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VK3ABK > SAT 17.02.03 05:21l 60 Lines 3027 Bytes #999 (0) @ WW
BID : 55288_VK3VSW
Read: DB0FHN GUEST
Subj: Re Antenna for AO-40
Path: DB0FHN<DB0RGB<OK0PPL<RZ6HXA<SP7MGD<ON0BEL<ON0AR<VK3TE<VK3SPB<VK3FRC<
VK3VSW
Sent: 030217/0407Z @:VK3VSW.#WEV.VIC.AUS.OC #:55288 [Geelong] $:55288_VK3VSW
From: VK3ABK@VK3VSW.#WEV.VIC.AUS.OC
To : SAT@WW
Hello Satellite Watchers.
Flavio, IW2NEF, has asked about dimensions for a 2.4GHz antenna, giving
circular polarization properties, for satellite AO-40.
I suggest a helical wound antenna would be the best, and simplest to make.
I have made several of these, consisting of five to 10 turns of 2mm diameter
copper wire, wound on a 40mm diameter former. A cardboard tube from a roll
of paper towels, aluminium or plastic foil, as used in the kitchen would
be ideal, and can then become part of the antenna support.
The required helix diameter is about 40mm (one wavelength circumference) and
the turns should be about 30mm apart. This spacing can be made just right
when the turns are pulled out from the wound form, and the wound diameter
will reduce to the required 40mm. The result will be a helix that fits well
on the cardboard former.
With or without the former, the helix is soldered to a type N connector
mounted on a metal disk (ground plane) of several wavelengths in diameter.
The lid of a large coffee can is suitable, with the connector centre pin to
the outer surface. The coffee can then makes a nice housing for a 2.4GHz
converter which 'connects' to the lid and antenna.
If you have a 2.4GHz receiver, (downlink only on 2.4GHz) the can may be
discarded, and a *short* coaxial cable link used instead. Better still,
maybe your receiver will fit in the coffee can, giving best noise figure
and lowest loss. A complete antenna and receiver in a can!
Power, audio, and antenna control cable will be required, and light duty
computer cable will do that job OK.
One end of the helix is bent to connect with the type N centre pin, giving
sufficient support for 2mm diameter wire. However wire of, say, 1mm or less,
can be supported by the cardboard former, which can then be glued to the
metal ground plane. Weather proofing the cardboard is easy. A plastic 'radome'
made from a variety of supermarket plastic detergent, or soft drink bottles,
can be mounted in place with a few spots of water proof sealer.
Getting a 50 Ohm match is important with GaAsfet or MMIC converter front-ends,
and this is achieved by bending the helix turn near the connector to get a
suitable capacitive component in the antenna feed point. A thin piece of
brass, cut in a strip and soldered to the last turn near the connector, may
be required. Aim for low converter noise, and freedom from oscillation.
The dimensions of these antennas are not critical, as in the case of dipoles
or yagis, and the only real decision to make is the direction of winding the
helix. Most 2.4 GHz uses Right Hand Circular polarization, ie., looking at
the input end of the helix, the winding goes right hand, or clockwise.
Instead of a small ground plane, better results can be found if the helix is
mounted at about the focal point of a paraboloid, or dish, reflector.
This antenna is easier to make than it is to explain in words!
Hope you get the picture. And the satellite!
73. Dick. VK3ABK.
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