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.