Electrical – Impedence Matching of Antenna and Coaxial Cable

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There is a scenario based question which is confusing for me
Antenna had four rods each 15cm arranged as a plus sign. its brass shielding was cut and it had two 75 ohm coaxial cables in it. one 10 cm long was connected to rod A and C whereas other was 17.5cm long connected to rod B and D. On measurement you found good impedance match between antenna and cable. both the cable got combined at the other end into one input connector

  1. Find Center Frequency
  2. Predict polarization
  3. Plot radiation pattern.

Can you tell me how impedance matching works, do length matters and how can we know if antenna with given length is having a good match with coaxial cable with given length

Antenna Question

Best Answer

Since this is homework, here are some hints:

  • The dimensions of the antenna elements determine its intended operating frequency.
  • Mismatched cable lengths create a phase shift in the signal. The phase angle is a function of the cable length and the operating frequency.
  • Crossed dipoles fed with different signal phases is one way to produce circular polarization.
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