A helically-wound dipole or monopole that is physically short in terms
of a free-space wavelength can be made electrically resonant at its
input terminals as a result of the inductance of the helical form of
the radiating conductor.

But that does not mean that it has all of the electrical
characteristics of a linear conductor that is inherently resonant,
without the need for inductive loading.

The radiation resistance of such a helically-wound radiator can be
much lower than a naturally resonant radiator, which can mean that the
percentage of transmitter power radiated by the antenna SYSTEM can be
much lower than when a naturally resonant radiator is used.

The link below leads to a page developing this point from Kraus'
Antennas For All Applications, 3rd Edition.

http://i62.photobucket.com/albums/h8...ndVertical.gif

RF

"Richard Fry" ha scritto nel messaggio
...

But that does not mean that it has all of the electrical
characteristics of a linear conductor that is inherently resonant,
without the need for inductive loading.

Yes, replying to Richard but also to Roy and Dave.. maybe i can't explain
very well, but the sense of my latest question is this:

if a half wave end-fed *monopole* antenna have the following primary
characteristics (if IIRC):

- High Z at the feedpoint (voltage maximum and current node);
- very small counterpoise lenght compared to the resonant wavelenght of the
antenna (typical 0.1-0.2 lambda)

can i mantain the same characteristics shortening the antenna in any way ??

Thanks for read and explain to those who want clarify my doubts.

-.-. --.-

On Thu, 1 Jul 2010 15:16:10 +0200, "-.-. --.-" wrote:


"Richard Fry" ha scritto nel messaggio
...

But that does not mean that it has all of the electrical
characteristics of a linear conductor that is inherently resonant,
without the need for inductive loading.

Yes, replying to Richard but also to Roy and Dave.. maybe i can't explain
very well, but the sense of my latest question is this:

if a half wave end-fed *monopole* antenna have the following primary
characteristics (if IIRC):

- High Z at the feedpoint (voltage maximum and current node);
- very small counterpoise lenght compared to the resonant wavelenght of the
antenna (typical 0.1-0.2 lambda)

can i mantain the same characteristics shortening the antenna in any way ??

Thanks for read and explain to those who want clarify my doubts.

-.-. --.-

Google EZNEC and find Roy's site.
Download & install the demo version- it is free!
Go through some of the simple examples like dipoles, 1/4 verticals
etc.
Bring your questions back to this group and we will all benefit.

Antennas are pleasantly addictive!
I consider myself a perpetual student...

John Ferrell W8CCW

On Jul 1, 8:16*am, "-.-. --.-" wrote:
if a half wave end-fed *monopole* antenna have the following primary
characteristics (if IIRC):

- High Z at the feedpoint (voltage maximum and current node);
- very small counterpoise lenght compared to the resonant wavelenght of the
antenna (typical 0.1-0.2 lambda)

can i mantain the same characteristics shortening the antenna in any way ??

With proper design you can maintain the resonance characteristic with
that short antenna, but not its characteristics of SWR bandwidth,
exact radiation pattern (directivity), or radiation resistance.

For a single frequency the only important difference will be radiation
resistance, unless the short radiator is used with a virtually perfect
(zero loss) ground plane.

A typical r-f ground loss even in a set of 120 each, 1/4-wave-long
buried radials is on the order of two ohms. So referencing the
example in the link to Kraus that I posted earlier, the radiation
efficiency of that helically-loaded monopole system with a two ohm r-f
ground would be about 0.6/2.6 = 23%, approximately.

The loss of a radial system using 0.1-0.2 lambda conductors would be
significantly higher, so the antenna system radiation efficiency then
would be significantly less than 23%.

A naturally-resonant, unloaded monopole about 1/4-wave high has a
radiation resistance of around 34 ohms. When it is used with a two
ohm r-f ground, the radiation efficiency of the antenna system is
about 34/36 = 94%.

RF

Richard Fry wrote:
A helically-wound dipole or monopole that is physically short in terms
of a free-space wavelength can be made electrically resonant at its
input terminals as a result of the inductance of the helical form of
the radiating conductor.

But that does not mean that it has all of the electrical
characteristics of a linear conductor that is inherently resonant,
without the need for inductive loading.

The radiation resistance of such a helically-wound radiator can be
much lower than a naturally resonant radiator, which can mean that the
percentage of transmitter power radiated by the antenna SYSTEM can be
much lower than when a naturally resonant radiator is used.
. . .

But a helically-wound radiator *is* a naturally resonant radiator if
wound to make it so. It just isn't straight.

Roy Lewallen, W7EL

Richard Fry wrote:
. . .
A typical r-f ground loss even in a set of 120 each, 1/4-wave-long
buried radials is on the order of two ohms. So referencing the
example in the link to Kraus that I posted earlier, the radiation
efficiency of that helically-loaded monopole system with a two ohm r-f
ground would be about 0.6/2.6 = 23%, approximately.

The loss of a radial system using 0.1-0.2 lambda conductors would be
significantly higher, so the antenna system radiation efficiency then
would be significantly less than 23%.

A naturally-resonant, unloaded monopole about 1/4-wave high has a
radiation resistance of around 34 ohms. When it is used with a two
ohm r-f ground, the radiation efficiency of the antenna system is
about 34/36 = 94%.

RF

Although 2 ohms is a reasonable approximation for 120 radial ground
system resistance, it varies not only with ground quality and frequency,
but also antenna height. For example, an NEC-4 simulation for vertical
radiators at 3.7 MHz with 120 radials, each a free space half wavelength
long buried 0.1 meter in average soil, shows 3.25 ohms ground system
resistance when the radiator is 0.24 wavelength high (nearly resonant).
When the radiator is shortened to 0.12 wavelength, the ground system
resistance increases to 4.30 ohms. And with a 0.06 high radiator, the
ground system resistance nearly doubles to 8.06 ohms. This decreases the
efficiency of the very short radiator by about an additional 3 dB beyond
what it would be if the ground system resistance were fixed at 3.25 ohms.

I believe the ground system resistance increase with short radiators is
due to concentration of the field very close to the antenna, resulting
in much higher ground currents in that region. It would probably be
useful to use a larger number of radials, which could be shorter, when
the radiator is very short.

Roy Lewallen, W7EL

-.-. --.- wrote:
"Richard Fry" ha scritto nel messaggio
...

But that does not mean that it has all of the electrical
characteristics of a linear conductor that is inherently resonant,
without the need for inductive loading.

Yes, replying to Richard but also to Roy and Dave.. maybe i can't explain
very well, but the sense of my latest question is this:

if a half wave end-fed *monopole* antenna have the following primary
characteristics (if IIRC):

- High Z at the feedpoint (voltage maximum and current node);
- very small counterpoise lenght compared to the resonant wavelenght of the
antenna (typical 0.1-0.2 lambda)

can i mantain the same characteristics shortening the antenna in any way ??

Thanks for read and explain to those who want clarify my doubts.

-.-. --.-

You can add a top hat to a vertical that's shorter than a half
wavelength, and bring it to anti-resonance (high input resistance with
no reactance). The impedance won't be as high as if the antenna were a
half wavelength high, and it will have narrower bandwidth, so it won't
be a perfect imitation. Or you can use a combination of loading
inductance and top hat to get a somewhat poorer imitation.

Roy Lewallen, W7EL

On Jul 1, 11:53*am, Roy Lewallen wrote:

But a helically-wound radiator *is* a naturally resonant radiator if
wound to make it so. It just isn't straight.

That is true, but a resonant, helically-wound, electrically short
radiator does *not* have the radiation resistance of a straight, self-
resonant radiator of about 1/4 wavelength -- which can make a large
difference between the two forms in the amount of available energy
radiated vs. that lost in the r-f ground system.

RF

Richard Fry wrote:
On Jul 1, 11:53 am, Roy Lewallen wrote:
But a helically-wound radiator *is* a naturally resonant radiator if
wound to make it so. It just isn't straight.

That is true, but a resonant, helically-wound, electrically short
radiator does *not* have the radiation resistance of a straight, self-
resonant radiator of about 1/4 wavelength -- which can make a large
difference between the two forms in the amount of available energy
radiated vs. that lost in the r-f ground system.

RF

Guess I just don't understand how a resonant, helically-wound antenna is
"electrically short". Suppose you helically wound an eighth-wave
vertical in such a way that it was resonant. Its physical length is an
eighth wavelength. What would its "electrical length" be? That is, how
"electrically short" would it be?

How do you determine what the "electrical length" of a vertical or
dipole is? Does a loading coil at the base alter the "electrical
length"? Does an L network at the base?

Roy Lewallen, W7EL

On Jul 1, 3:18*pm, Roy Lewallen wrote:
Guess I just don't understand how a resonant, helically-wound antenna is
"electrically short". Suppose you helically wound an eighth-wave
vertical in such a way that it was resonant. Its physical length is an
eighth wavelength. What would its "electrical length" be?

Its overall height in free space wavelengths. This is the definition
used by the FCC for the unloaded monopoles used in AM broadcasting

Do you expect your 1/8 WL high, self-resonant helical to have the same
electrical length and feedpoint parameters as a self-resonant,
straight radiator about 1/4 wave high in free space wavelengths?

According to Kraus and other authors, your example above still has
about the same radiation resistance as a 1/8 WL straight conductor --
not that of a self-resonant, straight conductor about 1/4 WL high.

An 1/8 WL high helical may be resonant, but it will not perform the
same in a practical antenna system as a straight, self-resonant
vertical whose physical height in free space wavelengths is about 1/4
wave.

RF