PCB - loop antenna best tuning - 433.92 Mhz

Hello edaboarders, I'm trying to tune a pcb loop-antenna for a remote control working @ 433.92 Mhz. I didn't design the rf module i buyed it on ebay but it needs external antenna design.
I copied the antenna filter topology from an old remote control and it looks like the one shown in the attached drawing; modifing the values like in the pic i managed to obtain a good level of dBuV (measured with a Lambda/4 antenna connected to a spectrum analyzer and varying the value of the components).
I know that i should use the smith chart to tune the antenna but i don't know how i can evaluate the impedance (inductive i suppose) of the antenna at 433Mhz... is there any way to evaluate the antenna impedance modifing the value of C3 and measuring dBuV values with my spectrum analyzer? Could be a good way for obtaining the max dBuV for my RC?
Which is the best way to obtain the values for the filter and which is the way usually adopted from experts to tune a pcb antenna like this?

P.S: I searched the entire net but i didn't find anything simple and clear to my poor rf knowledge. In these days i'll do other experiments on my board

TI application note AN003 shows some calculations for small loop antennas. You are right about the mostly inductive loop impedance, it can be compensated with a series capacitor. The interesting point is the remaining real radition resistance. It can be matched to the transmitter output with a pi matching network (possibly dual stage for very low radiation impedance).

**broken link removed**

It is not easy to tune a loop antenna without RF-tools. Can be complicated to reach good function even with tools. A VNA is to prefer but reading signals strength at a spectrum analyzer and a lot of systematic trial and error works also.
Even best tuned loop antenna is probably less good then a 170 mm long straight wire as monopole which not require any tuning at all.
A start can be to read resulting signal strength on your spectrum analyzer at a few meters distance, with the wire as TX antenna.
Compare this results whit what you can reach by tuning a loop as antenna. If you can reach a level 8 dB below that value with your proposed loop size is it max that can be expected for this type of antenna, when it is well tuned.

Worse performance of the loop design compared to folded monopoles is in fact also the summarized result of the TI application note. "Full size" λ/4 whips are much better anyway. I'm not sure about the reason, most likely the low radiation resistance.

A start can be to read resulting signal strength on your spectrum analyzer at a few meters distance, with the wire as TX antenna.
Compare this results whit what you can reach by tuning a loop as antenna. If you can reach a level 8 dB below that value with your proposed loop size is it max that can be expected for this type of antenna, when it is well tuned.

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Thank you very much for your reply, actually I'm far below the -8dBuV (respect Lambda/4). The wire antenna 170mm long measured 75dBuV on the s.a. @ 10meters of distance, the loop antenna at the same distance measured 57dBuV on the s.a... I think can improve the antenna a little bit more...

Can someone explain me in a simple way why when i keep in hand the remote contol i get 5 less dbuV than leaving it on a wooden table? How can i avoid this phenomenon?
I also noticed that sometimes if i tune the antenna without keeping the remote control in my hand, the tune isn't the correct for having the maximum radiated power when the remote control is in my hand. :shock:

Is there any human body model to consider :?:

Is there any human body model to consider?

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Good point. Generally, any antenna is de-tuned by interfering objects. An electrical small antenna has a high Q and is much more sensitive to de-tuning. There's no simple solution to the problem, except for tuning the antenna matching under reasonable enviromental conditions.

As FvM said does it not exist any simple solution. A common solution during tuning process is to slightly detune the antenna at free space position in opposite impedance-direction then how human body interferes, but that requires a VNA to find out how things should be adjusted. A minor detuning does not cost much of total antenna performance.
Your loop can however be much improved as it now is unbalanced as a big part of the loop is connected to ground.
It is probably ground that interferes more with a hand/body then what the antenna in itself do. A similar situation as for electrical monopole contra dipole antenna.
For reducing hand-effect is it not too hard to redesign your antenna to become balanced, were matching circuit also taking care of converting from balanced to unbalanced against radio.
A such redesign requires a bit more space as whole structure must be separated from ground as much as possible, I recommend more then 3 mm.