On 12/23/19 12:24 PM, Ignacy Misztal wrote:
I am getting JK Mid Tri 40 next month so this discussion is interesting.
Regarding F/B, one question is how to measure it across the band to see
whether reality reflect simulation.
I erected Spiderbeam at 45 ft as a temporary beam.and used a wideband noise
generator connected to a dipole some 200 ft away, with response measured on
See posting on Spiderbeam group at groups.io:
The Spiderbeam had very narrow F/B characteristics on 20m, with peak > 30db
but just 10db 120 KHz away. This is despite being 3 el. I think the
numbers are a few db below simulations in fee space. However the gain seems
This isn't surprising - a Yagi is a form of phased array, and just like
a 4 square, misphasing doesn't change the forward gain very much, but it
does affect the depth of the nulls/sidelobes/backlobes.
A signal that is -30dB relative to the front is 0.001 the amplitude. If
the F/B changes to -20dB, now that back signal is 0.01. That's only 1%
of the forward power (0.04dB change in forward gain).
On a Yagi-Uda, the current magnitude and phase in the parasitic elements
is set by mutual coupling from the driven element, which is quite
sensitive to the relative lengths of the elements. As a not very
accurate, but simple, conceptual model, imagine the parasitic elements
as all being the same length, but with a series L or C in the circuit
which changes the phase.
The current in a parasitic element that is "resonant" will lag by
approximately the distance between the elements (since that's when the
radiated field from the driven element gets there). By adjusting the L
and C loading on the two elements, you can adjust the relative phase,
and, as a side effect the amplitude.
You can imagine that a small change in amplitude (1%) or phase (a few
degrees) will ruin the "cancellation" in the backward direction, so
instead of cancelling to -30dB you only get -20dB or -10dB.
This is why "low sidelobe antennas" are so hard to make. Imagine an
antenna where the requirement is that the sidelobes be down -60dB. The
proverbial gnat's eylash on the antenna will probably screw that up.
It's also why almost all 3 element Yagis have about the same forward
gain - that's the easy parameter to optimize for.
Of course 200ft is not far away.
Will try the same with my Mid Tri when it is installed.
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