Actually, the information presented in the QST article was in reference to
the practical feeding of the driven element of yagi type antennae where the
normal feedpoint impedance can be quite low and may well be in the range of
8 to 15 ohms. So raising the impedance from 36 to 50 ohms with this method,
that is, with a beta or hairpin match, is trivial. (Actually a 'half-beta" I
suppose.)
A natural extension of this, and a technique perhaps more familiar, is the
"gamma" match where the feed line is attached via a parallel conductor to
the driven element a distance away from the grounded center of the element
and corresponding to a point on the element near 50 ohms. Then the inductive
reactance of the parallel conductor is tuned out by inserting a series
capacitor with the feed or shortening the driven side of the driven element.
The fact that many hams, commercial manufacturers, and my personal
experience, have used these methods successfully for more than half a
century provides me with enough empirical evidence that the techniques are
valid.
73,
Al, K9SI
Re: Ground plane yada yada
Posted by: "allan crites" [email protected] wa9zzu
Date: Tue Feb 23, 2010 4:29 pm ((PST))
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.
a.
<snip>
> "Al Wolfe" <k...@...> wrote:
>
> The straight skinny about the "beta" or "hairpin" match
> can be found in a QST article, April 1962, by Gooch and
> Gardiner. It explains how this matching scheme works. The
> driven element is shortened making it capacitive.
> Then the inductive reactance of the hairpin or beta
> section re-resonates the element by canceling the
> capacitive reactance of the shortened element while
> raising the feedpoint impedance at the same time. Theory,
> formulae, and practical examples are all in the article.
>
> The inverse is also desribed in the article where the element
> is lengthened to make it inductive and a series capacitor
> used to re-resonate the antenna.
>
> Both of these methods have been used for years to manipulate the
> feedpoint impedence of an antenna. The beta does have the
> advantage of presenting a DC ground.
>
> Al, K9SI
