Author Topic: RF Modules  (Read 1727 times)

0 Members and 1 Guest are viewing this topic.

Offline jonagikTopic starter

  • Full Member
  • ***
  • Posts: 63
  • Helpful? 1
RF Modules
« on: November 07, 2011, 09:34:42 PM »
Hi,

Firstly, can someone let me know if this is correct. Regarding the ANT pin on an RF module, is this supposed to be connected to the GND pin via a wire (the aerial)?

Secondly, I take it that a sinusoidal current flows out the ANT pin to GND. Therefore, is it viable to connect ANT to an amplifier (for example a BJT) to boost the signal strength?

Finally, when a radio transmission power is specified (e.g. 100mW) what does this refer to? Is it simply the voltage across the aerial times the current or is it more complicated? (I expect it is the latter)

Thanks.

Offline Soeren

  • Supreme Robot
  • *****
  • Posts: 4,672
  • Helpful? 227
  • Mind Reading: 0.0
Re: RF Modules
« Reply #1 on: November 08, 2011, 03:27:28 AM »
Hi,

Firstly, can someone let me know if this is correct. Regarding the ANT pin on an RF module, is this supposed to be connected to the GND pin via a wire (the aerial)?
No-no-no!
The aerial (ANTenna) should, in its simplest form, be a wire of a quarter (or half) of a wavelength going into the air (i.e. unconnected at the "far" end).
Shorting it to ground won't hurt a receiver (just decreasing reception to almost nothing), but a transmitter will die from that treatment!


Secondly, I take it that a sinusoidal current flows out the ANT pin to GND. Therefore, is it viable to connect ANT to an amplifier (for example a BJT) to boost the signal strength?
For a receiver, you can add a preamplifier, but a directional antenna can do magic without increasing noise.
For a transmitter, you can add a P.A. stage (Power Amplifier), but it will most likely be highly illegal and unless professionally designed, may well jam emergency frequencies by harmonics and sub harmonics.
Again, start with a better (directional) antenna.

To be brutally honest, asking the questions you do, you will not be able to make an UHF amplifier - the PCB layout in itself takes experience and knowledge, as does even the appearently simplest of UHF amps.


Finally, when a radio transmission power is specified (e.g. 100mW) what does this refer to? Is it simply the voltage across the aerial times the current or is it more complicated? (I expect it is the latter)
It's 100mW into the specified impedance.

What are you trying to achieve in terms of range (as a percentage of the present range) and at what frequency?
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline billhowl

  • Supreme Robot
  • *****
  • Posts: 376
  • Helpful? 32
  • Your success is in your hands.
Re: RF Modules
« Reply #2 on: November 08, 2011, 05:15:33 AM »
The Antenna may connected both the ANT and GND pin depend on the type of Antenna design.

Look at the Folded Dipole and the Loop Antenna

http://en.wikibooks.org/wiki/Communication_Systems/Antennas

Transmit power refers to the amount of RF power that comes out of the antenna port of the radio. Transmit power is usually measured in Watts, milliwatts or dBm.
The TX output was used to calculate radio link budget, lets you quickly compute the Free Space Loss, Received Signal Strength, Fade Margin , Distance and receiver sensitivity needed.

http://www.digi.com/technology/rf-articles/rf-basics
1 mW = 0 dBm
http://en.wikipedia.org/wiki/DBm
http://www.afar.net/rf-link-budget-calculator/



Offline jonagikTopic starter

  • Full Member
  • ***
  • Posts: 63
  • Helpful? 1
Re: RF Modules
« Reply #3 on: November 08, 2011, 05:49:48 AM »
I thought that the mechanism with which RF worked was that a sinusoidal current in a wire (the aerial) causes electromagnetic radiation of the frequency of the sinusoid to be emitted. If the ANT pin isn't grounded then (presumably) no current flows in the antenna and hence no electromagnetic radiation is emitted from the aerial. Presumably this is not the case, so how is it that the aerial works when not grounded?

If you could, I would appreciate any recommendations on reading into this area (radio communication). I'm currently in my first year of engineering going into electrical and electronic engineering next year, so getting a reasonable level of understanding of this subject matter can't hurt.

Thanks for your help.

Offline billhowl

  • Supreme Robot
  • *****
  • Posts: 376
  • Helpful? 32
  • Your success is in your hands.
Re: RF Modules
« Reply #4 on: November 08, 2011, 05:59:36 AM »

Quote
Marconi antennas are usually 1/4 wavelength long and require a path to ground. The ground plane itself acts as a reflector of energy, and combines with the directly radiated wave to create the overall radiation pattern. If the ground is dry or otherwise a poor conductor, a copper grid is generally laid out on the ground. The impedance of a 1/4 λ Marconi antenna is 36.6 Ω.


Quote
Notice that a Marconi antenna could be considered as a dipole antenna with one of the poles buried in the ground. The ground acts as a reflector to create the appearance of an buried antenna in the same way that a mirror creates the appearance of someone behind the glass. Increasing the antenna length has a significant impact on the radiation pattern:


Read more from here
http://en.wikibooks.org/wiki/Communication_Systems/Antennas

[youtube]lslHtCUSfN4[/youtube]
Directional Antennas
« Last Edit: November 08, 2011, 07:46:19 AM by billhowl »

Offline Soeren

  • Supreme Robot
  • *****
  • Posts: 4,672
  • Helpful? 227
  • Mind Reading: 0.0
Re: RF Modules
« Reply #5 on: November 08, 2011, 07:40:39 AM »
hI,

The Antenna may connected both the ANT and GND pin depend on the type of Antenna design.
Sure, but just as I didn't describe Yagis, discones and what not, to someone who's clearly a beginner, I wouldn't trust that he got the measures right on a folded dipole and if it goes on one of the tiny RF modules, I wouldn't give advice that could potentially kill, it if just a little misaligned (and yes, some of them are hyper sensitive to antenna matching).
It's all a question of judging the receiver (of the post :)).
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline billhowl

  • Supreme Robot
  • *****
  • Posts: 376
  • Helpful? 32
  • Your success is in your hands.
Re: RF Modules
« Reply #6 on: November 08, 2011, 08:22:00 AM »
No-no-no!
The aerial (ANTenna) should, in its simplest form, be a wire of a quarter (or half) of a wavelength going into the air (i.e. unconnected at the "far" end).
Shorting it to ground won't hurt a receiver (just decreasing reception to almost nothing), but a transmitter will die from that treatment!

Yes, mismatching can damage to the transmitter by the standing wave reflected back towards the transmitter.
http://en.wikipedia.org/wiki/Standing_wave_ratio



Offline dunk

  • Expert Roboticist
  • Supreme Robot
  • *****
  • Posts: 1,086
  • Helpful? 21
Re: RF Modules
« Reply #7 on: November 08, 2011, 06:54:21 PM »
hi Jonagik,
just to back up what Soeren is saying,
*most* (but not all) RF modules are designed to work with a 1/4 wave "whip" antenna.
the datasheet for your RF module should tell you what kind on antenna it is designed to work with.

a 1/4 wave "whip" antenna is just a straight piece of wire connected to the antenna pin.

the "1/4 wave" part is 1/4 of the wavelength of the radio signal where
speed = frequency*wavelength
speed in this case is the speed of light (and radio waves) in air.

amplifiers, directional antennas, etc are all possible but complicated.
other fractions of the wavelength (instead of 1/4 wave) are also possible if you are looking for particular signal properties.
better to keep it simple to start with though. 1/4 wave is easy and is usually what these modules are initially tested with.

a whip antenna works best when mounted at right angles to a ground plane.
this is a conductive sheet placed below the base of the antenna connected to your circuit's ground.
but for initial experimentation it is not required. you should get well over 50% of your modules advertised range without bothering about a ground plane.

so yea, just attach a straight piece of wire with nothing on the other end.


dunk.

Offline jonagikTopic starter

  • Full Member
  • ***
  • Posts: 63
  • Helpful? 1
Re: RF Modules
« Reply #8 on: November 08, 2011, 08:40:24 PM »
Thanks for your responses.

Can someone please explain to me how an antenna can work without current going through it? I assume that, because it isn't grounded, there is no current going through the wire.

Offline Soeren

  • Supreme Robot
  • *****
  • Posts: 4,672
  • Helpful? 227
  • Mind Reading: 0.0
Re: RF Modules
« Reply #9 on: November 09, 2011, 02:56:10 AM »
Hi,

Can someone please explain to me how an antenna can work without current going through it? I assume that, because it isn't grounded, there is no current going through the wire.

It cannot!
But luckily, there is a current going through it (or rather back and forth) or it wouldn't radiate.

You wouldn't doubt the same for a capacitor I'm sure and you can look at a dipole (not the folded variety) as a capacitor where you separated the plates by turning one of them (pivoting about its end) like this...

(Green=field lines - should have been more elliptic on the last drawing to better show how it works, but this was way faster :))
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


Get Your Ad Here