i had a doubt regarding which coaxial cable to use for connecting each helix to the feed. i am using an array of 4 helices.and to connect them i used a feed pattern which looks like +(plus) which i am going to etch on a pcb. and i want to place the helix on each terminal. But i need a specific height (18mm) for the helix so i need a cable(18mm) to join the pcb terminal and the helix. is it ok to use a normal coaxial cable of 50 ohms ( RG-series). or should i use a high frequency- low loss cable ( like LMR 300 etc) as i am designing for 2.4 GHz.
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The formula I gave for a helical antenna, and which you will find in many books, is based on the assumption the helix is round wire, and goes down to the ground plane. I worked out the dimensions for
900 MHz for you, since that is what you said you wanted. The diameter would be 106 mm, the pitch 77mm. A simple scaling to
2.4 GHz, as you now saying you are designing for, would be 106*900/2400=40 mm diameter, 77*900/2400 = 29 mm pitch.
What you show in your BMP is pitch of 14 mm, so I'm not sure where you are getting that from. Have you mid-interpreted the definition of the pitch, since the pitch you show (14 mm), is very close to half the value you need (29 mm). The odd 1 mm could easily be lost due to rounding errors, as I all my dimensions were posted with a precision of 1 mm.
What you have shown in the attached .bmp file is nothing like an axial mode helical antenna. A 19 mm gap between the helix and the ground plane is likely to be very significant at 2.4 GHz, as its 15% of the wavelength. It will no doubt change the characteristics a lot - I suspect it will reduce the gain. Also using a thin wide strap instead of a wire will change the characteristics in a way I don't know. The only way to optimize this setup is going to be to simulate it in a 3D EM simulator like HFSS, EMPro, XCcell, XFdtd, possibly CST Microwave Studio (?) or something similar. There is no way NEC, which you are using now, will be suitable for simulating a helix with a wide, thin strap like this.
What are your actual specifications in terms of the antenna? Tell us the exact frequency range, impedance required, gain required, polarization, size limitations, and we might be able to come up with something. But you originally asked about a helical antenna for 900 MHz, but now tell us the helix must be for 2.4 GHz, start 19 mm from the ground plane, and there will be 4 of the antennas, and connecting them via a PCB and coax and we see it is not made from wire, but a strap. You have rather moved the goal posts!
Dave