Cheap RF 433.92MHz design, PCB track antenna, values of capacitors

[abomin3v3l]

Brian. I connected a capacitor from colletor to ground. New schematic is attached.

I have some doubts:

(1) What shape of antenna of the of the pictures is better? curved or straight? Or do you recommend to try both?
(2) Do I need to place ground plane (red) where the components of the RF amplifier are placed? It is only where the antenna track is that I don't need to place ground plane? Ground plance seems to be correct in your view?
(3) From where to where do I need to connect the twist capacitor? From collector do GND? I think I will try it using my current board... I have thin enameled rigid wire here with 0.6 or 0.8mm of diameter. I use the same wire as the antenna of the receptor, currently I'm using its lenght as 1/2 of wavelenght of 433MHz, around 34.6mm.
(4) Do you know if the meter of your link measures capacitors of few pF? **broken link removed**
(5) I understood your calculations of XC.

 

[abomin3v3l]

The datasheet shows that pin 3 of RF receiver is a linear output.
I was monitoring this signal with the oscilloscope today, and the results are attached.
Now, I think I can measure the transmitter strenght through this signal.

 

[abomin3v3l]

Hi Brian.
Have you seen post # 21?
I saw on google, for that LCR meter the range of capacitance is from 25pF to above. Or maybe the minimum scale is 25pF, I do not have sure.

Do you think I shold try to prototype the two drawings of antennas from post # 21 or do you think that prototyping the straight antenna is sufficient?

Regards.

 

[betwixt]

I would think the 'straight' antenna would work best. Try both but my 'gut' feeling is there is no advantage to the second design.

Without a schematic it isn't possible to tell what the 'linear' output really is but as it appears to be a super-regenerative receiver I would guess it is the data output from the detector and the digital output is the same signal after passing through a data slicer. It is probably (I can't say for certain) representative of the received signal so if you make the data input at the transmitter permanently high so it transmits continuously, you should be able to measure the DC voltage at the linear output with a DVM as a signal strength indicator.

I checked my LCR meter and you are correct, that model doesn't measure below 25pF. There are other simple designs on the internet that can measure below 1pF accurately if you do a search for them, for example: **broken link removed**

Brian.

 

[abomin3v3l]

I'm going to produce new prototypes on next weeks, and I'll produce 3 versions of the transmitter board:
- Rev. B = original prototype with little modifications, but same antenna shape.
- Rec. C = new layout with curved/wiggle shape of antenna
- Rec. D = new layout with straight shape of antenna

After all the tests with them I will post the results here, and we will see which layout got more transmission power/distance.

Doubt: What value I can use for C5 as a starting point?
Doubt 2: Should I use ground plane (at other side of the PCB) where the RF amplifier is placed?

 

[FvM]

Meandered trace will slightly increase the inductance but I doubt that it has much effect on radiation efficiency. Tuning the LC circuit is however necessary. I agree with betwixt that using a variable capacitor (suggested "twist" capacitor or regular trimmer) is the best way.

I fear that adding a copper pour below part of the total loop area will reduce the transmitter power.

 

[abomin3v3l]

I will buy this meter
https://www.amazon.com/KNACRO-Preci...ords=LC+meter&qid=1552429075&s=gateway&sr=8-2