434 mhz antenna
I am searching for a formula to calculate the number of turns of a helical antenna for a handheld instrument.
The Helix has to be of very small diameter (max. 4.5mm). The wire diameter should be 1.0mm but could be less (0.8mm). The length must not be longer than 90mm.
The antenna should be resonant at 434MHz (most likely with 50 Ohm feedpoint).
I found several solutions for rather big helicals with big groundplane but none for those small dimensions.
I know that there won't be an universal formular and I don't need the perfect antenna. But it should be approximately a rather good solution.
Thanks for your valued hints in advance!
You should consider some simple facts.
- A monopole without a groundplane or some other "counterweight" can't operate as antenna.
- An electrical small antenna (monopole <λ/4) won't achieve 50 ohms without matching network
thanks for the reply.
I know that this is a rather poor antenna, didn't I mention it?
Of course there is some kind of groundplane but not an ideal one with 90 degree angle (just as radio amateurs use).
It is just a printed circuit board of limited size where I use one layer as ground layer and so acts like the groundplane.
I think of an antenna like that:
http://www.stecom.com/ste_2007/p_det...90002&classe=6
This is optimized for 433MHz but slightly too thick.
Somehow they did calculate the resonant frequency with this antenna too.
How can this be done?
[/url]
In a simplified view, a monopole shorter than λ/4 has a capacitive impedance, so if you add an inductor, either a lumped one at the foot or center of the antenna or a distributed one - resulting in a helical antenna, you can get a real valued impedance at a certain resonance frequency. As one major disadvantage, the antenna bandwidth gets rather small, as another, the input impedance is low, e. g. a few ohms.
The datasheet of the mentioned 433 MHz helical antenna (with a 30 mm respectivly λ/25) length is indicating low return loss or in other words, a near to 50 ohms real valued impedance. If this is a correct measurement, it would imply that most of the real input impedance is created by resistive losses rather than radiation impedance. Thus I don't think it's a preferred way to match a 50 ohms feed.
The properties of small normal mode helical (NMH) antennas are discussed e. g. in Balanis Antenna Theory. Because of the not exactly specified ground plane properties, it may be more easy to adjust the number of turns empirically for a given antenna length and diameter.
Thank you for the hints!
I guess it might really be the best solution to find some kind of optimum by trying different lengths an numbers of turns.
And I already tried to add an inductor at the feedpoint before, which in fact improved the performance .
Unfortunately I only have a simple spectrum analyzer where I can watch the signal strength and do my testing that way.
I would feel much better if I had some mathematical theory to get closer to the optimum.
But I will try to get an advanced antenna book as you mentioned to improve my poor knowledge in general.
The lack of proper ground-plane cannot stop the RF engineers to use Helix antennas even on two-way radios for VHF, UHF (or even 50MHz, where the wavelength is 6m !). Especially in a mobile communication generally never get the ideal ground plane that you want.
An empirical formula to calculate a Helix in normal mode is:
Diameter = (√2*S*λ) / Π, where S is spacing between turns.
In ?normal mode? where the diameter of the Helix is small compared with wavelength, the bandwidth and radiation efficiency of this antenna is small.
The input impedance is very dependent by pitch angle (angle of the helix wire and a plane perpendicular to the helix axis) and the size of the conducting wire, near the feed point.
If you have a signal generator and a coupler, together with your Spectrum Analyzer, you can find the return loss (or VSWR) of the antenna.
Thank you very much for your post!
This formular helps to get a rough value.
Rest can be done by varying length an turns around this empirical value.
I have a signal generator too, but unfortunately no coupler to measure the return loss which would of course help most to find the optimum.
But now I should get a suitable result in my design.
Thank you!