Amateur Dish Types

Dishes become viable alternatives to yagi sytems at the GHz frequencies. They are simpler than yagis but the dish is much heavier/db of gain than the yagi. Often the price of the dish is right (free), but the cost to mount and control is a major consideration.

At the 2401 MHz AO-40 receive frequency and above, the dish is the antenna of choice. It is important to understand several basic items about dishes.

The most obvious item is its diameter. Dishes come in all diameter sizes; they can be round, square, oval, or rectangular. The curvature is a concave parabola. The parabola has the characteristic of being able to focus parallel beams of light or RF to a precise focus point. Amateur dishes can be as small as an 18" Dish network dish that I call a "hubcap", to many feet in diameter.

Like a bulb in a flashlight, the dish must have a feed source looking into the surface of the dish. Some dishes are designed so that the feed source is mounted directly in front of the dish. This is referred to as a center fed dish. Other dishes are designed so that the feed source is off to one side, referred to as an off center fed dish, or just offset fed dish. The offset fed dish may be considered a quadrant of a center fed dish. The center fed dish experiences some signal degradation due to blockage, but this is usually an insignificantly small amount. The offset fed dish is initially difficult to aim, as the direction of reception is not the center axis like the center fed dish's. The attitude of the offset dish is more likely to be vertical, making it less susceptable to loss from snow accumulation. Offset fed dishes have difficulty pointing horizontally for terrestrial communications. One enterprising ham, W4WSR, mounted his offset fed PrimeStar dish upside down which matched terrestrial pointing better. Older dishes tend to be center fed; newer dishes are offset fed.

The dish's parabola can be designed so the focus point is closer to the surface of the dish, referred to a a "short focal length" dish, or further away from the dish's surface, referred to as a "long focal length" dish. To get the exact focal length, measure the diameter of the dish and the depth of the dish. The diameter squared divided by 16 times the depth is the focal length. The focal length divided by the diameter of the dish gives the focal ratio. Center fed dishes are usually short focal ratios in .3 to .35 range. Offset fed dishes are usually longer focal length such as .45 to .6. If you attach a couple of small mirrors (I use a couple of the wife's Mabelene eye shadow mirrors when she isn't looking) to the outer front surface of a dish and then point the dish at the Sun, you will easily find the focus point of the dish. Put the reflector of the patch or helix just beyond this point of focus.

The feeding of a dish has two major factors that determine the efficiency of a dish. Like a flashlight bulb, the dish feed source should evenly illuminate all of the dish, but none of the feed energy should spillover outside the dishes reflecting surface. No feed system is perfect in illumination gain achieved and/or spillover loss avoided. Typical dish efficiency is 50%. That's 3 db lost. A great feed system for one dish can be a real lemon on another dish. A patch feed system is very wide angle, but a helix feed sytem is narrow angle. A short focal ratio center fed dish requires a wide angle feed system to full illuminate the dish, making the patch the preferred feed system. But the offset dish with its longer focal ratio is overilluminated by a patch feed system, resulting in considerable spillover and sensitivity to off axis QRM, and does much better with a narrower beam many turn helix feed. Some think that they can get additional gain by using a high gain yagi to feed a dish. In fact they will lose, because the dish will be severely underilluminated.

Smaller dishes are usually constructed from solid aluminum or steel. Sometimes fiberglass with a conductive coating is used. Larger dishes are generally made from perforated steel or aluminum. When painted black, the dish with perforations is less easily seen, and slightly reduces the wind loading. If the perforations are made smaller than 1/10th to 1/12th wavelength, the dish is equivalent to solid in terms of signal reflection. At 2401 Mhz, this is about 3/8", making standard 1/4" fence wire a good option for a dish surface with low wind loading.

The Q of the dish's surface is quite low, so painting it, rusting it, or silver plating it will have little measurable effect. Recently a selection of medium sized dishes from Afganistan appeared in a magazine. The colorful dishes were made from old paint cans. The critical factor is maintaining the parabolic shape over the whole surface area.

Windloading is the major disadvantage of a dish. A 10' dish in an 80 mph wind has a windloading of 2000 lbs! A 30" x 40" oval PrimeStar dish mounted on a boom 1 1/2' out from a Yaesu G-5600 of G-5500 will break the azimuth gears during an 80 mph wind with 300 ft-lbs of torque, and break the weaker Yaesu G-5400 in a 55 mph wind, when producing 150 ft-lbs of torque! The open grill dishes greatly reduce windloading, but perforations tend to dam up the wind as they strain the wind through the perforations, and windloading is great at a high wind velocity in spite of the perforations. The Very Large Array (VLA) in Western New Mexico parks the 82' dishes pointing straight up when not in use to minimize wind loading to these 230 ton dishes. But not a good idea in case of heavy snow!

Performance of a dish can be spectacular even when the feed system sets the efficiency to 50%. At 10' dish at 2401 MHz would have 34.5 db of gain. The beamwidth at the 3db down points would be 2.8 degrees, calling for a very good positioning system. A linear fed 3' x 2' barbecue grill dish at 2401 MHz would have 22 db of gain and a beamwidth of 13 degrees, TV antenna rotators & Ham-M family rotators having adequate pointing accuracy. The 12 db difference is 2 S units, certainly a major reason to put the old TVRO dish into back into service receiving AO-40. The 30" x 40" oval PrimeStar dishes with a left circularly polarized 5 1/2 turn helix feed would have 28 db of gain when receiving the AO-40 signal, and 9 degree beamwidth, but much higher windloading. The PrimeStar dishes were orphaned when their company was bought up by Dish network and dropped as a service. Large quantities of these fiberglass dishes went to the scrapyard. Do your thing for Ecology and put them into Ham service.

Homemade Dishes

In the USA, there are large number of dishes that can be obtained either free or at low cost. But in some parts of the world dishes are not so plentiful, so hams make there own. ZS6BTD sent a picture of his dish that he built out of aluminum tubing and fence wire.

KG6IAL ordered an aluminum framework for a 4' parabolic dish from Sweden.. The picture at the right shows a full house satellite tracking system with 2401 MHz dish and yagis for 145 MHz, 435 MHz and 1269 MHz. Here is Rick's description of his semi-homemade dish:

The 1.2m dish is a computer-milled aluminum alloy frame which I obtained from Parabolic AB for $110 US. After attaching the ribs to the center support, I covered it with .25 inch hardware cloth. The feed is a G3RUH LHCP patch feed which is designed for dishes with a .3 to .4 f/d (this is a .3 f/d dish). The feed support is my own design, built from .5 inch hard conduit which is just the right size to go through the center hole of the dish mount where, on the back side of the mount, two adjustable clamps allow the conduit to be secured once the feed is properly positioned at focal point. I've attached a DEMI 13ULNA preamp to the patch feed and that assembly is secured to the feed support with a small radiator-style clamp which runs through two vertical cuts in the side of the conduit, encirles the outside of the preamp's 'N' connector and, when tightened, nestles the connector snuggly against a recess in the end of the conduit which was made with a .5 inch round file. Lastly, the connector assembly was sealed with a liberal application of silicone sealant to the outside of the assembly. A 5lb. weight can be seen on the back side view, used to counterbalance the dish on the boom. An 8 ft. length of RG-8X (run through the conduit of the feed support) connects the preamp output to 75 ft. of LMR-600 running into a Parabolic AB downconverter in the shack. The system noise figure is about 3-4 dB below the noise floor on the AO-40 transponder and, in combination with the ClearSpeech Base, gives "armchair copy" on all but the weakest downlink signals. I get 100% good telemetry decodes at apogee and at squints up to almost 40 degrees. As for the other antennas in the picture, they are all M2 antennas: (1) a 2MCP14, switchable for RHCP/LHCP, for 2 meters, (2) a 436CP30, switchable for RHCP/LHCP, for 70cm and (3) a 23CM35EZ for 23cm mounted for vertical polarization. I'm planning to add one more 23CM35EZ connected so as to provide switchable RHCP or vertical polarization.

Rick has recently switched over to a dual band circular patch (2401 MHz and 1269 MHz) for feeding his 4' dish. Construction details for building a dual band patch are found in the feed section of this webpage.

The science of Radio Astronomy was started 65 years ago when a ham radio operator built this big dish on display at the National Radio Astronmy Observatory at Green Bank, West Virginia.

Summary

So take your pick.

The Barbecue Grill comes ready to mount on the horizontal boom between the 2M and 70 CM antennas rotated by your Yaesu G-5500, attach your downconverter and receive AO-40 that afternoon. But adding 1/4" fence wire from your local hardware store, wire tied to the front surface, plus a circular patch feed can gain you a 4 db stronger receive signal for an evening's effort.

The Hubcap is simple to mount. The AO-40 2401 MHz transmit antennas are Right Circular polarized. Your dish reflection reverses polarity, so if you run a 5 1/2 turn Left Circular polarized helix feed (counter-clockwise wound looking forward from reflector) into the hubcap, you match theAO-40 transmitter, gaining 3 db over the linear feed of the usual barbecue grill, producing 23 db of gain, 1 db better than the barbecue grill used by most stations working AO-40. The single receiver hubcaps are plentiful due to a switchover to multiple receiver models being currently offered.

The PrimeStar with a 5 1/2 turn Helix feed comes in next at 28 db, an S unit over the Barbecue grill dish, but mount it very close to the rotator, because it could break the Yaesu's azimuth braking in a storm, and make sure your guy wire is strong if towertop mounting.

The 5' and 7 1/2' perforated aluminum dishes are really great portable antennas and offer good field day performance at 29 db and 32 db of gain on 2401 MHz respectively, 3db more if you use a Left Circular patch feed shown on this webpage. The aluminum panels distort very easily, so treat them gently. I have seen many abandoned due to the changeover to the hubcap dishes for receiving commercial TV. The 7 1/2' dish has enough gain on 435 MHz and 1269 MHz so that you can use your Satellite rig with a 1200 MHz 10 watt module directly without buying a $500 brick or 2C39 amplifier!

The 10' dishes were the backbone of the old TVRO world, and I have seen many advertised as "free to good home". The dish pictured at the left is made from perforated steel which is somewhat heavy, but survives things like hailstorms well. A fiberglass dish is usually found cream colored, and does not blend into the background as well as the perforated dishes. Fiberglass dishes are also very heavy. A 10' dish is not a project to be taken lightly, but when finished, it can really deliver some great signals.

I even got a 14' fiberglass TVRO dish free. The 14' dish has about 38 db of gain and a beamwidth of 2 degrees when I receive AO-40. 38 db is a power gain of ~6000! A 1 watt signal comes through like 6 KW. You can read the whole story on this dish by clicking on the picture. But the difficulty factor rises exponentially with size of the dish. Each time you double the diameter, you gain one S unit (6 db) on both receive and transmit and will expend 40 db more effort (and money).

Costs:

An automated AO-40 antenna system using a small dish (3' or less) will generally cost around $1300 by the time you include dish, downconverter, rotator, tracker, and a supplemental yagi for uplink, and take about 2 weeks to complete.

An automated AO-40 antenna system for a 4' to 10' dish will cost around $2000 and take about 1 to 2 months to complete.

The automated AO-40 antenna system using the 14' dish cost about $3500 and took 6 months to complete.