I quite often come across explanations of the Gamma Match (or Tee Match)
which suggest that a "tap" is being made along the dipole at a point
where the "feedpoint" Resistance is higher, and that the inductance
inherent in the Gamma rod is then being cancelled by the series
capacitor. That way of viewing it seems seriously flawed and can quickly
lead to some false conclusions.
Instead, picture a folded dipole as being the limiting case of a Gamma
Match (or Tee Match). We know that the feedpoint impedance does not
approach infinity - as you might expect from that flawed description -
when the Gamma Rod extends to the very tip of the dipole, nor does it
require a very high capacitive reactance to cancel what you might expect
to be a very high Gamma Rod inductive reactance.
We know that in the case of the folded dipole the impedance
transformation is determined solely by the relative diameters of the
wires and their spacing; for equal diameter wires we know that the
transformation will be a factor 4. Now, if we halve the length of one
side of the folded dipole - in other words halve the length of the Gamma
Rod - nothing changes: we still get an impedance transformation of 4 !!!
Seen from that perspective it is the Gamma Rod / Antenna diameter ratio
and spacing which determines the impedance transformation ratio, not
the length of the Rod.
The inductive element appears because by adding the Gamma components we
have introduced in parallel a short-circuit transmission line, the input
impedance of which is inductive if it's less than a quarter-wave long.
This becomes obvious when you look at the currents in the Gamma Rod and
realise they are out of phase with those in the parallel Antenna
element, which in turn are out of phase with those in the outer section
of the Antenna element.
To illustrate the change in thinking, here are a couple of series-form
input impedances taken from an EZNEC simulation of a Gamma Match (no
capacitor used) where I doubled the length of the Gamma Rod:
Gamma Rod = 1ft, Z = 17.5 +j53
Gamma Rod = 2ft, Z = 72 + j95.5
The "traditional" explanation would say that we have increased the
Resistance by a factor of about four, from 17.5 Ohms to 72 Ohms, by
"tapping" further along the antenna, and at the same time the longer
Gamma Rod has increased the series inductive reactance.
However, now convert those impedances to their parallel form:
Gamma Rod = 1ft, Z = 178 // +j58.8
Gamma Rod = 2ft, Z = 198 // +j149.8
Now our interpretation is quite different: doubling the Rod length has
change the resistance very little - it has simply increased the
**parallel** inductive reactance.
I hope that is useful. At the very least, transforming measured
impedances to their parallel form might give a better insight into what
adjustments to make.
One of the better papers on the topic is here:
n6mw.ehpes.com/N6MWGamma4.doc
73,
Steve G3TXQ
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|