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SdB 6dB

7 dB

8 dB

SdB 6dB

7 dB

8 dB

Parabolic Dish f/D

Figure 24 at 1.8 GHz and Figure 25 at 2.4 GHz. The f/D for best efficiency shifts with frequency, but an f/D of about 0.5 is good at both frequencies. I believe that some of the VE30NT EME tests used a short helix as the feed.

A longer helix would narrow the pattern, so that it would be suitable for a larger f/D dish. A paper26 studying helical feeds by numerical analysis for a DBS receiving application provides some interesting data on a 7 turn helix. With a normal helix reflector of 0.6 A. in diameter, the helix is suitable for an f/D of about 0.75, as shown in Figure 26. This might be a good feed for an offset dish; as an example, I5DTJ sent me a photo of the helix fed offset dish, which he uses at 2.3 GHz

The DBS paper also examined the helix with a reflector smaller than the normally suggested 0.5 X and found that the helix radiated in the other direction, toward the reflector, with small reflectors. Figure 27, for a reflector diameter of 0.35 X, shows low efficiency in the forward direction (solid line) and higher efficiency if used in the reverse, or "backfire" direction (dashed line). Making the helix reflector even smaller, 0.29 X diameter, makes it radiate predominantly in the backfire direction. In Figure 28, it appears that this combination would make a good feed for a dish with f/D of about 0.4.

The backfire helix feed is also quite broadband. An example was given covering 8 to 13 GHz, with the 10 GHz pattern graphed in Figure 29 showing best efficiency at an f/D of about 0.4. The dimensions of this helix were 7 turns at a pitch of 14° with a circumference of 25 mm, and a reflector diameter of 7.2 mm.

Note that the helix radiation plots are for circular polarization, not the two polarizations shown for the other feeds. Thus, the E- and H-planes plots are for the E-field in two orthogonal planes. If the polarization is not perfectly circu lar, there can be a difference in forward and reverse gain in the two planes.

Circular polarization is ideal for some applications, such as AMSAT. In an application involving reflections, such as radar, a helix is a poor choice because the reflection reverses the sense of the circular polarization, so echoes would be greatly attenuated by polarization mismatch.

Esoteric Radiotelescope Feeds

A large radiotelescope is rare enough that significant effort can be devoted to making a feed optimized for a single antenna, with no concern for manufac-

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