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Calculate dish effective area. Adjust beamwidth calculation for 10dB illumination taper and better correlation with measurement.
133 lines
4.7 KiB
Markdown
133 lines
4.7 KiB
Markdown
<h1>Antenna Tools Feature Plugin</h1>
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<h2>Introduction</h2>
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The Antenna Tools feature has a couple of calculators to help with antenna design and tuning.
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* A dipole calculator, for calculating the length of a half wave dipole for a given frequency.
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* A parabolic dish calculator, for calculating focal length for a dish with a given diameter and depth, as well as corresponding gain and beamwidth for a given frequency.
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Each calculator is contained within an individual tab. Settings in one tab do not effect calculations in other tabs.
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<h2>Half Wave Dipole Calculator</h2>
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<h3>1: Frequency</h3>
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When entering a frequency in MHz in this field, the calculator will calculate the total and element length for a half wave dipole at this frequency.
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When a length is entered in (3) or (5), this field will display the corresponding resonant frequency.
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<h3>2: Frequency Select</h3>
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Select how the frequency is chosen:
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* MHz - A frequency can be manually entered in (1) in MHz.
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* Device set N - The frequency automatically tracks the centre frequency of Device Set N.
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<h3>3: Dipole Total Length</h3>
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This field displays the total length of a half wave dipole corresponding to the frequency entered in (1) or element length (5).
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When a length is entered in this field, the calculator will calculate the corresponding frequency (1) and element length (5).
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The dipole length is calculated as: l=0.5\*k\*c/f
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<h3>4: Dipole Length Units</h3>
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This field selects the units lengths are entered and displayed in. This can be:
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* cm - centimetres.
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* m - metres.
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* feet - feet.
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<h3>5: Dipole Element Length</h3>
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This field displays the element length of each dipole for a half wave dipole corresponding to the frequency entered in (1) or total length (3).
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When a length is entered in this field, the calculator will calculate the corresponding frequency (1) and total length (3).
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<h3>6: End Effect Factor</h3>
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A half wave dipole in free space with total length being half the wavelength of operation has a reactance of 43 Ohms.
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To eliminate this reactance, the dipole should be shortened. The amount it needs to be shortened by depends upon the ratio of the diameter of the dipole to wavelength,
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with factors ranging from 0.98 for a thin dipole (0.00001 wavelengths) to 0.94 (thickness of 0.008 wavelengths) with a commonly used value of 0.95.
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The calculator doesn't use an analytical formula for this, as the reactance also depends on the enviroment (such as distance to ground), so some experimentation
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is needed in finding the true value.
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<h2>Parabolic Dish Calculator</h2>
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<h3>7: Frequency</h3>
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When entering a frequency in MHz in this field, the calculator will calculate the beamwidth (15) and gain (16) for a dish of the given dimensions (10 & 11) at the entered frequency.
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<h3>8: Frequency Select</h3>
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Select how the frequency is chosen:
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* MHz - A frequency can be manually entered in (7) in MHz.
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* Device set N - The frequency automatically tracks the centre frequency of Device Set N.
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<h3>9: Efficiency</h3>
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Specifies the efficiency of the dish in %. When entered, the calculator will calculate the gain (16), according to the entered frequency (7) and dimensions (10 & 11).
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This value should include losses for things like spillover, illumination loss, feed and support blockage, focus errors.
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<h3>10: Surface Error</h3>
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Specifies the RMS surface error of the dish in length units (as determined by (12)).
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<h3>11: Diameter</h3>
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When entering the diameter of the dish, the calculator will calculate the focal length (14), f/D ratio (15), beamwidth (16) and gain (17).
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<h3>12: Dish Length Units</h3>
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This field selects the units lengths are entered and displayed in. This can be:
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* cm - centimetres.
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* m - metres.
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* feet - feet.
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<h3>13: Depth</h3>
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When entering the depth of the dish, the calculator will calculate the focal length (14) and f/D ratio (15).
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<h3>14: Focal length</h3>
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Displays the calculated focal length.
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Focal length is calculated as: f=d^2/(16*D)
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<h3>15: f/D Ratio</h3>
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Displays the calculated focal length to diameter ratio.
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<h3>16: Beamwidth</h3>
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Displays the halfpower (-3dB) beamwidth in degrees.
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Beamwidth is calculated as: pi/180*1.15*lambda/D.
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Note that the constant 1.15 is dependent upon illumination tapering, which is determined by the feed.
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<h3>17: Gain</h3>
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Displays the calculated gain in dB.
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The nominal gain is calculated as: g0=10\*log10(eff/100.0\*(pi*D/lambda)^2)
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The gain is then adjusted for surface error with Ruze's equation: g=g0-685.81*(e/lambda)^2
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<h3>18: Effective area</h3>
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Displays the effective area in m^2.
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Aeff is calculated as: g*lambda^2/(4*pi)
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