small helix-antenna tunning
I have acquired a small helix antenna for 433Mhz. According to its datasheet, it is matched to 50Ohms with a very good VSWR 1:2. Of course, it has been tested on a PCB with an specific groundplane and matching networks to achieve this VSWR, but there was not information about that.
I have connect the small helix to my VNA and it it's not matched, it has a high capacitance as expected. A vertical groundplane - a piece of 5x5cm methacrylate - is placed under the helix but it doesn't improve, neither get worse. However, if the groundplane is placed parallel and close the antenna it is matched, even if I surround the helix with dielectric it works too. How is it possible? Does dielectric get an inductive effect? Antenna is connected directly to VNA, with out matching network.
Thanks
VSWR 1:2 doesn't exist.
VSWR ratio can only be equal or greater than 1.
Usually the helix antennas working in normal mode needs some impedance matching to get good VSWR.
http://www.pulseelectronics.com/down...9/w3127pdf/pdf
My guess is that tracks on the PCB provide some of the inductance for the antenna.
A mistake, I wanted to say 2:1.
There is no tracks on PCB, it is just a piece of methacrylate with a hole for the helix, and the helix is weld to an SMA and the SMA connected directly to VNA. No matching networks, only dielectric placed parallel close to antenna.
Is there anyway to make a normal-mode helix to be resonant? I mean, modifiying its diameter or length can be macthed to 50 Ohms without matching networks?
You are asking two questions in one. The answers are however different.
Yes, an electrical small (l < λ/2) helical antenna of give length can be made resonant (at least in principle) by tuning the diameter and number of turns.
No, the radiation resistance will be always smaller than 50 ohms, about 640*(l/λ)2. Unless you manage to add loss resistance to total 50 ohms (I guess you won't want to), you'll need a matching network.
The fact that the ground plane is usually far from being infinite changes the antenna type from electrical small monopole to asymmetrical dipole. This tends to increase the effective radiation resistance, but makes it also more difficult to tune the antenna because hand-effect becomes relevant.
To get resonance in normal mode, the helix diameter should follow the equation:
Diameter[m] = (SQRT(2*turns_spacing[m]*λ[m]))/3.14
So, for 433 MHz the diameter of the helix antenna should be about 5cm.
Thanks, so a bigger antenna can be self-resonant. Anyway, if I place a variable inductance before the RF feeding, tunning the antenna should be easy, doesn't?
Yes. If you add one or two capacitors, you can also match the impedance to 50 ohms.