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