>> How does the ladder line doublet antenna effect the patterns on different
bands? I suspect that the patterns can get somewhat chaotic on the upper HF
bands with say an 80 through 10 meter doublet?
Hi Tony,
In a multi-band doublet, the physical length of the antenna relative to the
wavelength of the band drive the patterns.
As the frequency of operation is increased from 1/2 to 1 antenna wavelength,
the pattern consists of a pair of bidirectional lobes that narrow with
increasing frequency.
As the frequency of operation increases from 1 to 2 antenna wavelengths, 4
new narrow lobes appear at ~45-50 degree angles to the main lobes and
continue to grow while the main lobes continue to narrow and ultimately
disappear.
As the frequency of operation increases from 2 to 3 antenna wavelengths, the
previous 4 lobes rotate towards the axis of the antenna and dissipate while
6 new and narrower lobes begin to develop and grow between them.
As the frequency of operation increases from 3 to 4 antenna wavelengths, the
previous 6 lobes rotate towards the axis of the antenna and dissipate while
8 new and even narrower lobes begin to develop and grow between them.
This process continues with increased multiples of wavelength. The
disadvantage of this is in the deep nulls in the pattern that develop
between the higher ordered lobes. Essentially one end up with a lot of
significant "holes" in the directional coverage when operating a doublet at
higher multiples of wavelength in frequency.
V-beam and rhombic antennas use this multi-wavelength pattern characteristic
by "folding" a multiple wavelength long doublet at an angle such that those
higher order lobes (at small angles off of the ends of the antenna) line up
and reinforce each other (while the broadside lobes cancel each other out),
thus producing a bi-directional gain antenna. In the case of the rhombic,
a proper resistance can be selected and placed at the far end of the antenna
to cancel out the reflected waves to achieve a single directional fixed beam
antenna.
Hope this helps.
Matt
KM5VI
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