On 9/21/22 2:04 PM, Jim W7RY wrote:
No kidding Gregg!
Do we really have to make rocket surgery about this?
Not if all you're doing is throwing up a dipole and want a "reasonable"
match - I just make it close, and use a tuner to take out the small error.
But if you're doing a phased array (like a 4 square) then you might care
a bit more, because the F/B and null depth is going to depend on small
changes.
And, to make a direct rocket example - I'm working on a project (SunRISE
mission) with 6 spacecraft orbiting the Earth just above GEO to make
radio observations of the Sun. We have 2 dipoles, a bit more than 5
meters long overall, on each spacecraft to receive from 0.1 MHz to 23
MHz. We care *a lot* about the exact impedance vs frequency for two
reasons:
1) the receiver is a high Z preamplifier with shunt C (parasitic), so
the circuit looks like a capacitive voltage divider. It's not practical
to test the antenna on a range (that 3km wavelength makes things tricky).
2) we will "see" about half the Earth, so all those FM and TV stations
are up near the resonances (30 and 90 MHz) and anti-resonances (60 MHz,
120 MHz) of the antenna. We don't want to overload the preamp.
So for this kind of thing, yeah, we DO care about small effects in the
model. And it's really useful to "know" what the changes are if the
antenna booms deploy to 2.7 meters instead of 2.6 or 2.8 meters.
Another "space" application of modeling, where we care about insulated
wires (and soil properties) is for a proposed radio observatory on the
far side of the Moon. There, the dipoles are 100m long and are
insulated wire laid out on the surface of the Moon.
73, Jim W7RY
On 9/21/2022 1:03 PM, Gregg Seidl wrote:
Goodness. It's not that hard. My experience is 468/ freq. Cut it a foot
longer and put it up. You might have to fold some back on the
insulators.
Just try it.
Gregg. K9KL
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