At 02:10 PM 4/6/01 -0700, John Kaufmann wrote:
...
>This is a very good point that is often forgotten when using open or
short-circuited coaxial stubs as reactive elements. These stubs operate
with infinite SWR and as a result, the line losses are MUCH higher than
they would be for a line with 1:1 SWR. The net result is that the
impedance presented by the stub back to the antenna can contain a large
series resistive component. The ARRL Antenna Handbook contains a section
which discusses how to use the Smith Chart to calculate the amount of
resistance with a lossy line. When you plug this resistance into an
antenna modeling program, you might be surprised how much it can degrade
the gain of a parasitic array.
Both K3LR's article (8/94 QST) and my own modeling take note of this
phenomenon. Initially, we tried using lossless NEC-2 transmission lines to
provide the loading, which modeled to provide excellent gain and mediocre
F/B. Substituting loads calculated with TLA as the equivalent to ~150
degrees of RG-8X reduced the gain somewhat, but also produced much-improved
F/B ratio, particularly well above the resonant frequency of the dipoles.
The latter phenomenon has me wondering -- perhaps ~150 degrees length of
coax doesn't load the reflector dipoles enough for optimum F/B at the
resonance of the driven dipole, but the added coax length required to
optimize the reflectors imposes too much additional loss.
73, Pete N4ZR
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