----- Original Message -----
From: "Al Williams" <firstname.lastname@example.org>
To: "towertalk" <email@example.com>
Sent: Wednesday, January 12, 2005 9:24 AM
Subject: [TowerTalk] back to antenna subjects
> Tonight I will be the presenter of the program at the Tacoma Radio
> Club. My topic will be antenna modeling (using EZNEC). I intend to
> include the results of a discovery that I made a couple days ago and
> will appreciate comments from this group if I may have made an "errant"
> I modeled an 80m 2 element wire yagi. I chose wire lengths and spacing
> to emphasize how the gain pattern can undergo a 180 degree reversal when
> the frequency is changed from one end of the band to the other end.
> My discovery was that the length of the driven element has little to do
> with the gain pattern. Even shortening the driven down to 20 feet or
> lengthening it to 100 feet made little difference in the gain pattern.
> After thinking about it, it seems quite reasonable to me that the driven
> element is primarily just a source for the frequency and phase
> relationship to the parasitic element to determine the pattern
> (except that the driven element length provides a multiple location
> source but that has much less
> influence on the pattern).
A fascinating example...
Here's one way to look at it. Consider the antenna system as a Pi network,
fed from one side, and open on the other. The elements of the network are:
Shunt element on driven side: Driven element self impedance
Shunt element on un-driven side: parasitic element self impedance
Series element: mutual impedance between the elements.
For a given drive, the current through the driven element is mostly a
function of that element's self impedance.
The current through the parasitic element is a function of the self Z of the
parasite AND the mutual Z.
Now think about the phase of the current in the parasitic element relative
to the voltage at the driven element feedpoint (it's I2 = Efeed *
(Y22+Y12) ) . When the combination of mutual and self Z is inductive, it
will lag, relative to when the combination of mutual and self Z is
The phase of the current in the driven element is just affected by the
driven element's self Z. (Note for rigor here.. yes, the Y11 of the driven
element IS affected by the impedance of the parasitic and spacing, etc., but
it's a lower order effect.. the driven element induces a current in the
parasitic, which then induces a current back into the driven element, etc.).
The Mutual Z (particularly for short elements) varies fairly slowly as a
function of the spacing and length of elements (in terms of wavelengths),
but the self Z changes quite rapidly (i.e. the element is fairly high Q).
So, as you go from 3.5 to 4 MHz, it's very likely that the phase of the
current in the parasitic element goes from leading to lagging that in the
driven element (or, more relevant to a pattern.. leading or lagging relative
to the physical spacing). (It's by adjusting the lengths of the parasitic
elements in a Yagi that you get the right current phase and magnitude).
A 12% change in frequency (500 kHz out of 4 MHz) is a pretty big difference.
Whether the current in the parasitic element is ahead or behind of the
wavefront from the driven element determines whether the beam is forward or
Jim Lux, W6RMK
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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