>From a Decibel Products engineer regarding the length of the radials on the DB-201:
"these values were determined empherically for best performance and may be due to the offset characteristics of the folded dipole." Chuck WB2EDV ----- Original Message ----- From: "skipp025" <skipp...@yahoo.com> To: <Repeater-Builder@yahoogroups.com> Sent: Wednesday, February 24, 2010 1:06 AM Subject: [Repeater-Builder] Re: Ground plane yada yada > allan crites <wa9...@...> wrote: > I appreciate the contribution of the information found in > the ARRL April 1962 QST article, which I have reviewed, > and the technical parameters of impedance matching provided, > however the information presented relates to the shortening > of a 1/2 WL simple dipole from the resonant freq. terminal > impedance of 72+j0 to achieve a terminal Z = 50-jXC and the > subsequent removal of the XC component with a shunt XL > Hairpin matching device. > The gnd plane antenna in the original discussion has a 1/4 WL > vertical radiating element terminal impedance of 36+j0. Any > reduction of the length will result in a lower R component as > well as to introduce shunt XC. If the 1/4 radiator was > reduced in length proportionally as the 1/2 WL antenna was, > it would have a terminal impedance of 50 divided by 72 or > 0.694 times 36, resulting in a R component of about 25 Ohms. > This is obviously is going the wrong way to achieve a suitable > match for a 50 Ohm system. There is no way no how that the 36 > Ohm 1/4 WL vertical radiator in a gnd plane antenna can be > made to match 50 Ohms by the addition of a shunt XL component. > Construction of the impedance on a Smith Chart verifies this. > Only a series transmission line with a shunt reactance can > get a impedance match to a 50 Ohm system. > I would be happy to submit a Smith Chart with the appropriate > series transmission line and location of the shunt stub > illustrated.Â
