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Author Topic: EFHW impedance  (Read 9107 times)
K9AXN
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« Reply #90 on: November 09, 2017, 08:27:59 AM »

... The testing that you suggest has already been done with an EFFW antenna.  The antenna has a 30 degree narrower pattern and 2+db gain over a dipole.  The fact that the driven end has radiating properties however meager, unlike the end of a dipole, should help to explain the reasoning. ...

Below is a NEC4.2 graphic of the radiation pattern envelope and the r-f current distribution for an "end-fed" full wave (EFFW), linear radiator in free space.

Note that the radiation pattern has two lobes, whereas there is only one lobe from a half wave, end fed antenna (EFHW).

Do you not accept antenna analysis results using MoM software, such as NEC?




I mistyped EFFW for CFFW.  Please run that and post the results.  My response to Glen had the correct comment CFFW.

Regards Jim
« Last Edit: November 09, 2017, 08:30:02 AM by K9AXN » Logged
K9AXN
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« Reply #91 on: November 09, 2017, 08:37:04 AM »

Quote from: K9AXN
The SURGE IMPEDANCE of a wire is the attribute from which all other characteristics of an antenna are built.
No.


Explain what Surge impedance in a resonant antenna is and what part it plays in antenna logic.

Regards Jim  
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W9IQ
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Posts: 1707




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« Reply #92 on: November 09, 2017, 08:53:03 AM »

Jim,

Here is my perspective on surge impedance.

Surge impedance is simply the ratio of voltage to current on a transmission line or linear antenna of a single wave propagating along  the line, away from the source. Surge impedance implies the absence of reflections. It is more commonly called characteristic impedance - particularly when dealing with transmission lines.

If there is a reflected wave, the characteristic (surge) impedance no longer applies as now the impedance of the element at any point along its length is determined by the interaction of the forward and reflected waves. This is commonly called the input impedance or steady state impedance.

While surge impedance is an important cardinal concept, I generally find that when analyzing a linear antenna, the steady state impedance at any point along the antenna is a more meaningful metric.

- Glenn W9IQ
« Last Edit: November 09, 2017, 08:59:15 AM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W9IQ
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Posts: 1707




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« Reply #93 on: November 09, 2017, 09:12:04 AM »

Jim,

I am still confused by your discussion of current distribution along an EFHW and the resulting radiation. Let me give my perspective and perhaps that will help you to clarify/contrast your position.

I would say that the ends of the EFHW antenna will be a current minima while the center will still be a maxima. Since radiation is relative to current, the center of the antenna will have the maximum radiation. The low current at the ends of the antenna are a result of the the forward and reflected waves causing a high input impedance. The (true) radiation resistance of the antenna is still ~72 ohms in free space despite the much higher feedpoint impedance at the end of the antenna.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KB1GMX
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« Reply #94 on: November 09, 2017, 09:15:28 AM »

I'm in the realm where experiments where proposed are tested and proven or disproven.

Truth in electronics and physics is repeatable.  Often I get to replay prior tests to verify
some published as fact or error.  Note I didn't say fiction as error often is the cause of
statements that are not fact.

For example: the pattern of a center fed dipole will be sine/cosine for any length
less that resonant length.    It is also the same for any frequency presented below
that resonant length. This is for a constant height in wavelengths.

RFRY,

The image you show is not that of a resonant half wave.  It is that of a off resonant
full wave and your own data in the image supports that.   For that case its not
supporting the overall argument of a feed impedance and pattern of a resonant
half wave.   The Z=472,J-718 is significant indicator.  Resonant would be a Z=x,j0
and anything else is a different discussion with different currents and potential
patterns.  Once we get over about .625 Lambda the discussion of pattern and
gain are not applicable to a half wave dipole regardless of feed construction.
That can be tested with a basic center fed dipole.

The NEC4.2 model is not that of a end fed half wave as the current minima is not at the feed
point.  It was extended to the end of the counterpoise.  The model requires adjusting the longer
wire length to be such that the feed point is j0 as then and only then is the feed resistance R
not a reactive Z value and the counterpoise section not included.  While the statement that an
end fed halfwave is an extreme example of a off center fed half wave is true it is also a boundary
case that requires far more care in modeling and analysis.

The easy test for this is a ~.5 (.480 for 26ga wire) meter antenna with 48mm counterpoise
at 300mhz at 1 meter height.   Everything stated for HF will scale accordingly including
wire diameter.  The real trick is noting the feed points effects due to cables, impedance
transformer and the cast of characters known as strays.   Why 300mhz  It allows for
gear we can build, model and construct without the need for acres of property and
trying to get the antenna to .5 or 1 lambda.   It also allows testing at distances outside
the reactive near field.  For HF that requires hundreds of meters  and knowledge of the
ground properties in the path to be close to accurate.

Real world antennas at HF are close to the ground and there is in just about every
antenna text graphing the effect of impedance of a resonant half wave dipole for
varying heights. All antennas the interact with the earth are influenced by that and by
simple extension that includes all antennas that have a reflection from the earth.

Lots of good discussions and thoughts.  Often its good to start with fundamentals
and build from there.

Allison
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W9IQ
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« Reply #95 on: November 09, 2017, 09:40:13 AM »

Quote
The image you show is not that of a resonant half wave.  It is that of a off resonant
full wave and your own data in the image supports that.

Richard made it clear that this is a full wave antenna and the diagram makes it clear that it is not truly end fed but rather OCF. Richard highlights the general "elephant in the room" issue that the feedpoint of an end fed antenna cannot actually be at the end point. We can theoretically discuss the end  fed antenna but when called upon to practically implement it, there are other factors at play that challenge the hobbyist to properly model or even generally describe it.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
RFRY
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« Reply #96 on: November 09, 2017, 09:46:02 AM »

...I mistyped EFFW for CFFW.  Please run that and post the results.  My response to Glen had the correct comment CFFW.

See below:

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KB1GMX
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Posts: 1496




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« Reply #97 on: November 09, 2017, 09:52:33 AM »

Glen,

Simple form: The current maxima is in the center and that is the region of peak radiation.
This is only true for a resonant half wavelength.  At harmonically related frequencies
we get multiple peak current locations and the pattern changes accordingly in a way
that is like a center fed dipole.  The actual magnitude for the values of resistance and
reactance vary with height for both center fed and end fed.

The radiation impedance is a function of antenna length and unaffected by feed point.
A dipole is a good model for that save for it is only a factor in feed point resistance
which includes resistive losses.  Ideal vs actual.  H. A. Wheeler presented a model
for that and ways to measure it.  For HF antennas its informative.

Reality is the center is offset by as much as a few feet but for practical HF
antennas less than 1 wavelength above the ground its is not a significant issue.
Real antennas are over uncertain ground with varying properties surrounded
by trees and houses.  Most models do not approach that level of reality. So any
model derived without those factors is informative but can be far from
measured in an imperfect environment.

Also for surge impedance the easiest model is to use a single current pulse that is
shorter in time than the possible reflection length.  That known amplitude pulse will
then have a current and voltage that would not include reflections until it is reflected.
It is a measurement in the time domain rather than frequency domain.

CST is a good tool for modeling that with the usual modeling caveats and TDR
(time domain reflectometry) is the way to measure that in the real world.


Allison





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W9IQ
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Posts: 1707




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« Reply #98 on: November 09, 2017, 09:58:32 AM »

Allison,

Thanks for your reply but I hardly need a basics course on antenna theory. My post was clearly directed to Jim as I am trying to understand his perspective.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KB1GMX
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« Reply #99 on: November 09, 2017, 10:10:45 AM »

The feed point and its analysis is measureable.  If its Z=x,j0 you there elsewise its somewhere likely close.

In the practical realm we work to transfer power by matching load to source.  Theoretical power
is fun but, I've not figured a way to apply it.

I like the elephant in the room analogy.  The elephant is always there but its size can be significant
or just a little ceramic thing on the end table.  Without knowing or defining environment we can only
say its there and not know if its moving the furniture.  The math says the effects are there.

With that I'll add this:  Adding a .05WL extension to an EFHW at first resonance is a good model.
WHen er go to 2F, 3F, its not an EFHW,  did we scale the extension as a function of frequency
(easy to do with 4NEC2 optimizer) or did we use a fixed length where at 2F and 3F its a significant
entity.  If we carry that far enough at 5F its an effective quarter wave and at 10F its an half wave.  
Those two points are now significant to have changes the color of the elephant and its size as well.


Allison

Allison
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KB1GMX
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« Reply #100 on: November 09, 2017, 10:17:57 AM »

Glen,

With I answered you it was trying to keep all of us on the same plane.
Hopefully Jim can then see both the elephant and appreciate its presence
effectively.

The other general point and its audience is that first principals hold
and we build on that.  Errors in models or conclusions drawn from them
need to be examined dispassionately.   Resolving the differences
between practice or observation and the maths is science.

Allison
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RFRY
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« Reply #101 on: November 09, 2017, 10:30:22 AM »

... Resonant would be a Z=x,j0 and anything else is a different discussion with different currents and potential patterns. ... 

My NEC model was not intended to show the parameters of a self-resonant x,j0 antenna, but rather those of an antenna of one lambda in physical wavelength (which it does).

Note that in free space, a thin-wire, center-fed dipole having an end-end physical length of exactly λ/2 has a feedpoint Z of 73 +j42.5 Ω.  To make that dipole self-resonant (j = 0), it must be shortened by several percent.  Doing so also reduces its radiation resistance from 73 to about 65 Ω.  Source = Antennas, Third Edition, page 182 (Kraus/Marhefka).

» The directivity/gain of the shorter version is almost identical to the full-size version, for all practical purposes.

» Very few of the antennas/arrays used by licensed AM broadcast stations are self-resonant -- they use matching networks at their feedpoints to provide a suitable Z to the transmission line connected there


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K9AXN
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« Reply #102 on: November 09, 2017, 11:17:32 AM »

...I mistyped EFFW for CFFW.  Please run that and post the results.  My response to Glen had the correct comment CFFW.

See below:



Is this a CFHW dipole or CFFW dipole?  Please model the 14MHz CFFW and the 14MHz CFHW dipole and post the results.  Need to compare the two.

Thanks et Regards Jim
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KM1H
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« Reply #103 on: November 09, 2017, 11:23:00 AM »

Quote
Is this a CFHW dipole or CFFW dipole?  Please model the 14MHz CFFW and the 14MHz CFHW dipole and post the results.  Need to compare the two.

All that and more can be answered by using one of the several excellent software programs; some are even FREE.

Then you can adjust all parameters on your own and see what really happens without continually questioning others results and/or ability.

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RFRY
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« Reply #104 on: November 09, 2017, 11:31:27 AM »

Is this a CFHW dipole or CFFW dipole?

It is a CFFW dipole, as shown by the text in the upper left corner of that NEC plot.

Quote
Please model the 14MHz CFFW and the 14MHz CFHW dipole and post the results.  Need to compare the two.

I have posted plots for the CFFW and CFHW already.  The 14 MHz plots for configurations with properly scaled conductors are the same as those posted earlier for those configurations.

Suggest you might want to download NEC yourself, and experiment with configurations for which you have an interest.
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