Application Note: 900 MHz vs 2.4 GHz
To determine whether you should use 900 MHz or 2.4 GHz RFd2d
technology for your products you need to answer the following:
- What is the maximum range your product needs to communicate?
- Will your product be deployed outside North America, Australia
or Israel?
What is the maximum range your product needs to communicate?
The major difference between 900 MHz and 2.4 GHz wireless OEM
modules is range. While MaxStream's 9XStream (900 MHz) offers
much greater range than the 24XStream (2.4 GHz), the 24XStream
is more accepted internationally. The following table lists specific
differences between the 9XStream and 24XStream.
| Feature |
9XStream
(900 MHz) |
24XStream
(2.4 GHz) |
| Pager & cell rejection |
70 dB |
Pagers and cell phones seldom interfere with this band |
Transmit power
|
21 dBm |
18 dBm |
Receive sensitivity
|
-110 dBm |
-104 dBm |
Range
(9600-baud module, line-of-sight)
|
Indoor: 600' to 1500'
Outdoor: 7 miles with dipole antenna, over 20 miles with
gain antenna |
Indoor: 150' to 375'
Outdoor: 1.4 miles with dipole antenna, over 4.5 miles with
gain antenna
|
Path loss
(for more information, see the section "Comparing Wave
Propagation of 2.4 GHz and 900 MHz Frequencies")
|
Because the 24XStream operates at 2.4 GHz,
it has +8.519 dB more path loss than the 9XStream which
operates at 900 MHz |
Cost
|
The 24XStream lists up to 34% less than the
9XStream |
| Unlicensed operation |
North America, Australia, Israel |
Worldwide |
TX current consumption
(when not in power-down mode) |
150 mA |
250 mA |
RX current consumption
(when not in power-down mode) |
50 mA |
90 mA |
Simply put, if your range requirements are beyond 375 feet indoors,
or beyond 1.4 miles outdoors, you should consider the 9XStream
over any other 2.4 GHz product. The 9Xstream proves to be a longer
range, lower power and lower cost solution than most 2.4 GHz products.
Will your product be deployed outside North America,
Australia or Israel?
Another difference between 900 MHz and 2.4 GHz solutions is that
the 900 MHz radio frequency band is for unlicensed use only in
North America, Australia and Israel. Worldwide (including North
America, Australia and Israel), 2.4 GHz is an unlicensed radio
frequency band.
If your product will be deployed worldwide, you should consider
24XStream 2.4 GHz wireless products. The 24XStream offers longer
range, lower power and advanced networking capabilities not offered
by other 2.4 GHz solutions.
Conclusion
9XStream 900 MHz products are best used in long range, low power
applications deployed in North America, Australia and Israel.
24XStream 2.4 GHz products are best used in applications deployed
worldwide.
Frequency Usage Table By Products
Each of the MaxStream radio modems uses as little of the available
RF spectrum as possible so as to minimize susceptibility to interference
from other RF systems as well as minimizing the impact of MaxStream
transmissions on RF systems already operating in the area.
Here is a table showing the specific frequencies used by the
MaxStream radio modems by product family:
| Product family |
First
Frequency
(MHz) |
Last
Frequency
(MHz) |
Frequency
spacing
(KHz) |
| X09 (USA, Canada) |
910.5 |
917.7 |
300 |
| XH9 (Israel, Australia, New Zealand) |
920.0 |
927.2 |
300 |
| X24 (25 channel) |
2450.15 |
2459.75 |
400 |
| X24 (75 channel) |
2450.15 |
2479.75 |
400 |
More technical information on wave propagation and range
calculations
Comparing Wave Propagation of 900 MHz and 2.4 GHz Frequencies
To demonstrate the basic difference in wave propagation of 900
MHz and 2.4 GHz waves, a quick look at path loss is provided.
As waves propagate out from the transmitter, some attenuation
of the signal takes place due to properties of the medium (air
in most cases). Path loss describes this attenuation as a function
of the wavelength of the operating frequency and the distance
between the transmitter and receiver. The path loss is derived
from the Friis transmission equation and is defined as:
Path Loss = 20 log(4*p*r/λ) dB
where r is the distance between the transmitter and receiver,
and ? is the wavelength . The table below shows how path loss
differs between 900 MHz transmitters (λ=0.33 meters) and
the 2.4 GHz transmitters (λ=0.125 meters).
NOTE: Path loss analysis does not account for effects such as
differing TX power outputs and RX sensitivities. See the "Range
of 9XStream (900 MHz) and 24XStream (2.4GHz) Modules" section
for more detailed range information.
| Frequency |
r = 10 Meters |
r = 100 Meters |
r = 1000 Meters |
| 900 MHz |
51.527 dB |
71.527 dB |
91.527 dB |
| 2.4 GHz |
60.046 dB |
80.046 dB |
100.046 dB |
Thus, the path loss is +8.519dB more over a given range for
the 2.4 GHz modules. Since the range doubles with every 6 dB of
reduced path loss, the 900 MHz modules have 2.67 times as much
range as the 2.4 GHz modules [2^(8.519/6) = 2.67].
Range of 9XStream (900 MHz) and 24XStream (2.4 GHz) Modules
A link budget analysis can mathematically predict the system range
based on the power output, receiver sensitivity, antenna gains,
path loss, and fading margin. The path loss equation represents
path loss (signal attenuation) as a function of distance between
the receiver and transmitter and the wavelength of the operating
frequency. This equation is derived from the Friis transmission
equation and is given by:
Path Loss = 20* log(4*p*r/λ) dB (Eq. 1), where
r = distance between transmitter and receiver
& = wavelength
The Friis transmission equation can be used to represent the
path loss as the sum of the other system factors leading to the
following equation:
Path Loss = P(t) + G(t) + G(r) - R(s) - F(s) dB (Eq. 2), where
P(t) = transmitted power
G(t) = gain of transmit antenna
G(r) = gain of receive antenna
R(s) = sensitivity of receiver
F(s) = fading margin, (experimentally determined to be 22dBm)
These two equations can be used to compare the maximum range
of the 9XStream and 24XStream modules.
- Consider the range of the 9XStream module:
= 0.33 meters (for f=900 MHz)
(Eq. 1) Path Loss = 113 dB = 20 * log(4*p*r/?)
(Eq. 2) Path Loss = 21dBm + 2dB + 2dB - (-110dBm) - 22dBm=
113 dB
By setting these equal to each other, a little computation
reveals that r=11848 meters, or a little over 7 miles.
- Now consider the 24XStream module:
= 0.125 meters (for f=2.4 GHz)
(Eq. 1) Path Loss = 105 dB = 20 * log(4*p*r/?)
(Eq. 2) Link Budget = 18dBm + 2dB + 2dB - (-105dBm) - 22dBm
= 105 dB
Once again, setting these equations equal leads to r=1768
meters, or just over 1 mile.
From this example, it is shown that operating at 900 MHz exhibits
a significantly longer range than is possible at 2.4 GHz.
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