RF modules - external components, correct value of antenna

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Vandal S

Joined: 02 May 2009
Posts: 50
Helped: 2

10 Jul 2009 20:29 RF modules - question

i have a rf transmitter and a receiver, ask, 433mhz, tx with 4 pins, rx with 8 pins (4 - long distance - 4) - if it usefull, i can upload the datasheet.
the questions are:
1. do i have to put any other external component?
or a 7805 which feeds the pic uC also for gnd and vcc? i saw once that someone put an inductor and a capacitor. are they really neaded?
2. i read that if it's 433Mhz, the antenna is around 16.4cm, but in the datasheet it says 30-35cm. way is that and which is the correct value?
thank you

vfone

Joined: 10 Oct 2001
Posts: 2328
Helped: 327

10 Jul 2009 22:01 Re: RF modules - question

Cannot tell what external components you need if I don't know what kind of module is.

If the antenna is 16cm could be a λ/4 monopole, and if is 35cm could be a λ/2 dipole.

Vandal S

Joined: 02 May 2009
Posts: 50
Helped: 2

10 Jul 2009 23:19 Re: RF modules - question

here are the datasheets.
so, those are the formulas for antenna, but which one do i use?
so, is it absolutely necessary to add other components if the TX is near a max232 and a 7805 and the RX is near a pic16f and a 7805?

this was said where i saw this information "ab47 uH inductor was used between the power supply and the transmitter unit to prevent the RF energy from shunting to the power rail, which was an AC ground and a 1nF cap was used between the transmitter and the antenna to prevent detuning of the signal. "

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TerryADS

Joined: 31 Oct 2008
Posts: 116
Location: USA

11 Jul 2009 1:43 RF modules - question

Yes,use external component for match antenna

ganesh_98422

Joined: 02 Jun 2007
Posts: 6

11 Jul 2009 7:00 RF modules - question

thanks

Vandal S

Joined: 02 May 2009
Posts: 50
Helped: 2

11 Jul 2009 8:31 Re: RF modules - question

sorry to ask again but are they absolutely needed? i mean if it's a difference from working or not, i will, but if it's a 10% difference in performance, i wouldn't change the layout

vfone

Joined: 10 Oct 2001
Posts: 2328
Helped: 327

11 Jul 2009 10:56 Re: RF modules - question

This is kind of module manufactured on a boat. There is no specification on the datasheet for RX-TX antenna impedance.
Give a try without matching components and luckily it will work for the distance you are looking for.

pranam77

Joined: 22 Apr 2008
Posts: 1071
Helped: 94
Location: Mangalore(India)

11 Jul 2009 15:20 RF modules - question

Nothing is so critical if you not manufacturing your product as per military specifications. For your hobby work or project, any wire piece will act as an antenna. You can power the module with the same 5V powering the PIC. But only powering the modules will do nothing. You have to feed the TX with a data which will be available at the RX end which is then required to be decoded. Good luck

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Vandal S

Joined: 02 May 2009
Posts: 50
Helped: 2

11 Jul 2009 18:03 RF modules - question

just transmitting 5-6 characters/second from the rs-232 (through max232) to the receiver on a toy car; 5meters will do just fine for a demonstration.
the problem is to work, as i have no decoupling caps, no Manchester encoding or stuff like that, just a U (10101010 in ASCII encoding) and then a character, w,a, etc.