"Antonio Vernucci" wrote in message
...
I wonder whether anyone can formulate a convincing answer to the following
question.

Let us assume you wish to design a 3-element Yagi antenna for good
performances, but with NO REGARD AT ALL TO ITS IMPEDANCE (i.e. assume that
you anyway intend to have a matchbox at the antenna terminals, so that the
antenna impedance is of no concern at all for you).

At this point you run a good antenna design program, giving it no
constrain in terms of antenna impedance. The program will give you the
"optimal" solution in terms of element length and spacing, typically also
showing the antenna impedance at the feedpoint, that we here assume to be
e.g. 30 ohm.

At this point you change your mind and, instead of using a 30-to-50 ohm
matchbox, you decide to match the antenna to the cable using the well
known hairpin technique, by which the radiator length is reduced somewhat
(to create some capacitive reactance in series with the antenna
resistance) and an inductor is put in parallel to the antenna (to resonate
the residual capacitance after resistance transformation).

Doing so the radiator length will result to be shorter that the length
calculated by the computer program, and one could then presume that the
actual antenna radiation characteristics (gain & front-to-back
performance, radiation pattern) do not correspond to those calculated by
the program any longer.

Even though the changes may be small, I would anyway be interested to
understand whether, in principle, the hairpin technique results in a
change of the antenna radiation characteristics.

If the change is confirmed, we would inevitably also come to the
conclusion that the antenna radiation characteristics depend on the
technique which is adopted to match the antenna to the cable (e.g.
matchbox or hairpin). This would sound real odd!

Thanks and 73

Tony I0JX


Tony,
by using hairpin matching, you take out portion of the highest current on
the element and fold it into the hairpin where it is taken out of antenna
"participation" for the price of match.

Better solution is to feed the element off center (K7GCO idea), find the 50
ohm point, insert the insulator and feed it with balun. This way you
preserve the full size of the driven element and its coupling to parasites
and higher antenna efficiency.

Better way is to use quad elements where the stacked elements, higher
impedance and full size elements produce most gain per boom length (up to 5
elements)

73 Yuri, K3BU