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Tuning an Meander monopole antenna in the Smith Chart and its behavior

Gregorio Tovar
Prodigy 240 points
Other Parts Discussed in Thread: CC1101

Hello everyone,


I have the following problem.


I am working with CC1101 radio, Johanson Technology Balun Matched Filter and use a monopole meander antenna at 915 MHz, figure 1.

 

Figure1 Meander Antenna

The card was built based on the DN024 and DN025 Design Notes.

 Using an RF analyzer to perform measurements on the antenna (figure 2) I have:

·       Smith Chart: 48 + j31

·       VSWR: 1.8

·       Log Mag: -10.3

 

Figure 2

My design has an Pi network to tune. Being in 48 + J31, my goal is to reach 48 - J21. The theory tells me to place on the PI network a 3.3pF capacitor in series to reach 48 - J21, figure 3.

 


Figure 3

 

Nevertheless, in practice the capacitor of 3.3pF has no vertical displacement in the Smith chart, instead it moves to the left: 18 + J1.6, Figure 4.

 

Figura 4

 

Q1. Is there some explanation for this case in which the theory does not match practice?

Q2. What is the reason for this behavior?

 

Thanks in advance.

Best regards,

Gregorio

  • 0
    TI__Genius 14940 points

    Answering your questions requires some additional information.  Most importantly, how are you measuring the final result?  Note that if you probe the antenna, your system will be loaded by the probe.  Your final validation can only be done by measuring the radiated signal.

    Jim Noxon

  • 0 in reply to Jim Noxon
    Prodigy 240 points

    Dear Jim,

     

    First, I apologies for my late answer.

     

    I tuned my antenna using the PI network with lumped components (with diferent values that the theory says, the reason of my last question) and cutting the lenght of my antenna to get to the desire frequency using a RF Network Analyzer (DN001).

     

    Once tuned my antenna inside the housings, both transmitter and receiver, I connect the transmitter module to the Smart RF Studio in a TX continous mode with the frequency, modulation and output power of my interest.

     

    At the RX side, I made the same as the TX module. I connect the receiver module at the Smart RF Studio in a RX continous mode with the frequency, modulation and output power of my interest. Also, I use an Handheld RF Spectrum Analyzer to corroborate the strenght signal. Following the section: antenna measurement (chapter 6) of the AN058.

    I compared my results with the whip antenna performance and I am close to the results.

     

    My question is:

    1. Is it a normal behavior that lumped components used to tune the antenna does not follow the path that the theory says in the Smith Chart?
    2. I consider that they should not be exact, but why differ to much in my measurements?

     

    This complicate the BOM of lumped components to carry out my tuning.

     

    Best regards,

    Gregorio