How can i design helical in 900MHz?

i have some question :
1. i have pameters, i got from the formulas λ = C/f , D = C/π , α = arctan S/C , L = C/cos〖α 〗 , G = k 12 Cλ2 n Sλ , long axis = n x S , then i simulate with supernec , i got the gain -2dB , and radiation pattern there's much back lobe. Is there somethin wrong with the formulas? and anybody have article bout count the helical parameter?

2. because i was out of curiousity, i tried to make the helical with the parameters before, i used copper as a convolution and the diameter the copper was 2mm. then i gave 5Volt. Then my helical design cant work, but VSWR measurement is 1. how the solution?



I am just learning about the antenna. I need the solution..

What do you mean by:

λ = C/f

? Is C supposed to be the circumference, or the velocity of light in a vacuum, or something else?

The usual approach is to make the circumference of an axial mode helical antenna equal to the wavelength. The pitch angle α should be between 12° and 14°. The helix is quite broad band, but I would decide on the centre frequency of your band of interest.

But based on 900 MHz, where the wavelength is 333 mm, a diameter of 333/Pi = 333/3.1416 = 106 mm should be used. Assuming a pitch angle of 13°. which is a reasonable angle, the spacing between turns S is

S=C*tan(α) = 333*tan(13°) = 77mm.

How many turns you need depends on the gain you require.

You need a groundplane - did you put that in your NEC simulations? Make that about λ square, so that's 333 x 333 mm.

Your best source of information on the helical antenna is likely to be books by Kraus, as the late John Kraus invented the helical antenna. The way he invented it is interesting - I'll wont spoil the fun of you finding that out for yourself!

Sorry, I don't understand you here. Can you explain that some other way.



Dave

@drkirkby


1. c = velocity of light in a vacuum ( 3 x 10^8 ) , me too, i got wavelength = 333mm , then i gave 5 coils, all parameter i put to superNEC simulation, but the result made me unsatisfied.


2. I make a helical antenna for jammer GSM, then, I applied to a device that has a voltage of 5V. And the helical can't work. the coils were made by copper diameter 2mm, the cable connector was 50 ohm. What should I change? helix that can work perfectly? What should I replace the coil? or Somethin else?


*confused*


by the way thank u drkirkby have answered my question

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.
helix.bmp

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

Why dont you use the antenna designer from hfss. It can create the model in few seconds and u will just need small tuning to fit ur requirements

i am using hfss. and i also got proper impedance matching.yes my gain is very low as the pitch is low.and i designed this kind of helix because when we insert a helix in a cavity it resembles a resonator. so our guide asked us to refer a helical resonator paper and ARRL book(chapter 11).
according to that when i designed and simulated few designs i found that with some impedance matching i am getting proper S11. but my gain is in negative. and also the antenna is not in axial mode.
so should i go with the ieee paper or just design a proper helix and insert it in cavity and work on the impedance matching (due to the capacitive reactance produced due to that cavity.)

I'd certainly not take too much from an ARRL handbook. Unless the whole chapter is written by someone who really does know about the antennas, then I would ignore it. There's a lot of **** in ARRL handbooks. IEEE publications are usually more relieable.

If you use a perfect electrical conductor, the peak gain can not be less than 0 dBi. But of course if you mess about with the position of the groundplane, you could well move the direction of peak gain away from the axis.

There is an IEEE paper about the gains of helix with different groundplanes. Whilst the author does not refer to them as a cavity, at least some appear to be a groundplane of size B x B, and with sides on the groundplane of height B/2. That had notibly higher gain than the flat groundplane.

You are changing the goal posts too much. It is hard to follow the thread when first you want something for 900 MHz, then you change to 2.4 GHz, then you show us a thin wide strap, then you tell us the helix needs to be 19 mm above a groundplane, then it has to be in a cavity. Originally you told us you were using NEC, now it is HFSS. Have you forgot to tell us the cavity is filled with mercury, or perhaps the thin strap which I assumed was a conductor is in fact PTFE?


Dave

oh no no..i think i should have started a new thread for my questions. i just totally understood the replies of this thread so i posted my doubt on the same thread. there is absolutely no relation between my project an the helix working on 900GHz. actually my structure is not yet simulated and fabricated by anybody. there is no ieee paper on that as well. thats why my work totally comes under research project.im just trying smthing new. i think i should increase the pitch and the circumference of helix only then ill get a proper gain. thanx any ways.