I will second that!
Yuri


"Roy Lewallen" wrote in message
...
Jerry Martes wrote:

I thought my statement that an antenna's Resonance is defined by that
antenna having an impedance thats purely resistive, was an accurate
statement.
. . .

It's accurate and complete.

Roy Lewallen, W7EL

On Fri, 26 May 2006 09:21:59 -0400, Buck wrote:

I have a wire dipole and an inverted Vee each cut to resonance on a
desired frequency. I would like to add a reflector and/or a director
(one at the time) to make each one a beam. Will adding the reflector
and/or director change the resonant frequency, or just the impedance?

Thanks
Buck
n4pgw


Maybe I used the wrong word, 'resonance', but I was referring to the
lowest SWR match, which I would assume for the example to be less than
1.5:1 on a given frequency.

To be more specific, assume a 20 meter inverted vee trimmed to minimum
SWR at 14.225. Would adding the reflector, and then the director
(assuming I match the coax tot he antenna impedance) raise or lower
the tuned frequency of the antenna?


--
73 for now
Buck
N4PGW

Buck wrote:


Maybe I used the wrong word, 'resonance', but I was referring to the
lowest SWR match, which I would assume for the example to be less than
1.5:1 on a given frequency.


Lowest SWR is not the same thing as resonance.

This is one of those cases where the Smith chart is a good visual aid
[1]. Here's a three-step guide:

1. The Smith chart is just another kind of "map projection". Instead of
plotting latitude and longitude, we are plotting the resistive and
reactive parts of complex impedances.

Geographical map projections are chosen to make certain features easy to
see correctly, eg distance, direction, or area. The peculiar shape of
the Smith chart is also designed to make certain things easy... once
you've learned how to read the map.

2. All purely resistive impedances are on the "meridian line", the
symmetry axis passing through the center of the chart. The resistance
scale is nonlinear so that resistances from almost zero to almost
infinity can be plotted. Impedances that lie anywhere off that meridian
line have a reactive component. Anywhere inside one half-circle is
inductive; anywhere in the opposite half-circle is capacitive.

3. At the center of the chart is your chosen system reference impedance
(usually 50 ohms resistive). When you plot an impedance anywhere on the
chart, the distance from the center point indicates its SWR, and the
direction indicates the phase angle [2]. This gives the Smith chart one
of its special features: you can draw circles of constant SWR.

It would obviously have been much easier to say that with the help of a
drawing [3] but hose three points are all you need to know in order to
use the Smith chart as a visual aid.

In particular, it is enough to *show* you the difference between
resonance and lowest SWR.

When you plot the R-X impedance of your antenna against frequency across
an amateur band, it traces a short arc on the Smith chart. If the
antenna is supposed to give "a good 50-ohm match", the whole arc has to
stay close to the center of the chart.

The frequency of minimum SWR is where the impedance arc passes closest
to the exact center of the chart. The resonant frequency is where the
arc crosses the meridian line (zero reactance). Those two frequencies
are generally not the same.



[1] Reg will disagree; but Reg also has the kind of mind that can lift
the same information straight out of the transmission-line equations.
The Smith chart is for "the rest of us" who need to think in more visual
terms. Fortunately those two different viewpoints are converging: we can
now have computer programs that solve the equations exactly in the
background, and plot them on the screen as a visual aid.

[2] Strictly speaking, radius is directly proportional to the magnitude
of the reflection coefficient. One complete turn around the chart
represents 180deg of phase angle, for the same reason that impedance
along a (lossless) transmission line repeat themselves every HALF
wavelength.

[3] The ARRL Antenna Handbook (current 20th edn) has a good chapter
about understanding and using the Smith chart... with real drawings.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Buck wrote:
Maybe I used the wrong word, 'resonance', but I was referring to the
lowest SWR match, which I would assume for the example to be less than
1.5:1 on a given frequency.

'Resonance' means where the reactance is zero.
'Lowest SWR' usually means SWR read on a meter calibrated
for 50 ohms. These two conditions may be quite different.
40+j10 is not resonant but has a 50 ohm SWR of 1.4:1.
10+j0 is resonant but has a 50 ohm SWR of 5:1.
--
73, Cecil http://www.qsl.net/w5dxp

On Fri, 26 May 2006 09:21:59 -0400, Buck wrote:

Will adding the reflector
and/or director change the resonant frequency, or just the impedance?


Wow, I sure generated a lot of complex discussion. This was supposed
to be a general question with a simple answer such as, "Yes, the
dipole or inverted vee will have to be lengthened." (or shortened).

I do gather from earlier posts that the driven element will shift
frequency with the introduction of parasitic elements.

Thanks for all the answers.

--
73 for now
Buck
N4PGW

Ian White, GM3SEK wrote:
"The ARRL Antenna Handbook (current 20th edn) has a good chapter about
understanding and using the Smith chart with real drawings."

Thank you Ian.

The 2006 ARRL Handbook also has Smith chart instructions which begin on
page 21.4.

Best regards, Richard Harrison, KB5WZI

Yuri Blanarovich wrote:

As you start adding elements,
they add capacitance to the system and lower the overall resonant frequency
and impedance. Single Dipole is around 75 ohms, 3 el. Yagi around 30 ohms.
The more elements you add, the more sensitive the design is and requires
more prunning for optimum performance. So just slapping elements to Inv Vee
dipole will not produce optimized antenna. EZNEC, 4NEC2, MMANA are an
excellent tools to demonstrate that and to optimize the design, give you
dimensions and impedance and understanding behavior of antennas, give
current distribution in elements, plots of impedance, gain and other
parameters.


I've never seen a yagi design center frequency move any particular
direction when another element is added. I am sure you have a specific
example you can give us.

thanks
tom
K0TAR

Yuri Blanarovich wrote:

As you start adding elements,
they add capacitance to the system and lower the overall resonant frequency
and impedance. Single Dipole is around 75 ohms, 3 el. Yagi around 30 ohms.
The more elements you add, the more sensitive the design is and requires
more prunning for optimum performance. So just slapping elements to Inv Vee
dipole will not produce optimized antenna. EZNEC, 4NEC2, MMANA are an
excellent tools to demonstrate that and to optimize the design, give you
dimensions and impedance and understanding behavior of antennas, give
current distribution in elements, plots of impedance, gain and other
parameters.


And I should have said resonance, not center frequency.

tom
K0TAR

"Tom Ring" wrote in message
.. .
Yuri Blanarovich wrote:

As you start adding elements,
they add capacitance to the system and lower the overall resonant
frequency and impedance. Single Dipole is around 75 ohms, 3 el. Yagi
around 30 ohms.
The more elements you add, the more sensitive the design is and requires
more prunning for optimum performance. So just slapping elements to Inv
Vee dipole will not produce optimized antenna. EZNEC, 4NEC2, MMANA are an
excellent tools to demonstrate that and to optimize the design, give you
dimensions and impedance and understanding behavior of antennas, give
current distribution in elements, plots of impedance, gain and other
parameters.


I've never seen a yagi design center frequency move any particular
direction when another element is added. I am sure you have a specific
example you can give us.

thanks
tom
K0TAR

No problem,
go to EZNEC, load NBS Yagi example
3el. resonant around 49.95 MHz
take director away, 2 el. resonant around 49.30
take reflector away, dipole left resonant around 51.05

You welcome

Yuri, K3BU

"Jerry Martes" wrote
So, I'd expect the antenna's "resonance" to be defined by its
"impedance".

Right you are Jerry.


Yuri Blanarovich wrote:
Antenna resonance is defined by its element's electrical properties,
"caused" by its physical properties/dimensions.
In case of 3 el "conversion" from Inv Vee, you have to picture elements as a
tuned circuits. If they are spaced within a fractions of a wavelength, they
have mutual impedance, affecting each other. As you start adding elements,
they add capacitance to the system and lower the overall resonant frequency
and impedance. Single Dipole is around 75 ohms, 3 el. Yagi around 30 ohms.

That's not quite accurate.

The mutual coupling and effect of additional elements can shift
frequency in any dorection depending on parasitic element tuning. The
effect isn't caused by the capacitance of additional elements, it is
caused by the complex interaction or reradiated power arrivibng with a
time delay back at the original source.

This is why, with various element tuning and spacings, feed impedance
of a Yagi can change all over the map.

Same for dipole height, the reflected wave from the earth will change
feed impedance as well as shift resonant frequency.

73 tom