Re: How can an antenna be used for both rx and tx?
- From: MassiveProng <MassiveProng@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 20 Apr 2007 18:53:05 -0700
On Fri, 20 Apr 2007 18:49:12 -0700, joseph2k <quiettechblue@xxxxxxxxx>
Gave us:
Anthony Fremont wrote:
Michael wrote:Oh goodness. Radar works by time division multiplexing transmit and
Hi - I was just thinking about this. How does a device in real time
share an antenna between rx and tx circuits? I'm thinking of something
like a cell phone, for example. My first thought was that the device
would allot a certain amount of time for sending and a certain amount
of time for receiving (say, every other 5 microseconds, or something
like that). But then making every other device match up with that
would be rather difficult methinks. So then my next idea was that the
It's way simpler than that.
device is always both sending and receiving and the received signals
are just extracted from the antenna's signal. But then I got to
thinking about exactly how this would be done. You'd want to subtract
the signal being transmitted from the overall signal present at the
antenna - but where would you be getting the antenna signal? Wouldn't
you be getting it from the antenna connection, where the transmitter
circuitry is connected to the antenna? So how would you be able to
pull out the rx signal?
Everything is happening at the same time usually (full duplex). First
off,
the transmitter and receiver are on different frequencies. This is
usually not enough to prevent the transmitter from overloading the
receiver, so
careful filtering prevents the transmitter from getting in. Google
"antenna duplexer".
I suspect this is a terribly uninformed question, but I'm really a
robotics/sensors guy - all this RF stuff seems like black magic to
me :)
When it comes to antennas, you're not too far off. ;-)
receive. Cell phones do so to a lesser extent and add band split, the
transmit frequencies a about 20 MHz apart. Older systems like the very
earliest radio data networks (google Aloha network, University of Hawaii)
all the transmitters and receivers were on the same frequency and time
shared (like a condo time share) transmit time. Depending on the case
involved code division multiplexing is also used. This allows multiple
transmitters to transmit at the same time on the same frequency and still
have each transmitters signal to be independently received. This is used
in GPS. Please try googling TDMA, FDMA, and CDMA; which stand for time,
frequency, and code, division multiple access. Please also check out
devices called circulators, 3 port and 4 port. All of the things make
sharing a single antenna much easier.
How else did these idiots think that a six element cell tower could
handle 300 connections at one "time"?
.
- References:
- How can an antenna be used for both rx and tx?
- From: Michael
- Re: How can an antenna be used for both rx and tx?
- From: Anthony Fremont
- Re: How can an antenna be used for both rx and tx?
- From: joseph2k
- How can an antenna be used for both rx and tx?
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