Steve, when I plug your numbers into a transmission line program I get these
results:283.8 j23.66 = 5.72 vswr (same as your example).
288.2 +j0 = 5.7 vswr which is just slightly lower than example #1 but
illustrates better vswr when j=0.
I did not look into the 40 +j4 case but in general, real antennas show a
reactance change much greater than a resistance change when they are in the
range of one half wavelength. I would expect a reactance of maybe 20 ohms
(positive) for an increase of 5 ohms in the resistive value. This would show a
vswr
of 1.55 which is higher than the 40 +j0 value of 1.25.
Perhaps someone can model a dipole to confirm or refute this.
73,
Gerald K5GW
In a message dated 11/14/2008 3:54:59 A.M. Central Standard Time,
steve@karinya.net writes:
Be aware that there are some "extreme" cases where minimum VSWR and
Resonance do not occur at **exactly** the same frequency.
For example, if I take a 34ft folded dipole, and am silly enough to feed
it through a 50 Ohm line, I get:
Minimum VSWR: 5.72 @ 13.62 MHz where Z=283.8j23.66
Resonance: Z=288.2+j0 @ 13.7 MHz where VSWR=5.76
It just depends how fast the resistive component is changing relative to
the reactive component as you move away from resonance. Using the
previous 40+j0 example, if away from resonance the impedance had become
45j4 the SWR would have fallen from 1.2 to 1.1, not risen. This rapid
shift of the resistive component tends not to happen on simple antennas
like dipoles, but can occur in more complex arrays.
73,
Steve G3TXQ
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