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where X is radius of the reflector Y is the,depth and f is the focal distance The shape. of the curve for example of a 7 diameter,dish with a focal distance to diameter ration. f d of 0 55 is shown in Figure 2 The,deeper a dish the shorter the focal distance. and the wider the beam width of the,optimum feed antenna. 7 Dish Curve,Fig 3 a The feed is located to one side.
Depth in inches,5 of the reflector b Relationship between. reflector f d and equivalent f d for feed,The offset dish besides having greater. 0 5 10 15 20 25 30 35 40 efficiency than a convention dish has an. Radius in inches,added advantage for EME It can be. Fig 2 Curve of 7 dish with f d 0 55 mounted very close to the ground and still. fully track the Moon,Offset Dishes Offset dishes are just a. portion of a parabola of revolution,conventional parabolic dish The antennas.
discussed in this paper use about a quarter,of a conventional dish reflector By using. only part of a normal full dish for the,reflector the feed antenna can be moved. away from the center of the reflector where,most of the RF energy is located The feed. can be located to one side of the reflector,where little or no RF energy is present as. shown in Figure 3a The feed must still be, located at the focal point of the parabolic SUPPORT.
curve The feed must also have higher gain, since it should ideally only illuminate the PIVOT POINT. reflector area As noted the offset dish is, only a fraction of the full dish Hence the Fig 4 The offset dish can be laid on. feed antenna must have higher gain to ground pointing up when not in use. produce a smaller beam The table in, Figure 3b shows the relationship between Consequently a relatively small mount can. the f d of a complete reflector and the f d be used and the dish can be laid on the. for which the feed should be designed to ground when not in use. properly illuminate an offset dish As can be, seen a deeper dish should be used to keep To partially overcome the problem of the. the focal distance a reasonable length longest piece the center hub of the. reflector can be extended Figure 1 shows to be reduced to a more convenient 5. that if you start bending at a 1 offset the length A rim around the outside of the. length of spoke can be reduced by about a reflector was made with 3 5 lengths of. third x wood modeling stock with two small,8 32 bolts as shown in Figure 6.
Construction To demonstrate the stress,offset dish concept a reflector with a radius. of 7 5 was fabricated for use on 1296 If,the reflector were complete it would. correspond to a dish of 15 in diameter In,the case of an offset dish only a quarter of a. conventional dish s surface is used This,surface was produced from just five 7 5. lengths of x wood molding stock,readily available at the local Home Depot.
These spokes were attached to a hub made,from a 1 radius wedge shaped quarter of a. circle piece of plywood with two bolts,Fig 6 An outside rim is formed from. 3 5 lengths of x modeling strips,The 3 5 length was chosen to produce a. reflector with an equivalent full reflector,f d ratio of about 0 3 This corresponds to a. feed beamwidth of about 90 This,beamwidth matches reasonably well a dual.
dipole feed Using equation 1 shows the,dish s focal distance is about 4 5. A 3 5 length of 2 x 3 lumber was used,for the main feed support. Fig 5 The spokes are attached to a,plywood center hub with two bolts. A 3 overlap was used It would have been,preferable to make channels into which the. wooden spokes could be inserted for,attachment I used this method of.
attachment for a 70 cm 20 portable stress,dish that I produced more than 20 years. ago This arrangement is stronger and,makes assembly and disassembly quicker. but with only five spokes the added time,was not considered significant The hub. could have been increased in size to 2 5 Fig 7 The dish is attached to the feed. This change would have allowed the spokes support by a 2 x 2 block. This piece was attached to the plywood for relatively low cost The screening was. center section using a small wood block first rolled over the top of the stressed dish. Nylon ropes were run from the feed support and cut to the required size The remaining. to eye bolts at the ends of each spoke The screening was flipped over to match the cut. length of these lines was adjusted so that the end with the shape of the dish and rolled. radius X distance of each spoke was 7 5 over the center portion of the dish This. The feed support was used for attaching the process was repeated a third time for the. dish to the mount bottom section see Figure 9,Feed Antenna Orthogonal dual dipoles. with a quadrature hybrid to produced,circular polarization were used as the feed.
antenna Dual dipoles were chosen because,of their relative small size An IMU horn. would be an excellent choice of feed for this,reflector but its size and weight could be a. problem if it is to be carrier as luggage,The feed was attached to about a 1 length. of 1 x 1 This was attached to a second,approximately 1 5 length of 1 x 1 using a. single 3 8 bolt which was in turn attached, to the feed support by another single 3 8 Fig 9 Aluminum screening is tie to the.
bolt Extra mounting holes were drill in the spokes using wire. feed support to allow the position of the, feed to be raised or lowered This One of the extra corner pieces from the top. arrangement provided several degrees of was used to cover the small remaining area. freedom in adjusting the position of the feed at the bottom vertex of the reflector The. for optimum performance Feed mounting screening is attached to the spokes using. details are shown in Figure 8 small gage 24 wire The wire is run. through the mesh and around the spokes and,then wrapped tied together The process. of attaching the mesh takes only a few,minutes The Aluminum mesh can be. removed and rolled around the 4 2 x 3,members for transport shipping. Use on 432 It is much harder to make a,case for the use of a dish reflector on 70 cm.
where linear polarization is almost, Fig 8 The feed is mounted using several exclusively used and the reflector diameter. supports to allow optimum positioning must become quite large 15 to. compete with even a single long yagi, Covering The dish was covered with Nevertheless an offset dish s ability to be. Aluminum screening This material is mounted near the ground low cost and ease. available in the US in 3 width by 25 long of construction merits its consideration To. rolls which is sufficient to cover the dish evaluate an offset dish on 432 the. equivalent of a conventional 20 dish is, under construction It uses a central hub weight and inexpensive but may be very. fabricated from a 4 x 6 piece of plywood time consuming to implement A wire. to allow spokes 16 5 in length to reach 20 spacing of about 1 is a good compromise. Seven main spokes and two optional smaller for 432 although even a 2 spacing should. spokes are used to produce the surface and work,are made from 1 x 2 redwood The outer. rim is made from 5 5 lengths of x Testing The offset dish for 1296 was. wood molding stock This concept is originally constructed in a single weekend. illustrated in Figure 10 a and b and tested for sun noise The dish worked. as planned and yielded 8 dB of Sun noise,This was 3 dB more than a 15 loop yagi.
that was used as a reference It is presently,assembled and being used for EME from. my backyard The bigger offset dish should,be assembled shortly and will also be tested. off the Moon from my backyard,Conclusion The offset dish designs shown. in this paper offer a relatively inexpensive,and simple way of producing antennas for. Fig 10a The spokes 6 long and 2 70 and 23 cm EME They are particularly. optional short are attached to a 4 x 6 useful for temporary and portable operation. plywood hub because they can be quickly assembled and. taken apart They also can be mounted very,close to the ground and with a relatively.
simple mount The 7 5 offset dish provides,performance equivalent to about an 8. diameter circular dish yet can be,disassembled into a small lightweight. References,1 A Katz Small Station EME on 70 and,23 cm Using JT44 65 http www. nitehawk com rasmit jt44 50 html, Fig 10b Side view of 70 cm offset dish 2 P Wade W1GHZ Online Microwave. Antenna Handbook Chapter 5 http www, A 9 length of 2 x 3 lumber was used for w1ghz cx antbook app 5a pdf.
the main feed support I have not yet, decided on the surface covering An 3 A Katz 20 Portable Stress Dish 432. appealing option is to use many parallel and Above EME Newsletter Oct 1980. wirers Since polarization is linear on 432 http www qsl net pa3csg Boek BoekH3 art. only conductors in line with the E field are 3 8 htm. stress offset dishes for 432 and 1296 MHz EME and describes their construction It shows that this type of antenna can be easily and inexpensively constructed and that the offset design offers advantages not only in gain for a given area but also in mounting as it can be placed very close to the ground Introduction This paper is an update of a paper written in 2004 on the construction of

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