"Mike Coslo" wrote in message
...
Fractenna wrote:

. Of course mesh parabolas

typically only have 80% of the gain of a solid dish,

I would seriously doubt the veracity of this statement and would be
gratified to see valid documented evidence of this. If the mesh
openings
are under 0.1 lambda the mesh will appear solid. The problem may be
achieving good surface accuracy with a mesh dish.Those of us using mesh
dishes on 1296 MHz EME are seeing excellent efficiencies with mesh
surfaces-
ultimately limited by the ability to properly illuminate the surface.
For
EME we tend to underilluminate the surface to take advantage of lower
side
lobes and less spill over, which means "seeing" less warm earth noise.

Dale W4OP


Hi Dale,

A wok has no focal 'point': it is not a true paraboloid. Its the
imperfection
in shape, not mesh:-) Ergo the loss in aperture efficiency.

For the purposes of radio signals, does the shape need to be a specific
paraboloid? Certainly at optical wavelengths a parabola is needed to
focus light from infinity, so a parabola is needed. But at radio
Frequency wavelengths a true parabola is not likely needed. In fact,
considering the difference between a sphere and a parabols, I would
suspect that anything out there is not a true parabola unless it is
arrived at by chance.

- Mike KB3EIA -


Hi MIke,
I am not certain I understand your answer- or Chip's.
The original poster made the statement that mesh parabolas are only 80% as
efficient as a solid parabola. That was what I was answering.

I can tell you that my 14' 1296 dish is within 5mm of being a true parabola-
this is well within the 0.1 lambda RMS error.

For my purposes- EME- an imperfect parabola (this includes sperical
surfaces) would lead to not only a degradation in gain, but much higher
sidelobes and therefore worse G/T. It is not a difficult exercise to measure
Tsys and therefore know just how well your dish is playing.
Spherical antennas certainly have their place- particularly when the surface
is fixed and the beam is then steered by moving the feedhorn- i.e. Arecibo.
But from memory, they take 2nd place to the parabola when one considers
dB/reflector area
Dale W4OP