On 4/11/2020 8:14 PM, Tox wrote:
Ft-991 (pre-A samples at least) has trouble tuning some multitrap verticals
for all the expected bands, this may explain why
The things we call "antenna tuners" do nothing at all to the antenna.
Rather, they are an adjustable network that we adjust so that the rig
(or power amp) supplies power to the transmission line, and so that the
output stage is "happy." In this context, "happy" means that the output
stage is operating in a "safe" region (not likely to overheat or fail),
and that it is producing minimum distortion.
If the antenna is poorly matched to the line, there are standing waves,
increasing the loss in the line, so less of that power gets to the antenna.
The fact that a radio doesn't work with a multi-band vertical says more
far about the multi-band vertical than it does about the radio. Perhaps
it was assembled badly, was damaged in some way, lacked radials (if a
type that needs radials), or was installed in proximity to a vertical
metallic object. Or the feedline was defective.
On 4/11/2020 7:57 PM, Mark Goldberg wrote:
> In addition, a Yagi operated at a non resonant frequency, especially
below
> it's design frequency is likely to not act like a Yagi at all. The
pattern
> depends on the elements being close to resonance.
Right. ALL antenna arrays (that is, those with multiple elements) only
work at their design frequency, or a narrow band around that frequency.
That's because they work on the basis of their electrical length in
degrees, and the electrical distance in degrees between the elements. So
that 10M Yagi is nothing more than a non-resonant dipole with a random
bunch of aluminum around it.
SWR is DEFINED by the match between the antenna and the line, and that
SWR causes additional loss. This loss causes the SWR to get smaller
along the line moving in the direction of the transmitter. When an SWR
bridge in the shack reads 5:1, the SWR at the antenna could be 10:1,
depending on the loss characteristics of the line and its length)!
So the antenna tuner does nothing to change the SWR or the antenna.
73, Jim K9YC
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