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Weatherproof antennas

Weatherproof UHF & microwave cavity antennas

Matjaz Vidmar, S53MV





1. Weatherproof antennas for UHF and lower microwave frequencies

1.1. Radio-amateurs, weather effects and antenna design

Quite frequently, radio-amateur antenna design is limited to a few popular antenna types. Directional-antenna designs are usually limited to Yagi antennas for the lower frequencies and parabolic dishes for the microwave frequency bands. The operation of a Yagi antenna is based on a collimating lens made of artificial dielectric like rods, loops, disks or helices. The basic design goal of all these slow-wave structures is to achieve the maximum antenna directivity with the minimum amount of material (metal).

The situation is actually made worse with the availability of inexpensive antenna-simulation tools for home computers. The latter provide designs with fantastic gain figures using little hardware. Unfortunately, these results are barely useful in practice. Besides impedance-matching problems, such designs are extremely sensitive to manufacturing tolerances and environmental conditions: reflections from nearby objects and accumulation of dirt and/or raindrops on the antenna structure.

The operation of a 2m (144MHz) Yagi with thin rods (or loops or helix) antenna will be completely disrupted if snow or ice accumulates on the antenna structure. Raindrops accumulated on the antenna structure will completely compromise the operation of a 23cm (1.3GHz) or 13cm (2.3GHz) Yagi antenna. Manufacturing tolerances limit practical Yagi antennas to frequencies below about 5GHz.

Professional VHF Yagi antennas use very thick rods to limit the effects of snow and ice on the antenna performance. All professional Yagi antennas above 300MHz are completely enclosed inside radomes made of insulating material that is transparent to radio waves. Since any radome includes some dielectric and the latter has a considerable effect on any Yagi antenna, a Yagi antenna has to be completely redesigned for operation inside a weatherproof enclosure. If some natural dielectric (radome) is added, then some artificial dielectric (Yagi rods) has to be removed to maintain the same focal length of the dielectric collimating lens.

Simply speaking, any serious design of a weatherproof Yagi antenna for frequencies above 300MHz is out-of-reach for most amateurs. Fortunately there exist other antenna solutions for amateur-radio equipment that has to operate unattended on mountain tops like FM voice repeaters, ATV repeaters, packet-radio nodes and microwave beacons. The most popular solutions are arrays of dipoles, quads, eights etc. The installation of the latter inside suitable weatherproofing radomes is much less critical than the weatherproofing of Yagis or helices.



1.2. Practical cavity antennas

Of course, a different antenna-design approach may provide much better results, like considering the weatherproofing issue right from the beginning! Cavity antennas may initially require more metal for the same decibels of antenna gain. On the other hand, a cavity antenna is relatively easy to weatherproof: most of the radome is the metal cavity itself and just a relatively small radiating aperture has to be additionally protected with some transparent material.

Before deciding for a particular cavity-antenna design, it makes sense to check well-known solutions. A comprehensive description of many different cavity antennas is given in the book [1]. A useful selection tool is the following directivity plot as a function of cavity diameter as published by Ehrenspeck [2]:

fig121
Fig.1.2.1 - Ehrenspeck 's directivity plot for round cavities.

Although Ehrenspeck 's diagram is a little bit optimistic regarding the achievable antenna directivity, it shows many important features of cavity antennas. The plot has local maxima and minima, meaning that not just every cavity size works fine. There are some cavity sizes that provide particularly good antenna performance. These fortunate cavity sizes may achieve aperture efficiencies beyond 100%!

It is interesting to notice that the real directivity plot does not come to an end as suggested by Ehrenspeck many years ago. As shown at the end of this article, the real directivity plot has at least one additional maximum, corresponding to the recently developed "archery-target" antenna [3]. Probably there are many more maxima at larger cavity diameters yet to be investigated.

Another important design parameter is the cavity height or (conductive) rim height surrounding the cavity. The maximum directivity is achieved at rim heights of one half wavelength or slightly above this value, regardless of the particular cavity diameter, as shown on the following plot:

fig122
Fig.1.2.2 - Directivity as a function of rim height.

As shown on the above plots, cavity antennas fill an important directivity gap between at least 7dBi and 20dBi. Since cavity antennas are simple to manufacture including weatherproofing, parabolic dishes become practical only if directivities of 22dBi or above are required.

Many practical cavity antenna designs follow directly the above-mentioned directivity plots. The most important are presented in this article including practical weatherproof designs for the amateur-radio frequency bands of 435MHz (70cm), 1.3GHz (23cm) and 2.3GHz (13cm). Some other important designs are omitted due to space limitations, like square cavities and cavities fed with microstrip patches.

All presented antennas were built and accurately tested many years ago at our outdoor antenna test range at the Department for Electrical Engineering of the University of Ljubljana, thanks to Mr. Stanko Gajsek. All directivity values and plots were computed from the measured E-plane and H-plane radiation patterns.

The measured radiation patterns shown in this article are all plotted on a 40dB logarithmic scale to have an excellent view of the sidelobes and any other side effects. Please note that it is relatively easy to hide antenna-design deficiencies by using a linear scale or a 20dB logarithmic scale!

All presented antennas include a transparent radome that is already part of the antenna structure. Therefore little if any additional effort is required for complete weatherproofing of these antenna designs. For terrestrial (horizontal) radio links the radiating surface is vertical, therefore rain drops, snow and ice quickly fall away if they ever stick onto the radome. Finally, the radiating surface is an equi-phase surface, meaning that a uniform coverage with ice or other foreign material does not defocus the antenna.

The presented antennas were initially used for 38.4kbps and 1.2Mbps links in the amateur packet-radio network in Slovenia. Some of these antennas accumulated 15 years of continuous operation in extreme climatic environments on mountaintops. During these 15 years, rain, snow and ice never caused any link dropouts.

A typical example is the packet-radio node CPRST:S55YCP installed on a mountain hut about 1840m above-sea-level and powered by solar panels. The antenna system of the latter includes a GP for 2m, two cup dipoles for 70cm, one cup dipole for 23cm, one SBFA for 23cm, one SBFA for 13cm and a webcam as shown on the following picture:

fig123
Fig.1.2.3 - Cup dipoles and SBFAs of CPRST:S55YCP.


 


2. Circular-waveguide horn

2.1. General circular-waveguide-horn design

The simplest cavity antenna is a waveguide horn. Waveguide horns are foolproof antennas: whatever horn of whatever size and shape will always provide some useful directivity. Waveguide horns are also among the best-known cavity antennas in the amateur-radio community, therefore they will be just briefly mentioned in this article.

The simplest waveguide horn is just a truncated waveguide, either of rectangular or circular cross-section. Such an antenna provides a gain of about 7dBi and a main lobe width (-3dB) of around 90 degrees. These figures are useful when a broad coverage is required and/or to illuminate a deep (f/d=0.3-0.4) parabolic dish.

At UHF and lower microwave frequencies such an antenna usually includes a coax-to-waveguide transition. A practical solution is shown on the following drawing:

fig211
Fig.2.1.1 - Circular-waveguide-horn design.

A quarter-wavelength probe is used to excite the fundamental TE11 mode in a circular waveguide. One end of the waveguide is shorted while the other end acts as a radiating aperture. The distance between the probe and the short should be around one quarter wavelength, to be adjusted for best impedance matching. The distance between the probe and the aperture should be at least one half wavelength to suppress higher-order modes inside the waveguide. One or more tuning screws may be added to improve impedance matching. A simple feed probe generates a linearly-polarized TE11 mode. Two feed probes fed in quadrature or a single feed probe and many more tuning screws inserted in a longer circular waveguide are required to obtain circular polarization.



2.2. Practical circular-waveguide horn for 23cm

Various-size coffee cans usually make useful linearly-polarized horns for 1.7GHz and 2.4GHz. A weatherproof waveguide horn for 1.3GHz (23cm) is a little bit too large for practical cans and will probably have to be purpose-built from aluminum sheet. The required dimensions for operation around 1280MHz are shown on the following drawing:

fig221
Fig.2.2.1 - Practical horn for 23cm.

The radiating aperture is covered by a disc of FR4 laminate (with any copper plating removed!) or thin plexi-glass that acts as a radome. The position of the tuning screw and the length of the feed probe correspond to the given design including all dimensions and the effect of the radome. If the waveguide section is made longer, the position of the tuning screw and the length of the probe will necessarily change!

If only simple tools are available, then it makes sense to build the individual components of the horn from aluminum sheet and bolt them together with small M3X4 or M3X5 screws. Aluminum is a good electrical conductor providing low losses in the antenna structure and does not require any special environmental protection. The required mechanical components for the horn for 23cm are shown on the following drawing:

fig222
Fig.2.2.2 - Mechanical components of the horn for 23cm.

The antenna is first assembled together using just bolts, making all necessary adjustments like feed-probe length and tuning-screw position. Afterwards the antenna is disassembled so that all seams can be sealed with small amounts of silicone sealant. Finally, do not forget a venting hole or unsealed seam in the bottom part of the antenna, where any (condensation) moisture can find its way out of the antenna!

The measured E-plane and H-plane radiation patterns of the prototype antenna are shown on the following plots:

fig223
Fig.2.2.3 - E-plane and H-plane radiation patterns of the horn for 23cm.

The measured patterns in both planes at a number of different frequencies were used to compute the directivity as shown on the following plot:

fig224
Fig.2.2.4 - Directivity of the horn for 23cm.

The operation of a simple waveguide horn is disrupted when higher-order modes are excited. In the case of a simple feed probe in a circular waveguide, the first disturbing mode is already the TM01 mode. The latter causes an unsymmetrical illumination of the aperture resulting in a squint of the direction of radiation.

The appearance of higher-order waveguide modes means that not every coffe can makes a useful antenna for the desired frequency range! The prototype horn for 23cm displays a large squint of the main lobe due to higher-order modes already at 1450MHz as shown on the following plot:

fig225
Fig.2.2.5 - Main-lobe squint due to higher-order modes.



 


3. Cup dipole

3.1. Basic design of a cup dipole

In order to increase the gain of a waveguide horn, the size of the aperture has to be increased without exciting too many higher-order waveguide modes. A rather simple solution is to make a smooth transition from the waveguide to the larger aperture in the form of a pyramidal or conical horn. Such a solution becomes unpractical at lower microwave frequencies and UHF, where the size of the horn is too large.

An alternative solution is to avoid exciting unwanted modes already at the transition from an arbitrary TEM feed line to the waveguide. Replacing a simple feed probe with a symmetrical half-wave dipole avoids exciting the unwanted TM01 mode while exciting the desired TE11 mode. Such a solution is called a cup dipole and is represented on the following drawing:

fig311
Fig.3.1.1 - Basic design of a cup dipole.

A cup dipole is a really compact antenna with a directivity of up to 12dBi and a very clean radiation pattern with very weak side-lobes. The directivity achieves its maximum at the second peak on the Ehrenspeck 's diagram [2]. Again, most of the metallic antenna structure can be used as a radome at the same time and just the radiating aperture needs to be covered with a transparent cover.

A cup dipole provides a -3dB beam width of about 50 degrees. Besides operating as a stand-alone antenna, a cup dipole also makes an excellent feed for a shallow (f/d=0.6-0.7) parabolic dish.



3.2. Cup dipole for 23cm

The construction of a practical cup dipole for 23cm (13cm) is shown on the following drawing:

fig321
Fig.3.2.1 - Practical cup dipole for 23cm (13cm).

If only simple tools are available, then it makes sense to build the individual components of the cup dipole from aluminum sheet and bolt them together with small M3X4 or M3X5 screws. Aluminum is a good electrical conductor providing low losses in the antenna structure and does not require any special environmental protection. The required mechanical components for the cup dipole for 23cm are shown on the following drawing:

fig322
Fig.3.2.2 - Mechanical components of the cup dipole for 23cm.

The front cover may be quite thick FR4 laminate or plexi-glass, since a dielectric plate in this position actually improves the performance of a cup dipole.

The measured E-plane and H-plane radiation patterns of the prototype cup dipole are shown on the following plots:

fig323
Fig.3.2.3 - E-plane and H-plane radiation patterns of the cup dipole for 23cm.

The measured patterns in both planes at a number of different frequencies were used to compute the directivity as shown on the following plot:

fig324
Fig.3.2.4 - Directivity of the cup dipole for 23cm.

The kink in the directivity curve will be explained later together with the same effect observed with the prototype for 13cm.



3.3. Cup dipole for 13cm

The design of the cup dipole can be easily scaled to the 13cm band. The required mechanical components for the cup dipole for 13cm are shown on the following drawing:

fig331
Fig.3.3.1 - Mechanical components of the cup dipole for 13cm.

The antenna is first assembled together using just bolts, making all necessary adjustments and checkouts. Afterwards the antenna is disassembled so that all seams can be sealed with small amounts of silicone sealant. Finally, do not forget a venting hole or unsealed seam in the bottom part of the antenna, where any (condensation) moisture can find its way out of the antenna!

A homemade cup dipole for 13cm that already provided several years of outdoor service is shown on the following picture:

fig332
Fig.3.3.2 - Cup dipole for 13cm.

The measured E-plane and H-plane radiation patterns of the prototype cup dipole for 13cm are shown on the following plots:

fig333
Fig.3.3.3 - E-plane and H-plane radiation patterns of the cup dipole for 13cm.

The measured patterns in both planes at a number of different frequencies were used to compute the directivity as shown on the following plot:

fig334
Fig.3.3.4 - Directivity of the cup dipole for 13cm.

As the frequency increases, the directivity plots of both cup dipoles for 23cm and 13cm include a kink. A further explanation of what is happening is given by the following two E-plane radiation patterns for both investigated antennas: the 23cm cup dipole at 1370MHz and the 13cm cup dipole at 2480MHz:

fig335
Fig.3.3.5 - Corruption of the radiation patterns of cup dipoles at high frequencies.

Both radiation patterns are badly corrupted due to the appearance of higher-order symmetrical waveguide modes. These are excited by a perfectly symmetrical half-wave dipole and are no longer suppressed by the waveguide. The size and directivity of a cup dipole therefore has a practical upper limit.

Beyond this limit different solutions are required to control the illumination of the aperture to obtain even narrower radiation beams and higher values of directivity.



3.4. Cup dipole for 70cm

The opposite happens at UHF and lower frequencies: all presented cavity antennas are physically too large to be practical. At low frequencies, a good hint is to look at the design of very compact coaxial-to-waveguide transitions. A practical solution in the 70cm band is a downscaled cup dipole as shown on the following drawing:

fig341
Fig.3.4.1 - Practical (downscaled) cup dipole for 70cm.

The aperture of this downscaled cup dipole corresponds to a simple waveguide horn. The expected radiation pattern is therefore similar to the much longer waveguide horn and the expected directivity is 8dBi or less. 8dBi may not seem much, but remember that this figure is achieved with a compact and weatherproof antenna at a relatively low frequency!

Since the half-wave dipole is installed rather close to the cavity wall, its expected radiating impedance will be very low. A folded dipole is therefore used for impedance transformation. The folded dipole is built on a printed-circuit board and requires a balanced 50-ohm feed.

The balun is simply a quarter-wavelength piece of RG-316/U thin teflon-dielectric coaxial cable forming a two-turn coil. The dipole is built on a piece of 1.6mm-thick FR4 laminate with 35um or thicker copper cladding. No etching is usually required. The copper cladding is marked with a sharp tip and the unnecessary copper foil is simply peeled off.

All of the folded-dipole components are shown on the following drawing, including two aluminum brackets used to bolt the dipole to the cavity wall:

fig342
Fig.3.4.2 - Folded-dipole dipole components for 70cm.

The measured E-plane and H-plane radiation patterns of the prototype cup dipole for 70cm are shown on the following plots:

fig343
Fig.3.4.3 - E-plane and H-plane radiation patterns.

The measured patterns in both planes at a number of different frequencies were used to compute the directivity as shown on the following plot:

fig344
Fig.3.4.4 - Directivity of the cup dipole for 70cm.





4. Short-backfire antenna (SBFA)

4.1. Design of a short-backfire antenna (SBFA)

As the diameter of the cup-dipole cavity increases beyond 1.4 wavelengths, additional symmetrical circular-waveguide modes are excited by the dipole feed and propagated in the waveguide. These modes spoil the aperture illumination, increase the side-lobe levels and decrease the antenna directivity. In order to build even larger cavity antennas, some means of controlling the amplitude and phase of all contributing modes has to be introduced.

The most popular solution to control the amplitude and phase of symmetrical modes inside a circular waveguide is to introduce an additional circular plate in the aperture plane. This plate is called the small reflector while the cavity is called the large reflector. Together with one or more feed dipoles, these two reflectors form a very efficient cavity antenna called the short-backfire antenna or SBFA:

fig411
Fig.4.1.1 - Design of a short-backfire antenna (SBFA).

The large reflector of a SBFA has a diameter D of up to 2.5 wavelengths while the small reflector has a diameter of about one-half wavelength or slightly more. The rim height h is usually around one-half wavelength. The directivity of such a simple antenna exceeds 16dBi with an aperture efficiency close to 100% corresponding to the third peak on the Ehrenspeck 's diagram [2]. Again, most of the metal structure can be used as a radome and just the radiating aperture needs to be covered with a transparent cover.

A SBFA provides a -3dB beam width of about 30 degrees. The whole antenna structure is not critical and is able to operate over bandwidths of more than 10% of the central frequency. Since the SBFA reflector structure is rotationally symmetrical, the polarization only depends on the feed. Two feed dipoles may be used for dual polarization or circular polarization.



4.2. SBFA for 23cm

The construction of a practical SBFA for 23cm (13cm) is shown on the following drawing:

fig421
Fig.4.2.1 - Practical SBFA for 23cm (13cm).

The front panel (radome) may act as a support for the small reflector, thus considerably simplifying the mechanical design of the antenna. The radome has some measurable effect on the SBFA and any dielectric should not be too thick. If only simple tools are available, then it makes sense to build the metal components of the SBFA from aluminum sheet and bolt them together with small M3X4 or M3X5 screws.

The front panel (radome) is a disc of 1.6mm (0.8mm for 13cm) FR4 laminate. The small reflector is a disc of copper foil (35um or thicker cladding). No etching is usually required. The copper cladding is marked with a sharp tip and the unnecessary copper foil is simply peeled off. The required mechanical components for the SBFA for 23cm are shown on the following drawing:

fig422
Fig.4.2.2 - Mechanical components of the SBFA for 23cm.

The measured E-plane and H-plane radiation patterns of the prototype cup dipole are shown on the following plots:

fig423
Fig.4.2.3 - E-plane and H-plane radiation patterns of the SBFA for 23cm.

The measured patterns in both planes at a number of different frequencies were used to compute the directivity as shown on the following plot:

fig424
Fig.4.2.4 - Directivity of the SBFA for 23cm.





4.3. SBFA for 13cm

The design of the SBFA can be easily scaled to the 13cm band. Since the SBFA is sensitive to the radome thickness, the latter has to be scaled to the higher frequency as well! The required mechanical components for the SBFA for 13cm including the radome from 0.8mm thick FR4 laminate are shown on the following drawing:

fig431
Fig.4.3.1 - Mechanical components of the SBFA for 13cm.

The antenna is first assembled together using just bolts, making all necessary adjustments and checkouts. Afterwards the antenna is disassembled so that all seams can be sealed with small amounts of silicone sealant. Finally, do not forget a venting hole or unsealed seam in the bottom part of the antenna, where any (condensation) moisture can find its way out of the antenna!

A homemade SBFA for 13cm is shown on the following picture:

fig432
Fig.4.3.2 - Homemade SBFA for 13cm.

The measured E-plane and H-plane radiation patterns of the prototype SBFA for 13cm are shown on the following plots:

fig433
Fig.4.3.3 - E-plane and H-plane radiation patterns of the SBFA for 13cm.

The measured patterns in both planes at a number of different frequencies were used to compute the directivity as shown on the following plot:

fig434
Fig.4.3.4 - Directivity of the SBFA for 13cm.

As the frequency increases, the H-plane side-lobes of both SBFAs for 23cm and 13cm increase. As the diameter of the antenna D exceeds 2.5 wavelengths, the side-lobes become so large that the directivity of the antenna starts decreasing.



4.4. Feeds for SBFAs and cup dipoles

A half-wave dipole is the simplest way to feed cup-dipole and/or SBFA cavities. Microstrip-patch antennas or rectangular or circular metal waveguides can also be used. A half-wave dipole requires a balun to be fed with standard 50-ohm coaxial cable.

Besides the directivity bandwidth of a cup dipole and/or SBFA, the gain bandwidth of these antennas is also limited by the impedance matching of the feed. In the case of a SBFA, the feed is enclosed between the large and small reflectors, resulting in a low radiation impedance and corresponding sharp resonance. The impedance-matching bandwidth is likely much narrower than the bandwidth of operation of the SBFA cavity.

An impedance mismatch results in a loss of antenna gain. In the case of microwave cavity antennas, this is the only significant loss mechanism, since the electrical efficiency of the cavities themselves is close to unity. A good estimate for the antenna gain is therefore just subtracting the impedance-mismatch loss from the directivity.

The antenna bandwidth can be improved by a broadband feed dipole. One possible solution is to build the feed dipole from semi-rigid coaxial cable and use its internal conductor as a reactive load to broaden the impedance-matching bandwidth. The same type of semi-rigid cable can also be used for the balun including a dummy arm. The wiring of such a dipole and corresponding balun is shown on the following drawing:

fig441
Fig.4.4.1 - Dipole & balun wiring diagram.

A practical solution is to build the feed dipoles and corresponding baluns from UT-141 semi-rigid cable (outer diameter about 3.6mm) in the 23cm frequency band and from UT-085 semi-rigid cable (outer diameter about 2.2mm) in the 13cm band as shown on the following drawing:

fig442
Fig.4.4.2 - Feeds for 23cm & 13cm cup dipoles and SBFAs.

A suitable coaxial connector for semi-rigid cable should be selected first. N connectors may be useful in the 23cm band. Smaller SMA or TNC connectors may be used in the 13cm band. While soldering semi-rigid coaxial cables one needs to take into account the thermal expansion of their teflon dielectric!

The dipole is made by cutting the specified length "A" of semi-rigid cable. Then "D" millimeters of outer conductor and teflon dielectric are removed at both ends. After this operation the outer-conductor copper tube is carefully cut in the center and both parts are pulled away to form the gap "C". Finally both dipole ends are filled with solder to connect the center and outer conductors.

The published 23cm SBFA with the described feed achieves a return loss better than -10dB over the whole 1240-1300MHz frequency band. The 13cm SBFA design has a slightly higher directivity resulting in a narrower bandwidth. The 13cm SBFA with the described feed achieves a return loss of -10dB over the frequency band 2300-2360MHz.

The cavities of cup dipoles represent a different load for the feed dipoles, therefore the dimensions of the feed dipoles are necessarily different from those used for the SBFAs. The impedance-matching bandwidth of cup dipoles is much broader than SBFAs and a return loss of -15dB can usually be achieved.



5. Beyond the SBFA directivity

5.1. Double-rim SBFA

The simplicity, efficiency and performance of the short-backfire antenna suggests to look for similar antenna solutions also for a cavity diameter larger than 2.5 wavelengths and directivity larger than 17dBi. Since a SBFA roughly looks similar to a parabolic dish, a possible extension is to modify the large reflector of a SBFA towards a parabolic shape. Several different solutions have been described in the literature [1].

The simplest but not very efficient extension is the double-rim SBFA. The latter includes a large reflector with two concentric rims. The inner rim is just a quarter wavelength high while the outer rim is one half wavelength high as shown on the following drawing:

fig511
Fig.5.1.1 - Double-rim SBFA.

The effect of two rims is barely appreciable. A maximum directivity increase of about 1dB can be expected when compared to a conventional single-rim SBFA. As the directivity increases, the antenna becomes more sensitive to environmental conditions including the built-in radome. The following plots show the difference between two double-rim SBFAs for 13cm: one antenna without radome and the other with the aperture covered by a 1.6mm thick FR4 laminate:

fig512
Fig.5.1.2 - Radome effect on double-rim-SBFA directivity.

As a conclusion, a double-rim SBFA without radome provides about 0.5dB more directivity than a conventional SBFA at 2360MHz. Installing a 1.6mm thick radome from FR4 laminate, the directivity drops by about 1dB and the final result is 0.5dB less directivity than a conventional SBFA. The double-rim SBFA is therefore an academic curiosity with little practical value.

A side result of all measurements is that a radome made from FR4 laminate has a considerable effect on the SBFA performance already in the 13cm band. Therefore it is recommended to reproduce the described conventional SBFA with exactly the same materials, using 0.8mm thick FR4 or slightly thicker plexi-glass for the radome.



5.2. Archery-target antenna

As the SFBA cavity becomes larger, additional circular-waveguide modes are excited. Rather than changing the shape of the large reflector, additional structures can be placed to control the amplitudes and phases of different modes. Thinking in terms of wave physics, a collimating structure in the form of Fresnel rings is required. The SBFA is already the first representative of such antennas, placing a small reflector in front of the feed dipole to control the lowest-order Fresnel zone.

A further evolution of the above theory is a collimating structure including one small-reflector disc and one annular-reflector ring. Such a structure results in a rather efficient "archery-target" antenna [3] as represented on the following drawing:

fig521
Fig.5.2.1 - Archery-target antenna structure.

The "archery-target" antenna described in this article achieves a directivity of 20.6dBi at an aperture efficiency of about 46%. The -3dB main-lobe beam-widths are about 13.8 degrees in the E plane and 10.2 degrees in the H plane. This new antenna is simple to manufacture, since the supporting structure for the small and annular reflectors can perform as a radome at the same time.

Probably the "archery-target" antenna could be further optimized. Some computer simulations suggest that both a directivity of 22dBi and better aperture efficiency could be achieved although at a reduced bandwidth. Last but not least, the structure could be extended further to include several concentric annular reflectors.

The successful "archery-target" antenna design presented in this article includes a large reflector with a diameter of about 5 wavelengths, much larger than in a typical SBFA. On the other hand, the small reflector has a diameter of 0.7 wavelengths and is comparable to the SBFA. The annular reflector extends from an inner diameter of 2.2 wavelengths to an outer diameter of 3.7 wavelengths. The reflector spacing and rim height are identical and equal to 0.7 wavelengths and this figure is also somewhat larger than in a typical SBFA:

fig522
Fig.5.2.2 - Archery-target antenna design.

The prototype antenna has a large reflector diameter of 570mm, an annular reflector with an inner diameter of 252mm and an outer diameter of 420mm and a small reflector with a diameter of 80mm. The reflector spacing and rim height are set to 80mm. The small and annular reflectors are carried on a large dielectric plate: 0.8mm thick FR4 laminate with a dielectric constant of about 4.5. Although thin, this carrier plate has the effect of decreasing the optimum frequency by as much as 100MHz in the S-band frequency range.

This prototype antenna achieved the best directivity of 20.6dBi at an operating frequency of 2640MHz. The measured E-plane and H-plane radiation patterns shown on the following plots:

fig523
Fig.5.2.3 - E-plane and H-plane radiation patterns of the archery-target antenna.

The measured patterns in both planes at a number of different frequencies were used to compute the directivity as shown on the following plot:

fig524
Fig.5.2.4 - Directivity of the archery-target antenna.

The first experiments with the "archery-target" antenna were made with a simple thin-wire half-wave-dipole feed. The dipole was positioned on the antenna axis of symmetry exactly half-way between the small and large reflectors just like in a SBFA. Since the thin-wire dipole had a poor impedance match to a 50-ohm source even over a narrow frequency band due to the antenna cavity loading, several other feeds were experimented.

Reasonable impedance matching (-15dB return loss over a 10% bandwidth) was obtained with a single wide-dipole feed built on a printed-circuit board as shown on the following image:

fig525
Fig.5.2.5 - Dipole feed of the archery-target antenna.

While experimenting with different feeds, small (up to +/-0.2dBi) but repeatable variations of the antenna directivity were observed as well. In particular, the directivity decreased when the wide-dipole printed-circuit board was installed parallel to the reflector plates. On the other hand, the directivity improved when the wide-dipole printed-circuit board was installed perpendicular to the reflector plates.

The feed radiation pattern can therefore contribute to a more uniform illumination of both annular apertures of the "archery-target" antenna. Effects of different feeds on other cavity antennas (cup dipoles and SBFAs) were not experimented yet.

The complete archery-target-antenna prototype is shown on the following image:

fig526
Fig.5.2.6 - Archery-target antenna prototype.





6. Selection of the most suitable antenna

Although the whole family of microwave cavity antennas is very large, many of these antennas are not known to the wider public. Little if any serious articles have been published in the amateur-radio literature. Therefore it was decided to write this article including the description of the most interesting microwave cavity antennas, their past experience, present performance and expected future developments.

fig61
Fig.6.1 - 23cm & 13cm SBFAs used for 1.2Mbps packet-radio access.

Unfortunately, most people select an antenna only according to its directivity or gain. WRONG! There are many more selection parameters and all of them need to be considered: width and shape of the main beam, side-lobe levels and directions, frequency bandwidth, sensitivity to environmental conditions and weather effects, ease of manufacturing etc.

Microwave cavity antennas may not provide the maximum number of decibels for a given quantity of aluminum. This may explain why they are not so popular. On the other hand, microwave cavity antennas may be simple to manufacture, insensitive to manufacturing tolerances, have low side-lobe levels, be reasonably broadband, easy to make weatherproof and insensitive to environmental conditions, quickly rejecting rain drops, snow and ice from accumulating on their radiating apertures.

While designing a radio link, the first consideration should be the antenna beam-width according to the desired coverage. Antenna arrays should only be considered in a second place, when a single antenna is unable to provide the desired coverage. The most common mistake is to select the antenna with the largest number of decibels. Its beam may be too narrow, its large side-lobes may pick interference and multipath and its deep nulls in the radiation pattern cause dropouts and pointing problems.

This article includes detailed descriptions of different cavity antennas: simple horn (7dBi & 90 degrees), cup dipole (12dBi & 50 degrees) and SBFA (16dBi & 30 degrees). All these designs are well tested and foolproof: it is just a matter of selecting the right design for a particular application. A SBFA is an excellent replacement and performs better than small (less than 1m diameter) parabolic dishes with poorly designed feeds at relatively low frequencies (below 2GHz).

On the other hand, a successful duplication of the "archery-target" antenna (20.5dBi & 12 degrees) and its likely future developments requires some skill and appropriate test equipment. The intention of this article was to show that there is still development going on in the antenna field, providing some hints to serious antenna experimenters.



References

[1] A. Kumar, H. D. Hristov: "Microwave Cavity Antennas", Artech House, 1989, ISBN 0-89006-334-6.

[2] H. W. Ehrenspeck: "A New Class of Medium-Size, High-Efficiency Reflector Antennas", IEEE Transactions on Antennas and Propagation, March 1974, pp. 329-332.

[3] Matjaz Vidmar: "An Archery-target Antenna", Microwave Journal, May 2005, pp. 222-230.


 
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