How can an antenna be used for both rx and tx?

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 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?

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 :)

-Michael

Reply to
Michael
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It's way simpler than that.

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".

When it comes to antennas, you're not too far off. ;-)

Reply to
Anthony Fremont

Wait - you're saying when an antenna is shared between rx and tx, that the rx and tx must be on different frequencies? Is that always the case? Surely, though, they're still at almost the same frequency, right?

Reply to
Michael

If they were on exactly the same frequency it wouldn't work - you'd have to switch from send to receive "Over"

They are sufficiently far apart that the receiver can ignore the transmitter.

Reply to
Homer J Simpson

No I didn't say that. :-) They could be on the same frequency, but then you would have to go with some type of half duplex as you described in your first scenario.

It depends, for example 2 meter (144-148MHz) HAM radio repeaters usually use a 600kHz offset where the transmitter and receiver differ in frequency by

600 kHz. 70cm (~450MHz) repeaters have a 5MHz offset. When you get up around 900MHz, the offset is usually around 45MHz IIRC. As you can see, as the frequency goes up so does the spacing. There would likely be many radio systems out there that would not comply exactly with what I just stated. For example, odd offsets are not uncommon in the commercial radio world.
Reply to
Anthony Fremont

Your intuition is very good Michael. Actually, both techniques are used. Various cell phone technologies incorporate both "Time Division Multiplexing" and "Frequency Division Multiplexing".

In TDM, the TX is on briefly, then the radio switches to RX mode in a cyclic manner. The transmitter and receiver antenna terminals are essentially connected to ports on a SPDT switch. The common pole is connected to the antenna. Timing and synchronization are critical to the success of this method. And enough "slop" to account for propagation delays must be factored in. The advantage is that lossy filters are not required to separate RX and TX. Instead, a less-lossy TX-RX switch may be used. This works extremely well as evidenced by the success of GSM technology which uses TDM. Another side benefit is that since the TX is active for only a small percentage of the time, excellent battery life can be obtained.

Other technologies, such as the archaic AMPS system and the various CDMA technologies, continuously receive and transmit at the same time, but at different frequencies (hence FDM). Filters are used to separate out the two frequency bands. One terminal of each of the RX and TX filters is coupled to a common antenna. The disadvantage is that these filters (aka duplexers) tend to be lossier than the switches used for TDM technologies. But it doesn't impose the timing/sync restrictions that TDM must account for.

Reply to
cledus

Different systems will use different methods, but the GSM frame structure is well documented, and you should be able to find answers to all you questions on the web. Search for GSM.

Tam

Reply to
Tam/WB2TT

The old analog cell phones shared the antenna on both ends, and transmitted and received simultaneously. Each end had a diplexer, that routed the transmit signal out and the receive sig in, based on frequency differences. Digital cell phones pack the voice signals into packets, and time-share the antenna so they don't have to transmit and receive at the same time.

Radar was the classic single-frequency, single-antenna system. Diplexers, using waveguide tricks and gas-discharge t-r switches, kept the megawatt transmit pulses from blowing out the delicate receive diode mixers.

John

Reply to
John Larkin

Ah, but haven't you wondered about how you could send an analog phone signal in both direction on one twisted pair? ;-)

Reply to
miso

This is a very good point, although to anyone thinking about it: The difference with radio antennas is that you typically have, e.g., one watt being transmitted and say, a microwatt being received. With telephone lines, the transmitted and received poweres are roughly comparable.

Reply to
Joel Kolstad

That uses a transformer-based "hybrid" that steers the signals. You can do the same thing with a simple opamp circuit. It does depend on a known line impedance, as a radar diplexer depends on a known antenna impedance.

John

Reply to
John Larkin

Intergalactic link open In fact for a reliable 2 way communication, say for example for digital control, a packet system could be used. Look up amateur packet radio for example. In short the transmitter / receiver is always in receive mode, monitoring the frequency. When it has data to send, the data is send in the form of a synchronous (SDLC or HDLC) protocol, first a syc pattern, then some data bytes, then a CRC check. The length of such a packet can be say 2 kB. The 'other side' receives and decodes the packet (the sync pattern is used to sync a PLL that generates a clock to extract the data), and if the CRC is correct an ACK message is transmitted after waiting for the channel to be free for a random amount of time (in case others also want the channel). If the CRC was not correct a NAK message is send in the same way. The original transmitter will repeat after some time if it hears nothing, or send the next packet if it gets a ACK, or repeat the last packet if it gets a NAK.

For robotics this is likely the only safe scheme. You want to make sure the command (to do something) was received correctly, and possible control more then one robot (individually), and allow feedback from sensors all at the same frequency.

Alien13297432673284768324 Intergalactic link closing

Reply to
panteltje

Oh goodness. Radar works by time division multiplexing transmit and 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.

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Reply to
joseph2k

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