The hairpin (or BETA) match requires that the DE be slightly capacitive
(i.e. with an impedance approx. R -j15 ohms). The shunt inductance of the
Beta match and the series capacitance of the DE cancels out (along with
other stray bracket capacitance and pigtail inductance) to produce either a
50 or 200 ohm balanced input. More DE shortening is required to match to
200 ohms. The value of "R" can be anywhere from 10 to 49 ohms. The
hairpin will be very short at 10 ohms and close to a quarter-wavelength at
49 ohms. (Of course if you are at 49 ohms, you don't need the hairpin for
matching, but it does make a good DC short) I don't like to match anything
below 10 ohms because of resistive losses.
The "T" match also requires that the DE be slightly capacitive. Each "T"
conductor and part of the DE make a section of transmission-line, shorted
at one end. Since each of these lines is shorter than a quarter-wave, they
have inductive reactance. This inductance can be cancelled out by making
the antenna shorter and more capacitive or by adding series capacitors to
the input terminals of the "T" match. The length of the "T" sections and
the length of the DE are adjusted to match 200 ohms.
I don't know of any reference that compares the amount of shortening that
each match requires. The main problem with this is that the hairpin needs
a split dipole DE while the "T" match needs an unbroken length of DE.
There are variations, however that defy this requirement, however. The
Hy-Gain 66DX 6 meter beam uses a split-DE with a "T" match and half-wave
coaxial balun. The "T" in this case is more like a VERY short but very
WIDE hairpin.
To answer your second question, the parasitic element lengths and spacings
should not be changed. They determine the gain and radiation patterns,
mostly independent of the type of DE. When designing a beam antenna, the
DE is tuned for low VSWR last, after the parasitic lengths and spacings
have been determined. Also, the input impedance and bandwidth of the DE is
directly affected by the parasitics, so if they are changed, the DE has to
be retuned.
73 Roger WB0DGF
Telex Communications, Inc.
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|