I want to design one of such antennas for home use. It is intended to be a 24 dBi for the 2.4 GHz band. The feed will be a classic dipole. The question is: is the spacing between reflector wires determinant? I know that, if they are really close, it is'nt. Nevertheless, I'm not so sure in the case spacing is at the range of the wavelength.
The spacing among the wires must be much less tan 1/4 wavelenght to avoid resonance.
The dimension of 1/10 wavelenght seems to be fair; take in account to prepare a strong frame to avoid modification of the shape of the grid, so not to decrease the gain.
Better results are obtained with the grid made by a net where at the crossing the two wires are soldered togheter.
I have consulted sevreal amateur radio handbooks. The British RSGB suggests 0.1 wavelength. The US ARRL suggests 0.06 wavelength. Since hams usually are more interested in cost than the last dB of performance, these values are probably real limits.
The gain of reflector antennas depends on the dimensions being exact. If the surface of the reflector is off, the gain is reduced. You may find it easier to control the flatness of a corner reflector than the dimensions of a curved surface. For 20 dB gain, the surface must be held to about 1 mm in the 2.4 GHz frequency range.
yes 1/10 Lambda is good and sufficient value. But dont use a dipole
as feed, a loop will do much better for linear polarisation. A very
broadband radiator with good efficency for f/d=0.5. Easy to build.
Be shure to keep the parabolic form of the reflector, not the easy...
but there are a lot of cheap comercial dishes for TV..
Illumination efficiency is affected by the radiating pattern of the feed. Commercial microwave dishes lose 3-4 dB from the theoretical maximum gain even though they use horn feeds. The two sources of this loss are the feed radiating into angle areas where there is no reflector. The other source of loss is the feed having reduced gain at angles towards the edges of the reflector. Back in the 1939-1945 war, Allied radar dishes were fed with a two element yagi. The UHF TV antennas usually have two bow tie dipoles in front of a screen reflector. At these frequencies, the dimensional tolerances for the dish are in the cm range which is easy to meet.
why not make a wave guide like a shower rail or a folded alford slot
to a circle
placed at the proximity point of the radius using a dip ed dish back in
like a splash shape on water of a droplep then you can mount rx and tx assembleys at any phase relativity using a simple poleriser
The Alford slot is omnidirectional and therefore would send most of the energy outside the reflector. The parabolic reflector antenna depends upon a point source feed. This is why horns are used. They are a close approximation to a point source. They do not have an ideal radiation pattern and produce the 40%-50% feed efficiency. The ideal pattern is constant field for all angles up to the edge of the dish and then infinite sidelobe suppression. This is equivalent to the brick wall filter problem and has the same approximations used to get close.
any antenna is onmi directional that stand upright
so has a donut shape
an alford slot is specificaly designed to penitrate by shaping the tx signal downward while extending its range by flattening the donut of a normal dipole radiation pattern
so.... therefore
much like a dipole as you bend the radiator round you change its impedance or Z
and the radiation pattern
also its proximity to the isotropic norm
by 90deg
this changes its Z to 50R from 75R
if you make a circle with a dipole this is a delta loop
working on a different magnetic cohesive varient flux fields set and not using atmospheric flux media
but instead clinging tot he mangentic subshelf within the ground wave field
making distance in a solid rather than a gas greater
so given that a sat dish is listening to a vacum much like listening to a bell in a drum
you will be able to offset the actual resonance of nothing from the radio wave you receive
given that a 1/4 ring ..... is also a 1/4 wave
then a simple stub is all that needs taken away
to balance the impedance to 50 R again
a disk has a focal point
but also has a variance field and also more important
can be resonant its self BY density and weight atomicaly
lots of research was done into using mesh made from glass fibre
for parabolic dish
but a better system is to use a ring of dipoles each polerised at 10 deg angles as bowties made on a substrate
for conventional mounting this is fine
using a lip round the dish add stabilities
and also gives a place to add gain like a shell from the sea.....
the dish lip carrying the dipole array
this way many thousands of dipoles each get a shot
and the conventional one is used to give referances being made of four bowties
this way you can target with a still dish using just polerisers on the ring
also adding extra coverage and surface area
however i fully agree with 1 1/10 of a wavelengh uniform tollerance at 10 cm band this is within 1cm all over to avoid breakups of week signals from wind
the best dish is bilt underground using a few to focus a bigger beam width
dishes arenet so good
i used to listen to many satelites using just a reflector and a circular spiral yagi
at 10 cm is good ..... at 15 cm was very good ..... this is also an alford slot design
try the arrl handbook
you will find drawings of such antenna
i think nec also has several demos of them you can simulate....
as for sat stuff
there is a huge dish basicaly underground in the top of a volcano in hawia ?? spelling isnt my strong point
it is a radio telescope for the 10 - 21 cm bands
this partial qualifies as the underground approch
but if you simply put a dish underground and feed its foward face with reflections from any source {you can use a xtal to focus this and redirect it and also place xtal mixer to do a simialr job by bifringance technique using blocks of lattice}
deep space {pre hubble} days way done this way
the americans carrying out many tests pre and post war on underground antennas for this you can read the declassified military sww archive avalible washington dc....
or i think even on the net {youll need to hunt }
there are many methods to reflect gathered points
using an upside down pymarid shape {you can rotate etc on a gimble}
maybe better to use a wave guide and a dish or use a mobile phone antenna and cut it down a bit 5/8 over 5/8 lambda or usually quad tripple 5/8 for 24dbi tripple 5/8 is needed for vertical polerisation
can anyone help where or space from parabolic dish for my 12 element yagi (freq 2.4ghz) . My yagi reflector element have space about 25mm from dipole, and ,my parabolic dish (i use old indoor tv receiver) 21cm (diameter) , f = 11cm
please help...