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RULES FOR PATCH ANTENNAS
Extract from AMSAT
Phase 3D Antenna Design Review
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Rules for Patch Antennas
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Use only air dielectric. Air (or
space vacuum) has the lowest loss and a dielectric value of
unity. A dielectric constant, E=1.0, makes the patch element
full size which gives maximum gain. Teflon with a E=2.45 reduces
the size to 64 percent and with no loss reduces the maximum gain
by 3 dB. This is caused by the wider beam width of the smaller
patch.
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Mount patch higher not lower. The
height of the patch above the groundplane should be
approximately two percent of the width of the patch. With air
dielectric a half wave square patch should be a minimum 0.01
wavelengths above the ground plane. Lower heights result in
higher Q and high currents resulting in higher losses.
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Design for maximum bandwidth. The
bandwidth of a patch antenna is direct function of it's height.
The limiting factor is mutual H-plane coupling in a close spaced
planner array. The higher the elements the greater the spacing
required between elements. Minimum edge spacing for 20 dB
isolation between elements is 0.12 wavelength for a height of
0.04 wavelength.
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Use coaxial not stripline feed.
Patch antennas and striplines are not compatible on the same
dielectric material. Strip lines prefer a high dielectric
substrate and minimum height to work properly. 50 ohm strip
lines also require a 1/4 wave transformer to match the edge of a
patch.
With these rules in mind a 0.435 GHz six
element circular patch array was designed for the Phase 3D
Spacecraft. They are supported by a central grounding post and a
dielectric honeycomb under each element. Each element is operated in
a RHCP mode. Element center-center spacing is set at 0.69
wavelengths (470mm) and is limited by the size of the available top
plate area of the spacecraft. The original, and most basic of these
six element arrays is a hexagonal pattern. With equal power to all
elements, the array is set for maximum gain. All elements are fed in
phase and no phase changes are required.
The 1.269 GHz antenna is a Short Back
Fire (SBF). This antenna is two wavelengths in diameter and has a
1/4 wavelength high outer ring with a 1/2 wavelength high post in
it's center supporting a turnstile at 1/4 wavelength high and a 1/2
wave circular reflector. The antenna has a gain of 15 dBic and has a
very smooth pattern. The antenna fits well on the spacecraft and is
within the Negotiable volume set by ESA. This antenna also had the
maximum gain per unit of area for any antenna tested.
The 2.401 GHz antenna is a 500mm dish
with a gain of 18 dBic. The first prototype was built by K5SXK and
weighs in at 1.3 Kg or 3 pounds for the rest of us. The feed is a
turnstile backed by a reflector. K5SXK builds space qualified
antennas in his line of work and expects to deliver a space
qualified antenna ready for our coax connector. Nice Work.
The 5.6 GHz antenna could be a 250mm
dish or a multi-element array. W3TMZ is working on a 5.6 GHz
receiver and antenna array. The antenna would be a low profile
design be 250mm in diameter. AMSAT has received a 250mm spun
aluminum dish from a group in Belgium. It weighs 175 grams an would
have a gain of 20 dBic.
The 10 GHz antenna is now a single 20
dBic circular horn. OH7JP and his group from Finland are well along
on their design. The original design using four horns with a
separate amp on each horn has been changed to a single horn with
multiple amps and a waveguide feed.
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last revised 12 March 2022
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