wrote:
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.

Same for dipole length, the reflected wave from the ends of the
dipole will change feed impedance as well as shift resonant
frequency.

The feedpoint impedance of any standing wave antenna is the
superposition of all the E-field voltages divided by the
superposition of all the H-field currents and is the result
of the interference pattern. Doing anything to change any of
the voltage or current components will change the feedpoint
impedance.
--
73, Cecil http://www.qsl.net/w5dxp

Yuri Blanarovich wrote:

"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



Ok, that clears it up. You said originally lower, I said no particular
direction, so the answer is the second, but you probably meant it could
go either way, correct?

I also normally work with YO on yagis, so I really don't pay much
attention to the resonance, just the real portion of the impedance.
Then I fix it when I build the DE and match.
tom
K0TAR