I am working on a model of a transmitter. The transmitter is attached to the wheel of the vehicle and thus constantly changes it's height. In other words capacitance between antenna and ground is changing.

What happens when the transmitter is in most up and most down position. Is there significant loss of the signal? Which effects are of crucial importance?

In particular case, I am dealing with TPMS (Tire Pressure Monitoring System). Transmitters are attached on the wheel inside the tire. The receiver is somewhere inside the car. The operating frequency is 433.92 MHz (ISM band). So, the wavelength of 70 cm which is comparable with the radius of the wheel.

  • 1
    $\begingroup$ What frequency are you working on? What is the height of the top and bottom positions? And what are the expected rpms of the wheel? $\endgroup$
    – Davidmh
    Oct 28, 2014 at 9:17
  • $\begingroup$ 433.92 MHz. The average wheel of a car is 60 cm and RPM depends on the speed of the vehicle. I wouldn't count on RPM influence since the frames sent from transmitter lasts for 10 ms. The angle of the transmitter thus changes from 5 to 65 degreees (for the sppeds up to 150 kmh) during transmission. $\endgroup$
    – Nexy_sm
    Oct 28, 2014 at 12:11
  • $\begingroup$ what type of antenna is used? $\endgroup$
    – Nikos M.
    Oct 28, 2014 at 12:35
  • $\begingroup$ Well, I don't know the exact type, but the sensor is really small and the length of the antenna is approximatelly 10 times less than wavelength. Maybe even smalle, 3 cm at most. $\endgroup$
    – Nexy_sm
    Oct 28, 2014 at 13:04
  • $\begingroup$ The idea is to have some approximation of isotropic radiator, so the radiation pattern is not of any particular directional shape. $\endgroup$
    – Nexy_sm
    Oct 28, 2014 at 13:20

1 Answer 1


i will give a more general answer unless other information is added (then i can update)

It would depend on type of antenna, the frequency/wavelength of transmission and how this wavelength compares relative to the height of the wheel and rpm (i would say the height mostly).

if the wavelength is comparable to the height of the wheel it will have serious distortions, if not the difference is neglizible.

Lets say we have a simple example of wavelength $a$ and wheel height $b=\frac{2}{3}a$ (comparable)

Then the antenna at the heighest point will have a height of $\frac{5}{3}a$ which effectively alters the antenna transmission range.

On the other hand lets say the wavelength is $a$ but the wheel height is $b=10^{-5}a$,

in this case any difference in transmission wavelength will be of the order of $10^{-5}$, neglizible.

You might want to check this answer as well

A basic primer on antennas

  • $\begingroup$ i am sorry, should I add a comment here as an update or to edit main post? $\endgroup$
    – Nexy_sm
    Nov 18, 2014 at 13:02

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