Forums

Sharing an antenna between Rx and Tx RF modules ?

Started by Simon Clubley January 2, 2008
I'm currently doing some experiments with 433MHz license free RF modules.

Is it practical to share one antenna between a Tx and Rx module in a
half-duplex environment, and if so, what techniques are used ?

I would need a microcontroller to be able to switch the antenna between
the Tx and Rx modules as required.

I know that the proper solution would be to use a transceiver with Tx and Rx
enable pins, but I am wanting to reuse some modules that I have available.

In case it matters, this is in a hobbyist environment.

Thanks for any information,

Simon.

-- 
Simon Clubley, clubley@remove_me.eisner.decus.org-Earth.UFP
Microsoft: Bringing you 1980's technology to a 21st century world
On 2 Jan 2008 06:34:13 -0600, in comp.arch.embedded
clubley@remove_me.eisner.decus.org-Earth.UFP (Simon Clubley) wrote:

>I'm currently doing some experiments with 433MHz license free RF modules. > >Is it practical to share one antenna between a Tx and Rx module in a >half-duplex environment, and if so, what techniques are used ? > >I would need a microcontroller to be able to switch the antenna between >the Tx and Rx modules as required. > >I know that the proper solution would be to use a transceiver with Tx and Rx >enable pins, but I am wanting to reuse some modules that I have available. > >In case it matters, this is in a hobbyist environment. > >Thanks for any information, > >Simon.
ISTR an antenna switching scheme on the nordic semi site, but it was for 2.4G.The site is a bit difficult to navigate. You really need to be able to switch each module on and off quickly via a logic input. There may be a problem of the device taking, say 100mSeconds to stablise if you try to switch the power. You could use external switches http://www.macom.com/psc/jsp/ListParts.jsp?dataFile=switches.txt martin
On Jan 2, 7:34=A0am, clubley@remove_me.eisner.decus.org-Earth.UFP (Simon
Clubley) wrote:

> Is it practical to share one antenna between a Tx and Rx module in a > half-duplex environment, and if so, what techniques are used ?
Yes, RF R/T switches are readily available. NEC among many other vendors. Very simple to use. If you look on Digikey for RF switches you should find many in in the sub-$3 price range (one-off qty). Typically they are in a SOT23-6 package.
"larwe" <zwsdotcom@gmail.com> wrote in message 
news:3ec42c37-59ca-4944-97e6-1c0937ac8330@i29g2000prf.googlegroups.com...
On Jan 2, 7:34 am, clubley@remove_me.eisner.decus.org-Earth.UFP (Simon
Clubley) wrote:

> Is it practical to share one antenna between a Tx and Rx module in a > half-duplex environment, and if so, what techniques are used ?
Yes, RF R/T switches are readily available. NEC among many other vendors. Very simple to use. If you look on Digikey for RF switches you should find many in in the sub-$3 price range (one-off qty). Typically they are in a SOT23-6 package. How do analog cordless phones work? Don't they transmit and receive simultaneously on different frequencies?
On 2008-01-02, Moon Shine <mshine@msn.biz> wrote:

> How do analog cordless phones work?
Back when I worked on cellular phone designs, they use a duplexer (a directional RF antenna coupler).
> Don't they transmit and receive simultaneously on different > frequencies?
Yes. As do digital cordless phones and mobile phones of all sorts (cellular and pre-cellular). -- Grant Edwards grante Yow! I'm wet! I'm wild! at visi.com
On Jan 2, 11:21=A0am, "Moon Shine" <msh...@msn.biz> wrote:

> How do analog cordless phones work? > Don't they transmit and receive simultaneously on different frequencies?
If you share the antenna like that, you just make sure the filter on the Rx will keep the Tx out of your signal. Not difficult. But it's not what the OP was asking about. For a half-duplex system on a single frequency (the type of device I work with mostly), simple SPDT RF switches are an uncomplicated answer.
On Jan 2, 7:34 am, clubley@remove_me.eisner.decus.org-Earth.UFP (Simon
Clubley) wrote:
> I'm currently doing some experiments with 433MHz license free RF modules. > > Is it practical to share one antenna between a Tx and Rx module in a > half-duplex environment, and if so, what techniques are used ?
If the transmitter power is low enough that it would not damage the receiver input, you may be able to simply connect them and de-power the transmitter in order to receive. However, it is possible that element in the receiver front end (such as rectifier diodes, intentional or parasitic) might distort or attenuate the transmitted signal. Diodes for example would likely produce spurious emissions at overtone frequencies, which could well be illegal. Actually the type acceptance of the transmitter is probably invalidated if you connect anything other than the specific antenna. RF switches as others have mentioned are a safer if more complicated solution.
Simon Clubley wrote:

> I'm currently doing some experiments with 433MHz license free RF modules. > > Is it practical to share one antenna between a Tx and Rx module in a > half-duplex environment, and if so, what techniques are used ? > > I would need a microcontroller to be able to switch the antenna between > the Tx and Rx modules as required. > > I know that the proper solution would be to use a transceiver with Tx and > Rx enable pins, but I am wanting to reuse some modules that I have > available. > > In case it matters, this is in a hobbyist environment. > > Thanks for any information, > > Simon.
The simplest switch is either a suitable RF relay or a 1/4 wave element and a pair of fast switching diodes in the input of the receiver. The latter arrangement protects the receiver from the transmitter output and allows the weaker signals for the receiver input to be seen. If you need to transmit and receive at the same time (on different frequencies of course) then very sharp pass-band filters for the transmitter and a sharp pass-band and notch filter for the receiver is required. [diagram by email] -- ******************************************************************** Paul E. Bennett...............<email://Paul_E.Bennett@topmail.co.uk> Forth based HIDECS Consultancy Mob: +44 (0)7811-639972 Tel: +44 (0)1235-811095 Going Forth Safely ..... EBA. www.electric-boat-association.org.uk.. ********************************************************************
In article <3ec42c37-59ca-4944-97e6-1c0937ac8330@i29g2000prf.googlegroups.com>, larwe <zwsdotcom@gmail.com> writes:
> > Yes, RF R/T switches are readily available. NEC among many other > vendors. Very simple to use. If you look on Digikey for RF switches > you should find many in in the sub-$3 price range (one-off qty). > Typically they are in a SOT23-6 package. >
Thanks. These are the kinds of things that I am looking for, but I wasn't sure what they were called when I was looking for them, or if there were issues with them that I was not aware of. The comment that was made about invalidating the type certification is also of interest, as I'm in the UK and OFCOM appear to be rather uptight about what exactly you can and cannot do with the licence free modules. Thanks to everyone for their replies, both here and in email. Simon. -- Simon Clubley, clubley@remove_me.eisner.decus.org-Earth.UFP Microsoft: Bringing you 1980's technology to a 21st century world