The antenna is electrically a half wave on 10M to 40M, the electrical length of an element is not directly related to it's physical length. That is how they make a "halfwave" in a stick shorter than a physical quarter wave.

The line "full size halfwave on 2/6 meters" indicates that on those two bands the antenna is not loaded, but actually physically a halfwave antenna. The manual indicates that as initially built to dimension, before adjustment, the antenna is centered on 51 MHz on 6M, and 146 MHz on 2M, so my assumption s it is acutally a halfwave on those two freqs.

"Token" ha scritto nel messaggio
...

The antenna is electrically a half wave on 10M to 40M, the electrical
length of an element is not directly related to it's physical length.
That is how they make a "halfwave" in a stick shorter than a physical
quarter wave.

Ok... could you please explain me how i can build an, e.g., electrical half
wave for the 20 meters in a 3 meters stick ??

I repeat, i'm a great newbie on antenna theory and related arguments, so the
question i pose to you now is not ironic but really posted to increase my
knowledge, if is possible.

Thanks in advance also to other people that may contribute to this question.

-.-. --.-

-.-. --.- wrote:
"Token" ha scritto nel messaggio
...

The antenna is electrically a half wave on 10M to 40M, the electrical
length of an element is not directly related to it's physical length.
That is how they make a "halfwave" in a stick shorter than a physical
quarter wave.

Ok... could you please explain me how i can build an, e.g., electrical half
wave for the 20 meters in a 3 meters stick ??

I repeat, i'm a great newbie on antenna theory and related arguments, so the
question i pose to you now is not ironic but really posted to increase my
knowledge, if is possible.

Thanks in advance also to other people that may contribute to this question.

-.-. --.-

"Electrical half wave" doesn't have much meaning when applied to an
antenna, so there isn't a good answer to your question. There's no way
to make a short dipole behave exactly like a half wavelength dipole in
all respects. But if you mean you want to make a short antenna resonant
(one of the properties of a physically nearly half wavelength dipole),
you can make it or an antenna of any physical size or shape resonant by
putting an impedance transforming (matching) network at the feedpoint.
Presto, a resonant antenna. If you want the same feedpoint resistance at
resonance as a physical half wavelength antenna, you can get that too by
adjusting the network component values. The pattern of a dipole of any
length shorter than a half wavelength will in practice be
indistinguishable from that of a half wavelength dipole. What will
happen is that you'll have increased loss due to the larger currents and
voltages in the shorter antenna and the matching network, the amount
depending on the design. And if the losses are kept reasonable, the
bandwidth of the shorter antenna will be narrower than the bandwidth of
a half wavelength antenna.

Roy Lewallen, W7EL

The antenna is electrically a half wave on 10M to 40M, the electrical
length of an element is not directly related to it's physical length.
That is how they make a "halfwave" in a stick shorter than a physical
quarter wave.

Ok... could you please explain me how i can build an, e.g., electrical half
wave for the 20 meters in a 3 meters stick ??

I repeat, i'm a great newbie on antenna theory and related arguments, so the
question i pose to you now is not ironic but really posted to increase my
knowledge, if is possible.

The short explanation is "reactive loading".

The simplest way to achieve what you're looking for, is to take a
three-meter stick, and wind it with a spiral of wire. The wire should
be insulated, to prevent adjacent turns from shorting together. You
would feed the antenna in the center, just as if it were a full-sized
half-wave dipole.

The exact number of turns required (and thus the total length of the
wire you'd need) can probably be calculated, but I lack the detailed
information to know just what the calcs are. I'm sure that there are
examples shown on the Web, and/or in the ARRL Antenna Compendium
books. You can see one example of this approach at

http://www.w0ch.net/travel_antenna/travant.htm

There are a bunch of design alternatives, divided roughly into

(1) Wind the wire evenly along the whole length of each half of the
shortened dipole.

(2) Run the wire straight along the pole for part of the way from the
center to the end, and wind turns over the rest.

The "travel antenna" is of the latter sort - it puts most of the added
inductance (the coiled turns) down near the feedpoint. He designed it
as a shortened quarter-wave, but you could take two of these and
stick them back-to-back and have a shortened half-wave.

If you measure the resonant frequency of this shortened half-wave and
find that it resonates at too low a frequency, then you've got too
many turns... remove some and run the wire straight along a portion of
the pole (or space all of the turns further apart). If it resonates
at too high a frequency, you need more turns (more inductance).

The behavior of this sort of shortened dipole will be similar to that
of a full-length dipole, with several differences:

- Slightly less directional gain

- Higher electrical losses in the dipole

- Lower radiation resistance

The latter two factors result in a loss of electrical efficiency...
more of your transmitter power turns into heat in the antenna itself,
and less is radiated.

The feedpoint impedance is likely to be different than a full-sized
half-wave, too... it may be lower (due to the lower radiation
resistance) or higher (due to the additional loss resistance) or
nearly the same (if these two factors cancel out).

The approach I've described uses inductive loading - you add
inductance in series with the antenna in order to resonate it.
Another approach is capacitive loading - you add additional capacitive
coupling at the ends of the antenna. This can be done by adding a
circular metal "hat" at each end, or a set of radial wires sticking
out at a 90-degree angle.

The MFJ antenna under discussion actually uses both techniques - it
has an inductive loading coil, and a "capacity hat" of wire spokes, at
each end of the antenna (actually, one per band that it's supposed to
tune). The combination of added inductance, and added capacitive
loading, creates the necessary resonance on each band.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

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 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