1
\$\begingroup\$

In RFID (according to this source: https://blog.acdist.com/understanding-rfid-and-rfid-operating-ranges) there are:

Low Frequency RFID:

  • has the shortest read distance, about 10 centimeters between the reader and the tag.
  • Low frequency covers ranges from 30 to 300 KHz.

High Frequency RFID:

  • The read distance ranges from 10 centimeters to 1 meter.
  • Operates in a frequency range from 3 to 30 MHz.

Ultra-High Frequency RFID:

  • Can read at ranges as far as 12 meters with a passive RFID tag, whereas active tags can achieve ranges of 100 meters or more.
  • The operating frequency of UHF RFID tags ranges from 300 MHz to 3 GHz.

So, in summary: the lower the frequency, the lower the transmission distance.

However, I thought in radio frequency the lower the frequency, the greater the transmission distance.

enter image description here

picture source: https://terasense.com/terahertz-technology/radio-frequency-bands/

Why, in radio frequency, the lower the frequency, the greater the transmission distance, but in RFID, the lower the frequency, the lower the transmission distance?

30 to 300 KHz -- 10 centimeters
3 to 30 MHz -- 10 centimeters
300 MHz to 3 GHz -- 12/100 centimeters

I'm sure there's something I misunderstood. So could you help me understand?

\$\endgroup\$
3
  • 2
    \$\begingroup\$ The lf and hf rfid use magnetic coupling - effectively the reader and card form a transformer. This is so enough energy is transferred to power the card. \$\endgroup\$
    – Kartman
    Feb 17 at 1:09
  • 1
    \$\begingroup\$ Antenna length: RFID generally rely on near-field, coupling, which falls off exponentially. This is considered evanescent, i.e., dropping to effectively nothing beyond a certain distance. Antennas in the 10-15 kHz bands, used to reach submarines, are kilometers long and use hundreds of kW power. en.wikipedia.org/wiki/VLF_Transmitter_Cutler \$\endgroup\$ Feb 17 at 1:26
  • \$\begingroup\$ en.wikipedia.org/wiki/Free-space_path_loss \$\endgroup\$
    – Jun Seo-He
    Feb 17 at 18:40

1 Answer 1

1
\$\begingroup\$

There is a misunderstanding of what the results of the wavelength and range are when there are actually several other factors contributing to the range of the signal.

I also do not understand the graphic for what the 100km, 10km, 1km... 1mm represents. I know that the wavelenght of 3kHz is not nearly 100km, nor is is the range. The same goes for the 300GHz is 1mm which does not represent either the wavelength or range, which are longer.

The basic concept you are addressing, is correct. The higher the frequency, at the same power, the shorter the range. But range is also strongly influenced by the power of the broadcast source. Radio stations broadcast at a massive power to reach audiences. Boats also get nearly the same range, at a lower power by broadcasting at a lower frequency. Also note, the higher the frequency, the more water absorbs the wave vs reflecting it, so low frequencies are much more effective at sea. As soon as you get your high-frequency cellphone by a lake, what happens to its signal?

The next concept affecting range is analog or digital. The range of analog signals for transmitting are far longer than digital.

Once we are on the reduced range of digital, its shortcomings can be enhanced by different 'signing'. In a digital transmission such as RFID's use, the analog waveform is interpreted to a digital signal by modulations in the wave. https://www.britannica.com/technology/amplitude-shift-keying Signing is what patterns are used to determine 1 or 0.

The interpretation of a 1 or 0 can be the high/low point of a wave (ASK), or more commonly a rising/falling wave at the tick point (PSK). Also at higher frequencies, the pattern is set to repeat a number of times to ensure data integrity. A bit chain of 101, sent PSK, would be 3 rising, 3 falling, 3 rising waves. The receiver takes the best 2/3 wave results. I apologize for the crude drawing:

Visual representation

By using a series of waves to represent a single bit, communications can become clearer to distant receptions at higher frequencies or lower powers. This is why a 5GHz wifi does not transmit at 5Gbps, but 1.3Gbps. As transmitter is sending 1 bit, 3 times, and a sync-bit in between every 4 bits so the receiver can keep the pulses straight. Also the further away you are, the lower the bandwidth as the transmitter is increasing the number of waves used to transmit a single bit as the signal drops off.

So the effective range of a transmission is effected by:

  • Frequency: reduced range for increased frequency.
  • Power: increased range by increased power.
  • Analog or Digital: increased range for Analog, but reduced clarity.
  • Signing: increased range for PSK, increased range for multi-wave signals.
  • Reception: The gain and filtering that can be applied by the receiving device.
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.