‹header›
‹date/time›
Click to edit Master text styles
Second level
Third level
Fourth level
Fifth level
‹footer›
‹#›
A tour of Electromagnetic Fields in and around antennas, particularly feed horns.
Reading Appendix 6C first is recommended
The near field is inside and around the antenna. The far field is beyond the Rayleigh distance, 2D**2/lambda
The near-field distribution at the aperture and the far field radiation pattern are a Fourier Transform pair (like time and freq domains)
An open-ended rectangular waveguide is a simple antenna whose properties are well-known
Simple ideal case: no field outside of aperture
Extends beyond guide. Field high on wide walls but not narrow walls creates edge currents on aperture wide edges.
Guide wavelength (inside) is longer than free space wavelength (outside)
E-plane field reaches waveguide walls, H-plane field falls off to zero at walls
Large field at E-plane wall creates edge currents which cause sidelobes
High field along rim of wide wall induces edge currents, which radiate and cause sidelobes
Unwanted lobes in far field from edge currents at aperture
H-fields (magnetic) are always present, but don’t usually add a lot to visualization. Note large field at rim. E x H = Power
E-plane edge currents radiate to sides and back. Good simple feed, but others produce higher efficiency.
High E-field at rim of circular horn in E-plane
Edge currents are additional sources which interfere with main lobe to form sidelobes
Penny feed has two distinct sources, one from each slot, and a null between them. Radiation pattern must have interference effects.
Asymmetrical fields due to two sources
If edge currents produce sidelobes, how can they be reduced?
Choke ring reduces backward radiation from edge current
Choke ring reduces sidelobes from currents in rim of central circular waveguide
Multiple choke rings work over wider bandwidth
Changing the choke ring position adjusts the far-field pattern for different f/D
W2IMU dual-mode feed generates second mode (TM11) to cancel E-field near aperture rim
Two modes combine to cancel field at edge of aperture – field is in center of aperture
G3PHO 10 GHz W1IMU-style dual-mode feed from plumbing fittings
Unmodified plumbing fitting: Improper taper dimensions put modes in wrong relation and break up fields
Slot-fed cavity feed: pins next to slot generate second mode
Diagonal horn by A.W. Love generates more symmetric field than rectangular or cylindrical horn
Diagonal horns have good field symmetry, produce symmetrical E-and H-planes but worse in 45 degree planes
Gradual taper works well when larger aperture is desired
Larger flare angle shortens horn
Excessive flare angle has high field at corners
Diagonal horns have high edge currents at corners, producing E-plane sidelobes
Moderate flare is better
Excessive flare angle has high field at corners
Plain flared horns have predictable performance and are easy to design and fabricate
Conical horns have high edge currents in E-plane, making far-field pattern asymmetric
Conical horns have high E-field edge currents at the rim
Rectangular horns have high E-plane edge currents, but flares can be adjusted to make far-field pattern symmetrical
Black surplus horn -- what do you want for $5?
Reverse feeds are fed by waveguide through center of conventional prime-focus dish. Mechanically great, low feedline loss.
Asymmetrical fields due to two sources radiation – one at each slot. Low efficiency.
Pins on penny splasher improve performance somewhat
Bending and shaping the penny splasher doesn’t help. Note large field at tip of splasher
Circular guide with disk splash plate. Note large field at rim of splasher – diffraction around the edge.
Diffraction around edge of small splash plate
Larger splasher disk reduces diffraction. Still asymmetric.
Rim changes fields completely. Radiation appears to emanate from point between rim and guide; E & H asymmetric with phase error
E-plane and H-plane fields not in phase – this feed has low efficiency due to phase error
Waveguide fed dipole and reflector on metal strip at end of guide. Poor front-to-back. Asymmetric E & H fields like any dipole feed.
Waveguide-fed dipole with cup reflector. Asymmetric E & H fields like any dipole feed.
Interesting surplus horns
Chaparral-like choke rings shaped for oblong dish. Originally used vertical polarization at 11 GHz
We would prefer horizontal polarization at 10 GHz; still works pretty well
Rectangular sectoral horn – corrugations in E-plane work well to reduce edge currents
Corrugations in H-plane: no field near corrugations so they don’t do anything, large edge currents on plain E-field side
E-plane corrugations reduce field and current at rim, H-plane corrugations aren’t needed
Dual-band design is difficult even with fancy software!
Dual band feed works like simple coffee-can feed at 10 GHz
At 24 GHz, fed from rear waveguide, still works well as oversize coffee-can – very little sign of additional modes
Choke rings improve performance slightly at 10 GHz
Choke rings reduce performance slightly at 24 GHz
Dual band experiment – works like coffee-can at 2304 but step from rear waveguide feed at 5760 generates unwanted modes
Dual-band feed works like simple coffee-can feed at 3.456 GHz
Dual-band feed attempts to act as dual-mode feed at 10 GHz – second mode is obviously destructive
An alternative, or supplement, to lots of words of explanation
Nevermind all the details – tell me the answer!
Sometimes we lose sight of our objective…
Very good efficiency, easy to fabricate, work well without adjustments.