On Wed, 22 Jul 1998 sbest@cushcraft.com wrote:
>
> The above statements are perhaps the biggest misconception in the
> amateur hobby regarding antennas. Antenna VSWR does reduce antenna
> gain. No question. If you feed an antenna with a 20:1 VSWR you will
> give up 7.41 dB of radiated power. VSWR results in a mismatch loss
> regardless of cable attenuation.
I think there is some confusion on nomenclature. I disagree that VSWR
affects antenna gain, but agree that VSWR affects *system efficiency*.
Antenna gain is only a function of antenna geometry. System efficiency
includes antenna gain and line loss, but the two are independent.
To wit, assume an antenna feedpoint of 300 ohms, which will result in a
line VSWR of 6:1 if fed with 50-ohm line. If the 50-ohm line has 3dB of
loss at the frequency in question, the non-unity VSWR will result in an
additional 2.5dB of loss for a total of 5.5dB total line loss.
If that same antenna is fed with 300-ohm transmission line, even if the
base line loss remains 3dB, net system efficiency will improve by 2.5dB
because VSWR is 1:1 and there are no reflections. This ignores any losses
that might be incurred by changing the transmitter's output impedance to
300-ohms by using an external tuner or readjusting the output matching
network.
Has the antenna gain changed? Not at all! Nor has feedpoint impedance
changed. System efficiency *has* changed.
If system efficiency with the initial configuration (300-ohm antenna and
50-ohm line) was unacceptable, there are several independent ways to
rectify the problem. One, get another 300-ohm antenna that has 2.5dB more
gain. Two, change the antenna feedpoint impedance to 50-ohms. Three, get
better cable with 2.5dB less loss. Four, change the line impedance to
300-ohms. And so on.
Note also that the term "antenna VSWR" has *no meaning* unless a specific
feedline impedance is specified or assumed. An antenna is what it is and
has no VSWR until a transmission line is connected.
73, Ward N0AX
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