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This PCB antenna circuitry supposedly is able to transmit on both 406MHz and 121.5 MHz. I can clearly see a micro-strip printed antenna.

Facts: Dual-band 406MHz, 121.5MHz antenna

My Assumptions:
1. 1/4 wave monopole dual-band antenna

Questions:
1. What is that inductor looking thing?
2. How can they transmit 121.5 MHz with such a small antenna?
3. What is the point of the zig-zaged strips? Filter? add length?

FYI
121.5MHz:   Wave length is 8.0988 ft, 1/4 Wave is 2.0247 ft
406MHz:     Wave length is 2.4236 ft, 1/4 Wave is 0.6059 ft

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2 Answers

up vote 2 down vote accepted
  1. I guess the inductor looking thing is actually an inductor
  2. is answered also by 3
  3. It is (also) an inductor. The inductors probably act as loading coil to adapt the antenna which is to short to be a quarter wavelength for 121.5 MHz.

BTW: 121.5 MHz and 406 MHz are reserved for emergency signals. If you play with that you might get some unexpected visitors.

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Yes, this is an EPIRB. Someone will most certainly show up at my doorsteps if I broadcast on those freqs. Thanks –  hassan789 Jan 7 at 16:29
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Yes, the component you show is clearly a coil, which is almost certainly there to provide inductance.

The impedance of a antenna varies widely with frequency and even relatively small geometry changes. Ideally a antenna looks resistive at the desired frequency, but it is often impossible to make the bare antenna look resistive in a size-constrained device. Instead, reactive components like inductors and capacitors can be connected in front of the antenna in a network such that the whole thing looks resistive to the transmitter or receiver. The inductor you show and the zig-zag traces above it are most likely part of a network that presents a different impedance to the RF circuitry than the bare antenna does. The zig-zag pattern probably also radiates, so is both part of the antenna and part of the impedance matching network.

If you want to delve into this further, look up something called a Smith chart. That is a old graphical device to help design the impedance matching networks for antennas. Given the bare antenna impedance and the impedance you want to achieve, it helps you determine how and what inductances and capacitances need to be used.

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