Using one antenna for RF transmitter and receiver

SZ_78

I just wanted to know is it possible to connect only one antenna to both RF transmitter and RF receiver. I'm using a TWS434A (6 pin package) and an RWS 434A with a Whip antenna.

Thanks

FearTurtles

You would need to install a duplexer to block the RF from the Transmitter to feed back into the reciever.

Manetheren

A circulator would also work.

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Manetheren

jmalaysia

What you really need is just an antenna switch,· which can either be a relay or just a couple of diodes.· The receive path needs a normally forward biased diode and the transmit path needs a normally reverse biased diode.· During transmit,· you switch the bias of the diodes,· connecting the transmit path to the antenna and blocking the receive path.

If the transmitter and receiver are in close together the signal will still induct into the receiver so if that's a problem you will have to use 2 different frequencies.

Carl Hayes

A·circulator would work, but a duplexer·would not, unless the transmitter & receiver are on different frequencies.··These are all rather expensive·solutions, though, and finicky to adjust.· Could you just use a relay instead?

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· -- Carl, nn5i@arrl.net

Post Edited (Carl Hayes) : 4/25/2009 3:54:00 AM GMT

FearTurtles

Yea thats true the duplexer needs two different frequencies. I was under the impression that is what the kits contained.

Carl Hayes

FearTurtles said...
Yea thats true the duplexer needs two different frequencies. I was under the impression that is what the kits contained.

If the frequencies are wildly different (one is more than twice the other), a small, inexpensive·($40 to $80) duplexer requiring no adjustment·(of the kind often called a diplexer) will serve.

If on the other hand the frequencies are close together, you'll need a different sort of duplexer, large and expensive and requiring adjustment with exquisite precision.· It won't fit in a shoe box, but (depending on frequency) might fit in the shipping carton for, say,·a microwave oven.

Jmalaysia suggests that you could use diode switches, but this is true only at very low power levels; and if you use it with a transmitter it carries the risk of generating spurious signals that would interfere with other radio services.

Me, I'd use a simple relay.



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· -- Carl, nn5i@arrl.net

jmalaysia

Diodes are used exclusively in 2-way radios up to 60 watts rf.· Over that then you need a relay.· A relay will certainly accomplish the same thing but isn't required at such a low level.

Carl Hayes

Yes, there are some transceivers (by no means all)·that use diode TR switches at those levels.· There are pitfalls, though, and the design must be a careful one if you're going to avoid putting out a really dirty signal.· Would you really recommend this to a questioner who asks whether it's possible at all to use the same antenna for transmitting and receiving?

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· -- Carl, nn5i@arrl.net

jmalaysia

"What you really need is just an antenna switch, which can either be a relay or just a couple of diodes."

FearTurtles

@Carl Wouldn't the low power of those transmitters help to insure a cleaner signal? What considerations should be looked at in order to use diodes?

Carl Hayes

FearTurtles said...
@Carl Wouldn't the low power of those transmitters help to insure a cleaner signal? What considerations should be looked at in order to use diodes?

I'm not familiar with those transmitters, but low power in general makes it easier to get a clean signal through diode switches.· When the diode switch is on, it is on because a forward DC current is flowing through it.· The RF signal is then imposed atop the DC, so that at any instant the instantaneous current is the sum of the (always positive) DC current and the (sometimes negative, sometimes positive) instantaneous current resulting from the AC delivered by the transmitter.

The trick is to assure that the DC current you impose (to turn the switch on) is much larger than the instantaneous peak current of the radio-frequency signal.· "Much larger than" means, say, twice as large anyway.

So you must·be able to calculate that instantaneous·peak RF current.··This is not too tough if you know the carrier power and the kind of modulation.·

Here's an example:

Assume an amplitude-modulated signal, at 1 watt carrier power, in a 50Ω system.

Because it's AM, the rms power at a modulatiion peak envelope will be 4x the carrier power, or 4 watts.

The signal voltage (rms) at a modulation peak·we calculate from p=e²/Z so with 4·watts in 50Ω· we get 14 volts rms.

The peak signal voltage·is 'prox 1.414 times this, or about 20 volts.

The peak·signal current, then,· is 20v/50Ω or 0.4 amperes.· To switch the diode on you need at least twice this, or 0.8 amperes.· Not too tough.

For other modulation types·the calculation of peak RF power would be different.· For FM, as an example, the peak and average RF power are identical, so you wouldn't multiply by 4.· For radar signals, usually the peak RF power is what's specified anyway.· And like that.·

All of that is if my arithmetic is correct.· No guarantees.

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· -- Carl, nn5i@arrl.net

Post Edited (Carl Hayes) : 4/27/2009 3:27:27 PM GMT

Manetheren

You could also use a transistor and the relay. The important part is to isolate the receiver section from the transmitter section as the receiver will use less input power than the transmitter puts out and will have the chance of being fried with the full output power of the transmitter. Isolation is the Key.

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Manetheren

Robert Kubichek

SZ_78 said...
I just wanted to know is it possible to connect only one antenna to both RF transmitter and RF receiver. I'm using a TWS434A (6 pin package) and an RWS 434A with a Whip antenna.

Thanks

It is doable, but by the time u factor in time/material, you are much better off getting a transciever module that does the same thing and makes life easier for you.