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Parabollic Grid antennas.

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Darkseed

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parabolic dish ring feed by dl4mea

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.

Any ideas?

Thanks in advance.
 

flatulent

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ringfeed ghz

The number I have heard is 0.1 wavelength or less separation. Also the wires should be in the same direction as the electric field.
 

Darkseed

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This is the version I know it works but I have seen comercial grid antennas where the wavelenght is at the range of the element spacing.
 

FANT

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Antenna grid

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.

Mandi
 

flatulent

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ham sources

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.
 

Darkseed

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Thank you all for your help! :D
 

flatulent

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one final thought

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.
 

Darkseed

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Good point. I didn't realised that. This means you have saved
me some hours of not-so-usefull work.

Thanks! :D
 

freddy_microwave

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Hi,

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..

Loop: http://www.qsl.net/dl4mea/ringfeed.htm

Freddy
 

flatulent

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feed efficiency

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.
 

S

sick_man

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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
 

flatulent

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alford slot

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.
 

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sick_man

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i dont think you understand

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


but yet is completely tube directional

very strange ey????
 

flatulent

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published reference

Can you give me some references to published work on this?
 

S

sick_man

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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}
 

thisthat

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Anyone got any good designs for 24 dBi or better for the 2.4 GHz band dipole antenna.
Send and receive.
1km distance required.

thisthat
 

S

sick_man

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dipole 24 dbi lol:) from a dipole hahaha lol:)

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
 

2009kucing

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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...
 

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