Hello,

not long time ago i was caught by a doubt, partially solved in my mind, but
i think can be interesting for the folks here reply to a antenna newbie (me)
about newbie questions

The doubt was (and is): a monopole, end feeded, that is 1/4 wave phisical
lenght, is obviously also 1/4 wave electrical lenght, in respect of the
classic "book image" that show the wave travelling (or standing, here i see
different opinions...) cross the monopole just for 1/4 of the full sinewave.
And this in my mind is OK.

Now, how i do treat a 5/4 wave phisical lenght monopole, in respect of
electrical lenght ?? It is even a "1/4 wave configuration" and 1/4 wl
electrical lenght, because the sinewave travelling (or standing) in the
monopole terminate in both cases at 1/4 of the wave, or 5/4 is just 5/4 wl
and 1/4 is 1/4 wl, electrically and phisically ??

Note: i'm wondering about the impedance, not about radation patterns. I'm
aware that radiation patterns are singular in respect of the singular
electrical lenght of the monopole. Or better, that is what i learn, right or
wrong

The second question is about 1/4 wave vs 1/2 wave monopoles. I googled
around, but no one define exactly why "1/2 wave antenna don't need ground
plane". The most technical explanation i found is that due the high
impedance at the feed point, the 1/2 wl antenna don't care about the ground
plane. But i'm not so convicted about this, and a basic explanation of the
1/2 wl "no-ground-needed" theory is appreciated.

Apologize for grammar errors and lenght of the post. Regards and 73.

-.-. --.-, Cristiano, Italy.

Hello Cristiano,

I put comment between your text.

On 22 abr, 09:40, "-.-. --.-" wrote:
Hello,

not long time ago i was caught by a doubt, partially solved in my mind, but
i think can be interesting for the folks here reply to a antenna newbie (me)
about newbie questions

The doubt was (and is): a monopole, end feeded, that is 1/4 wave phisical
lenght, is obviously also 1/4 wave electrical lenght, in respect of the
classic "book image" that show the wave travelling (or standing, here i see
different opinions...) cross the monopole just for 1/4 of the full sinewave.
And this in my mind is OK.

A physical 1/4 wave, behaves is an electrically 1/4 wave, with some
additional end-capacitance due to the "end effect" (also called
"fringe effect"). To compensate for the end-capacitance, you have to
make the physical 1/4 wave radiator somewhat shorter (to get resistive
input impedance).

Note: input impedance of quarter wave over an infinite good conducting
ground plane is about 35 Ohms (from image theory). A quarter wave fed
over 3 or 4 radials has somewhat lower input impedance.

Now, how i do treat a 5/4 wave phisical lenght monopole, in respect of
electrical lenght ?? It is even a "1/4 wave configuration" and 1/4 wl
electrical lenght, because the sinewave travelling (or standing) in the
monopole terminate in both cases at 1/4 of the wave, or 5/4 is just 5/4 wl
and 1/4 is 1/4 wl, electrically and phisically ??

You can see the 5/4 wave as "end-capacitance" + 0.5+0.5+0.25 wave.
When you make the first 0.5 wave somewhat shorter to compensate for
the end-capacitance, you will find a low input impedance (resistive),
but somewhat higher then for the 0.25 wave (due to more radiation loss
of the forward en reflected wave). On the resonance frequency, the
0.45+0.5 wave section behaves as a high input impedance, therefore the
input input impedance of the 0.25 wave section is low (quarter wave
transformer formula).

You have the length correction for "end-effect" only ones. So when you
have to cut 2cm for the 0.25 wave case, you should do almost the same
for the 5/4 wave case.

When you change frequency (starting from resonant frequency), you will
notice that inductive or capacitive component changes faster than in
the 0.25 wave case. So roughly spoken, the shape of the impedance
curve around the resonant frequency is same as for the 0.25 wave case,
but the exact figures are different.


Note: i'm wondering about the impedance, not about radation patterns. I'm
aware that radiation patterns are singular in respect of the singular
electrical lenght of the monopole. Or better, that is what i learn, right or
wrong

The second question is about 1/4 wave vs 1/2 wave monopoles. I googled
around, but no one define exactly why "1/2 wave antenna don't need ground
plane". The most technical explanation i found is that due the high
impedance at the feed point, the 1/2 wl antenna don't care about the ground
plane. But i'm not so convicted about this, and a basic explanation of the
1/2 wl "no-ground-needed" theory is appreciated.

Thin 0.5 wave antennas have high input impedance (some kOhms). So at
about 100W input current is about 0.3A, while input current for a 0.25
wave is about 2A. (difference is 16 dB).

The total impedance as seen from the coaxial feeder is the sum of the
antenna impedance and the impedance of your (floating) ground (for
example some quarter wave radials). As input impedance of thin HW
radiator is high, many commercial antennas do not have radials. They
rely on the ( 1 kOhm) impedance of the screen of the coaxial feeder
or the mast/pole/roof structure, etc.

Sometimes your mast/pole or cable may show resonance that increases
the ground impedance as seen at the antenna base to a value not far
below 1 kOhm. This will lead to relative high RF current in mast/pole/
cable screen, and because of this, significant change in radiation
pattern.

I have a document solely on HW monopoles he http://www.tetech.nl/divers/HWmonopoleNL1.pdf.
It is in Dutch Language, but the comment in all graphs is in English
Language.

Apologize for grammar errors and lenght of the post. Regards and 73.

-.-. --.-, Cristiano, Italy.

Distinti saluti,

Wim
PA3DJS
www.tetech.nl
Please remove abc when using the pm.

The electrical length of a monopole is longer than its physical
length, because the velocity of propagation along the monopole is
slower than in free space.

Here are some parameters for two cases of base-fed, self-resonant, 1
MHz monopoles, with two ohms in the r-f ground connection, over a
perfect ground plane:

1/4-WAVE

Height = 70 meters
Width = 1.5 meters (solid cylinder)
Input Z = 38 + j0 ohms
Peak Gain = 4.9 dBi at zero degrees elevation
Number of Lobes in Elevation Pattern = 1

5/4-WAVE

Height = 368 meters
Width = 3 meters (solid cylinder)
Input Z = 63.5 + j0 ohms
Peak Gain = 7.57 dBi at 57 degrees elevation
Number of Lobes in Elevation Pattern = 3

Antenna current always is very nearly zero at the top of a monopole of
every height, and distributes itself below the top of the monopole
approximately in the form of a sine wave.

Monopoles of every height need a very low-loss return path for the
displacement currents they generate in the nearby earth, extending out
to about 1/2-wavelength. Current densities in the earth vary within
that distance for monopoles of different heights, and are
proportionally less very near a 1/2-wave monopole than very near a 1/4-
wave monopole. But for best system efficiency a 1/2-wave monopole
still needs a low-loss ground plane to work against.

An example of a low-loss r-f ground connection is 120 buried radial
wires extending about one-half of a free-space wavelength in three-
degree steps around the base of the monopole, and bonded together at
the base. Such a radial system has an r-f loss of 2 ohms or less.

RF

-.-. --.- wrote:
Now, how i do treat a 5/4 wave phisical lenght monopole, in respect of
electrical lenght ??

The free demo version of EZNEC will answer your question.
1/4WL above the 5/4WL monopole's feedpoint, there is a
current node ('x' minimum point). Of course, there is another
current node at the top of the monopole. Looking at the
monopole with your head turned 90 degrees, the current
distribution looks something like this.
_ __
\ / \
fp----x------x------x
\ __ /

Since it is fed at a current maximum point, you need
a counterpoise/ground-plane for such an antenna.
You didn't ask, but the take-off-angle for a 5/4WL monopole
is about 58 degrees, not a good idea above 10 MHz.

The second question is about 1/4 wave vs 1/2 wave monopoles. I googled
around, but no one define exactly why "1/2 wave antenna don't need ground
plane".

A 1/4WL monopole is half of an antenna. The ground plane
is the other half. The feedpoint is at a current maximum
point which is a magnetic field maximum. The high intensity
magnetic field directly encountering the earth is a lossy
condition.

A 1/2WL monopole is a complete resonant antenna. The
feedpoint is at a voltage maximum point which is an
electric field maximum. A pure electric field encountering
earth ground is less lossy than a pure magnetic field
encountering earth ground. Since the feedpoint impedance
is very high, some hams simply use a ground rod for a
counterpoise.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

ha scritto nel messaggio
...

The total impedance as seen from the coaxial feeder is the sum of the
antenna impedance and the impedance of your (floating) ground (for
example some quarter wave radials). As input impedance of thin HW
radiator is high, many commercial antennas do not have radials. They
rely on the ( 1 kOhm) impedance of the screen of the coaxial feeder
or the mast/pole/roof structure, etc.

Ok. Have some right input now.. so, reading also the other posters (that
here i thanks for all the inputs...), i assume that:

- thin HW monopoles have high impedance;
- by the way, high impedance mean a Z transformer, or auto-transformer, that
match the ~1kOhm Z of the antenna to the 50 Ohm of feed line;
- radials are ininfluent or useless because HW monopole is a complete
resonant antenna;

What is not clear for me is "They rely on the ( 1 kOhm) impedance of the
screen of the coaxial feeder or the mast/pole/roof structure, etc."
What means? That because i have a complete resonant antenna the concept of
"ground" is only a concept of "Z in regard of what??" and because
pole/coax/mast present for sure a much lower Z (at monopole resonant
frequency) than the monopole itself, we (and the antenna) assume that
pole/coax/mast is "ground zero" for us ?

Sometimes your mast/pole or cable may show resonance that increases
the ground impedance as seen at the antenna base to a value not far
below 1 kOhm. This will lead to relative high RF current in mast/pole/
cable screen, and because of this, significant change in radiation
pattern.

And if i guess right in the precedent statement i wrote, in this case
(resonant mast/pole/feed line) i lose the "ground 0" reference and in this
case i do provide a low impedance reference to the antenna. Maybe, adding
some radial. But this is my guess, i'm not sure if is the right thing to do.

I have a document solely on HW monopoles he
http://www.tetech.nl/divers/HWmonopoleNL1.pdf.
It is in Dutch Language, but the comment in all graphs is in English
Language.
Read and appreciate for what i understand inside

I have one more (in pure newbie fashion ) question that i append directly
to Cecil post, but is very appreciated also a reply from all the readers of
this thread.

-.-. --.- , Cristiano, Italy

"Cecil Moore" ha scritto nel messaggio
...

A 1/4WL monopole is half of an antenna. The ground plane
is the other half. The feedpoint is at a current maximum
point which is a magnetic field maximum. The high intensity
magnetic field directly encountering the earth is a lossy
condition.

A 1/2WL monopole is a complete resonant antenna. The
feedpoint is at a voltage maximum point which is an
electric field maximum. A pure electric field encountering
earth ground is less lossy than a pure magnetic field
encountering earth ground. Since the feedpoint impedance
is very high, some hams simply use a ground rod for a
counterpoise.

First thanks for replies abt the 5/4 WL monopole, i'm trying right now to
use eznec and simulate what you saw me.
One more newbie question: 1/4 WL and 1/2 WL monopoles are both end feeded.
So my feed point is exactly at the begin of monopole.
Assuming there is no feed line, or also a 1/2 WL feed line (if IIRC 1/2 wl
is the line lenght that preserve the phase of my RF wave from generator to
the end of the line...) what decide if i'm feeding the antenna on a current
maximum point or in a voltage maximum point ?? On a dipole, with the imagine
of the wave "superimposed" to the dipole in my mind, is quite easy for me
figure out if i'm feeding it on a current node or on a voltage peak (e.g
center feed vs. off center feed dipole). But since in both 1/4 and 1/2 wl
monopoles i'm at the *begin* of the antenna, what decide the feed point
modality (I or V) ?? The impedance i have at the specific point ??

Regards,
-.-. --.-., Cristiano, Italy.

"-.-. --.-" ha scritto nel messaggio
...
Assuming there is no feed line, or also a 1/2 WL feed line (if IIRC 1/2
wl is the line lenght that preserve the phase of my RF wave from generator
to the end of the line...)

Oops. Sorry.
1/2 wl feed line preserve the impedance seen at both ends of the line. Not
sure about phase.

-.-. --.-

-.-. --.- wrote:
... what decide the feed point
modality (I or V) ??

1/4WL down from the tip is a current maximum point.
1/2WL down from the tip is a voltage maximum point.
3/4WL down from the tip is a current maximum point.
4/4WL down from the tip is a voltage maximum point.
5/4WL down from the tip is a current maximum point.

i.e., odd multiples of 1/4WL are current maximums.

i.e., even multiples of 1/4WL are voltage maximums.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

On 22 abr, 20:07, "-.-. --.-" wrote:
ha scritto nel ...

The total impedance as seen from the coaxial feeder is the sum of the
antenna impedance and the impedance of your (floating) ground (for
example some quarter wave radials). *As input impedance of thin HW
radiator is high, many commercial antennas do not have radials. They
rely on the ( 1 kOhm) impedance of the screen of the coaxial feeder
or the mast/pole/roof structure, etc.

Ok. Have some right input now.. so, reading also the other posters (that
here i thanks for all the inputs...), i assume that:

- thin HW monopoles have high impedance;
- by the way, high impedance mean a Z transformer, or auto-transformer, that
match the ~1kOhm Z of the antenna to the 50 Ohm of feed line;
- radials are ininfluent or useless because HW monopole is a complete
resonant antenna;

What is not clear for me is "They rely on the ( 1 kOhm) impedance of the
screen of the coaxial feeder or the mast/pole/roof structure, etc."
What means? That because i have a complete resonant antenna the concept of
"ground" is only a concept of *"Z in regard of what??" and because
pole/coax/mast present for sure a much lower Z (at monopole resonant
frequency) than the monopole itself, we (and the antenna) assume that
pole/coax/mast is "ground zero" for us ?

Sometimes your mast/pole or cable may show resonance that increases
the ground impedance as seen at the antenna base to a value not far
below 1 kOhm. This will lead to relative high RF current in mast/pole/
cable screen, and because of this, significant change in radiation
pattern.

And if i guess right in the precedent statement i wrote, in this case
(resonant mast/pole/feed line) i lose the "ground 0" reference and in this
case i do provide a low impedance reference to the antenna. Maybe, adding
some radial. But this is my guess, i'm not sure if is the right thing to do.

I have a document solely on HW monopoles he
http://www.tetech.nl/divers/HWmonopoleNL1.pdf.
It is in Dutch Language, but the comment in all graphs is in English
Language.

Read and appreciate for what i understand inside

I have one more (in pure newbie fashion ) question that i append directly
to Cecil post, but is very appreciated also a reply from all the readers of
this thread.

-.-. --.- , Cristiano, Italy

Hello Cristiano,

Please go to figure 5 of http://www.tetech.nl/divers/HWmonopoleNL1.pdf.

Look to the source at the bottom of the coaxial structure. As the
impedance of the source is assumed 50 Ohms, and the cable is 50 Ohms,
you may take out the source and place it between the radiator and the
screen of the cable.

Then you get the circuit model in the right part of figure 5. Now
you can see that the source (power comes from your transmitter) is
between the radiator (Zrad) and three other impedances (that makes the
ground impedance as seen from the source). So the source experiences
the sum of Zrad and the three low impedances in parallel.

To avoid that the impedance seen by the source depends on the ground
impedance, you want to have Zrad (Zscreen//Zgpr//Zgpl). For the
quarter wave case (low impedance) you need Zgpr and Zgpl (impedances
of two radials) to have low "ground" (counterpoise) impedance.

For the HW case, Zrad can be 1 kOhm, in that case the radials (Zgpr,
Zgpl) can be removed as the impedance of the cable screen/mast, etc is
mostly below 1 kOhm.

More about the currents
For simplicity think of a coaxial feeder with solid screen far thicker
then the skin depth. In such a case Ic = Isi (figure 5). Isi is
pulled out of the two radials and the outer of the coaxial screen.
Because in this case Zgpl and Zpgr is low (quarter wave radials) Isi
is pulled out of the radials, hence you have less common mode current
(Iso) on your coaxial feeder.

When you don't have radials, Isi will flow on the outside of the
coaxial feeder (figure 6). As this current is as high as the radiator
current (Irad), this may lead to severe problems. Figure 7 shows the
situation for the HW radiator. Due to the impedance transformation,
Irad' is far below the quarter wave situation, hence Iso (=Isi'). Here
Iso is also the common mode current (that contributes (positively or
negatively) to the overall radiation pattern of the antenna).

It is not uncommon that Irad' is about 8 times less then the current
in the middle of the HW radiator.

The advantages of a HW radiator with respect to lower common mode
current disappear in case of thick radiators. This is because the
input impedance of HW radiators depends on the thickness/lambda
ratio.

So whether you need radials (or other provision) depends on the
situation. Design goal is to get Zrad (HW case) Zground
(Zcounterpoise).

Saluti,

Wim

"Cecil Moore" ha scritto nel messaggio
...

5/4WL down from the tip is a current maximum point.
i.e., odd multiples of 1/4WL are current maximums.

i.e., even multiples of 1/4WL are voltage maximums.

Yep
My error is seeing RF "flowing" from my feed point only through the monopole
and up , not considering that an RF generator is a bipolar thing, and
feeding an antenna is ever related in respect both begin and end of the
entire antenna.
In case of my monopole, i simply forget about the other part of my antenna

Sorry for the dumb question. Hope to ask more serious things.

73,
-.-. --.-, Cristiano, Italy.