antenna-theory.com

[nurul]

hello,

How can we know/estimate the area covered by an antenna. Say an antenna with a gain of 2 dB, how far is the distance that it can cover?

Thanks a lot

[Schubert]

Saying an antenna has a gain of 2 dB gives no information on what distance it covers.

What you do need to know:
- The frequency of operation: higher frequencies attenuate quicker than lower frequencies (see Friis Transmission equation: http://www.antenna-theory.com/basics/friis.php).

- The type of environment: are you in deep space (no obstructions) or in an urban environment (with buildings and trees greatly increasing path loss)

- The sensitivity of the receiver you are trying to communicate with

- The power transmitted

Without this info, the question you presented is meaningless

[nurul]

Schubert, thanks.

so, the range covered by an antenna depends on its transmitter gain and power as well as receiver gain (Friss).

on average, what is the ~minimum Pt (received power-friss transmission formula) required by mobile devices?

[Schubert]

You're asking what transmit power is typical?

For mobile devices, transmit power is limited by SAR:
http://www.antenna-theory.com/definitions/sar.php

For WIFI, the transmit power is typically around 10 mW (10 dBm). For cellular radios, the transmit power is typically around 100 mW (20 dBm).

The power cannot be increased to maintain the link, or the manufacturer cannot sell the product.

[wolberine]

Schubert,

Just to get a rough perspective on cellular systems and range performance, do you know off hand:

1) Typical transmitter antenna gains?
2) Typical receiver (cell tower) antenna gains?
3) Typical receiver sensitivity?

My guesses:

1) ~0dBi because they need omni-directional antennas
2) 10dBi because the antennas are directional and only have a specific direction they need to cover.
3) -110dBm because they're industrial-grade and can work in very controlled or isolated environments / mechanical housings (there are not many restraints to the mechanical/electrical design). However, this may be offset by the requirement for high data rate transfer

[Schubert]

Hi Wolberine,

I can give you some typical numbers on mobile phones.

1) Antenna gains: because the devices are handheld (small), they are basically lambda/4 (lowband: 900 MHz) to lambda/2 i(highband: 1800 MHz) n size. Hence, the directivity is low, typically 3 dB for the lowband and 4-6 dB in the highband. The efficiencies are typically -7 to -1 dB (loss due to small bandwidth, mismatch, and electronics lower the efficiency). Hence, the gains are typically close to 0 dB.

2) Cell tower gains...probably not quite 10 dBi. If you divided sectorially into two hemispheres, you'd have +3 dB antennas. If it was 4 equal regions, then it would be +6 dB. Three regions then would probably be roughly +4-5 dB.

3) Receiver sensitivity....yeah about -110 dBm sounds pretty accurate. I don't think there is a correlation between higher data rate transfer and lower sensitivity. (yes, larger bandwidths have larger noise floors which can lower throughput). But for cellular systems the bandwidths are already fixed by the agreed upon communication techniques. The throughput depends on the Signal to Noise Ratio. For the downlink, the maximum power can get very high (relative to the mobile phone transmit power), because there are no SAR requirements or battery constraints.