The Quadifilar Helix Antenna as a 2.4 GHz dish feed.
As I was reading the W1GHZ Online Microwave Antenna book when looking for a suitable feed for my offset dish, the thought occurred to me that a half-turn W2DU quadrifilar helix antenna (QHA) might have some interesting qualities if used as a dish feed for AO-40 S-band reception. The QHA has a good pattern, with a very good front-to-rear ratio. Both the E and H plane patterns are symmetrical. It doesn't require a reflector as does a turnstile, and being longer than it is wide, the QHA would result in very little feed blockage. And it is circularly-polarized, an important 3db advantage for AO-40 reception.
I developed an EZNEC file for Walt Maxwell's half-turn QHA some years ago. Given that EZNEC doesn't provide phase information, I reformatted the file as a NEC2 file. You can see that the calculated pattern closely matches the pattern as measured by W2DU.
I took the output from NEC2 and plugged it into W1GHZ's feedpatt.exe and phasepatt.exe. As you can see, the W2DU 1/2 turn QHA should make a good feed for a deep dish with a f/D of .25 to .3. Because of it's small diameter, the feed blockage is small, which is important when feeding small parabolic reflectors. The intrepid experimenter is invited to give it a go.
CM quadrifilar dish feed W2DU.NEC CM 1/2 turn W2DU QHA CM two feed points CM 90 degrees out of phase CM by VA3RR from W2DU description CE ********************************************* GW 1 15 0.087 0 0.28 -0.087 0 0.28 0.003 GW 2 3 -0.087 0 0.28 -0.082 0.03 0.309 0.003 GW 3 3 -0.082 0.03 0.309 -0.067 0.056 0.338 0.003 GW 4 3 -0.067 0.056 0.338 -0.044 0.075 0.367 0.003 GW 5 3 -0.044 0.075 0.367 -0.015 0.086 0.397 0.003 GW 6 3 -0.015 0.086 0.397 0.015 0.086 0.426 0.003 GW 7 3 0.015 0.086 0.426 0.044 0.075 0.455 0.003 GW 8 3 0.044 0.075 0.455 0.067 0.056 0.484 0.003 GW 9 3 0.067 0.056 0.484 0.082 0.03 0.513 0.003 GW 10 3 0.082 0.03 0.513 0.087 0 0.542 0.003 GW 11 15 0.087 0 0.542 -0.087 0 0.542 0.003 GW 12 3 0.087 0 0.28 0.082 -0.03 0.309 0.003 GW 13 3 0.082 -0.03 0.309 0.067 -0.056 0.338 0.003 GW 14 3 0.067 -0.056 0.338 0.044 -0.075 0.367 0.003 GW 15 3 0.044 -0.075 0.367 0.015 -0.086 0.397 0.003 GW 16 3 0.015 -0.086 0.397 -0.015 -0.086 0.426 0.003 GW 17 3 -0.015 -0.086 0.426 -0.044 -0.075 0.455 0.003 GW 18 3 -0.044 -0.075 0.455 -0.067 -0.056 0.484 0.003 GW 19 3 -0.067 -0.056 0.484 -0.082 -0.03 0.513 0.003 GW 20 3 -0.082 -0.03 0.513 -0.087 0 0.542 0.003 GW 21 15 0 -0.087 0.282 0 0.087 0.282 0.003 GW 22 3 0 0.087 0.282 0.03 0.082 0.311 0.003 GW 23 3 0.03 0.082 0.311 0.056 0.067 0.34 0.003 GW 24 3 0.056 0.067 0.34 0.075 0.044 0.37 0.003 GW 25 3 0.075 0.044 0.37 0.086 0.015 0.399 0.003 GW 26 3 0.086 0.015 0.399 0.086 -0.015 0.428 0.003 GW 27 3 0.086 -0.015 0.428 0.075 -0.044 0.457 0.003 GW 28 3 0.075 -0.044 0.457 0.056 -0.067 0.486 0.003 GW 29 3 0.056 -0.067 0.486 0.03 -0.082 0.515 0.003 GW 30 3 0.03 -0.082 0.515 0 -0.087 0.544 0.003 GW 31 15 0 -0.087 0.544 0 0.087 0.544 0.003 GW 32 3 0 -0.087 0.282 -0.03 -0.082 0.311 0.003 GW 33 3 -0.03 -0.082 0.311 -0.056 -0.067 0.34 0.003 GW 34 3 -0.056 -0.067 0.34 -0.075 -0.044 0.37 0.003 GW 35 3 -0.075 -0.044 0.37 -0.086 -0.015 0.399 0.003 GW 36 3 -0.086 -0.015 0.399 -0.086 0.015 0.428 0.003 GW 37 3 -0.086 0.015 0.428 -0.075 0.044 0.457 0.003 GW 38 3 -0.075 0.044 0.457 -0.056 0.067 0.486 0.003 GW 39 3 -0.056 0.067 0.486 -0.03 0.082 0.515 0.003 GW 40 3 -0.03 0.082 0.515 0 0.087 0.544 0.003 GM 0 0 0 0 0 0 0 -0.542 GE 0 ' EX excitation at segment 8 of both wires 11 and 31 ' if you swap the last input on the two EX cards, the ' main lobe changes by 180 degrees. EX 0 11 8 0 1.0 -1.0 EX 0 31 8 0 1.0 1.0 'LD 5 0 0 0 3.72E+07 PT -1 RP 0 19 3 1500 0.0 0.0 10.0 45.0 EN
Last update: 07 August 2003
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