follower output stage having a 50 ohm
resistor in series with the output port can have an output Z of very close
to 50 ohms over a broad frequency range.

73, Mac N8TT
--
J. McLaughlin; Michigan, USA
Home:
"Owen Duffy" wrote in message
...
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

Whip antennas with coils
 

On Aug 29, 8:45*pm, John Smith wrote:
On 8/28/2010 7:23 PM, Art Unwin wrote:

...
Art

Having locked myself away for months, wife will claim years, and
searching for the "best" shortened stealth antenna(s), I have been
forced to employ coils. *In any loaded antenna (hide the JD!) the coil
is best at the bottom with a .62 L/D ratio. I currently use DLM's from
Robert Vincents' patent, it allows me to break the laws ... others
research and mileage may vary.

The loading coil adds +J to compensate the -J from a short antenna ...
however, old hams taught me that the coil adds electrical degrees, God
bless them ...

Regards,
JS

this one sounds like a religious argument to me... to phase shift or
not to phase shift, that is the degrees of shortening... weather tis
nobler to count the degrees or accept the time delay traveling waves,
one way or another there is still high voltage at the top and high
current at the bottom, and never the twain shall meet.

Whip antennas with coils
 

On 8/30/2010 4:40 PM, K1TTT wrote:

...
this one sounds like a religious argument to me... to phase shift or
not to phase shift, that is the degrees of shortening... weather tis
nobler to count the degrees or accept the time delay traveling waves,
one way or another there is still high voltage at the top and high
current at the bottom, and never the twain shall meet.

From your posts here, over many subjects, I would deduce everything
here sounds religious to you. Or, as if it comes from evil spirits.
Or, it comes from an oracle sniffing the vapors of petroleum. Or, some
drug which has affected the mind ... etc.

Regards,
JS

Whip antennas with coils
 

On Aug 30, 6:36*pm, "J. Mc Laughlin" wrote:
* * I add a vote for avoiding the arguable position about the "output Z" of
a transmitter (note, transmitter) and instead using words similar to those
used by Mr. Owen Duffy: * a transmitter designed to operate into a load Z of
50+j0 ohms

I'll drink to that. Consider a transmitter connected to an antenna
tuner by a piece of 50 ohm coax where the SWR is an ideal 1:1 on that
short piece of coax. There are no reflected waves on the coax.
Therefore, no reflected energy reaches the source. Energy transfer is
a one-way process on that flat piece of coax. Except for efficiency,
the source impedance doesn't matter. Whether the source impedance is
10 ohms, or 50 ohms, or 200 ohms, dissipative or non-dissipative,
reactive or resistive - just doesn't matter. All that matters is the
voltage delivered to the 50 ohm Z0-match at the tuner input.
--
73, Cecil, w5dxp.com

Whip antennas with coils
 

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