900MHZ Phone Question
Your name
Hello, I just have a few questions about the difference between 46MHZ
cordless phones compared to 900MHZ phones. My understanding is that 900MHZ
radio waves will "bounce" off walls better than 46MHZ waves will.Why is
this?Also why do 900MHZ waves travel though buildings that have a lot of
metal and concretet.Also will 900MHZ waves penetrate things like steel and
concrete or will they bounce off them.Thanks for any info.
hd...@hcol.humberc.on.ca
You are correct...900 MHz signals bounce off of walls and other objects
much better than signals at 46 MHz.
46 MHz will penetrate but usually be absorbed by concrete. 900 MHz will
indeed bounce off concrete, both would be reflected off of steel but
again, some of the 46 MHz signal would become attenuated by absorption
of the steel where 900 MHz would mostly bounce (be reflected).
A 900 MHz signal will travel thru buildings not by penetrating the structure
but rather by bouncing around so much that it finds its way around and
thru aperatures such as window and door openings. Firefighters and other
public services are moving up to 800/900 MHz for this reason.
Without getting into physics, it has to do with the wavelength of the
signals. The wavelength of 46 MHz is roughly 7 meters where 900 MHz
is about 33 centimeters in length. (Frequency is a function of
wavelength).
Generally speaking, the longer the wavelength (lower the frequency),
the more penetrable the signal. What might also work for radio users
like Firefighters would be to use radio equipment in the Very Low
Frequency range (VLF) as the signals there would penetrate concrete,
wood and other structures however, the power required and length of an
antenna on a handheld unit would be unfeasible...it makes more sense to
go the other way and let the signals bounce around and find their way out
of a buidling using UHF.
Just for interest sake, submarines in WW2 used to use Very Low Frequency
when they were submerged (not too deep) as the long wavelength could
penetrate thru the water to communicate. (They towed balloons that suspended
their antennas when surfaced.)
Now to throw a wrench into the above statements..the opposite occurs
when "skip" conditions arise...
Frequencies below (typically) 50 or 60 MHz are susceptible to being
reflected off the various layers of the Ionosphere as they are radiated
toward the horizon. (The density of the Ionosphere is affected by many
things such as time of day, sunspot activity ect..)
The bounced signal comes back to earth at some location hundreds or
thousands of miles away. Communication can take place at both ends.
As you go up in frequency, the signals refract or penetrate the
Ionosphere. Thats why Microwave is a good band for satellite
communication...it is less vulnerable to Ionospheric density (although
dense weather can play a factor ...yet another story).
All of this is explained by the density of the ion particals in the
various layers of our atmosphere....the lower freqs (longer wavelength)
get reflected off the dense clouds of ion particles but the higher
frequencies (shorter wavelength) sift past the ions to penetrate off
into space. The dense ion clouds act like a wire mesh reflector...the
bigger waves bounce, the smaller ones get thru.
Bet you didn't anticipate a long winded explanation like this. did you:)
Funny what a beer and a few minutes of spare time after putting the kid
to sleep can do to you !
Cheers...
Hugh Duff VA3TO Toronto
(Corrections, comments ect.. welcome )
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ş NFX v1.3 [000]
J. Lee Blanton
In article <you-030796...@204.249.225.247>, y...@usa1.com (Your
name) wrote:
> Hello, I just have a few questions about the difference between 46MHZ
> cordless phones compared to 900MHZ phones. My understanding is that 900MHZ
> radio waves will "bounce" off walls better than 46MHZ waves will. Why is
> this? Also why do 900MHZ waves travel though buildings that have a lot of
> metal and concrete. Also will 900MHZ waves penetrate things like steel and
> concrete or will they bounce off them. Thanks for any info.
An advantage of 900 MHz is that the signals propagate better through
enclosed spaces (hallways, rooms, etc.) because of the shorter
wavelength. At 46 MHz (wavelength = 21 feet) enclosed spaces bounded by
steel-reinforced building materials may be near or below the 'waveguide'
cutoff frequency and the signals won't propagate very well.
RF signals don't pass through steel. They may pass through concrete with
relatively low loss if there is no steel reinforcement in the concrete, or
if the wave polarization is orthogonal to the rebar orientation. There
was a paper in the 1996 IEEE National Radar Conference on this subject.
Hope this helps.
mtau...@flash.net
Your name wrote:
>
> Hello, I just have a few questions about the difference between 46MHZ
> cordless phones compared to 900MHZ phones. My understanding is that 900MHZ
> radio waves will "bounce" off walls better than 46MHZ waves will.Why is
> metal and concretet.Also will 900MHZ waves penetrate things like steel and
> concrete or will they bounce off them.Thanks for any info.900Mhz has more penetration that lower freqs. The wavelength is shorter
and "sneaks" through holes easier. 46 will out travel it in the open.
Neither likes steel but 46 will be more adversly affected.