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Modifiying an antenna

 
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garry
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Joined: 19 Jul 2011
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PostPosted: Wed Jul 20, 2011 7:41 am    Post subject: Modifiying an antenna Reply with quote

I'm new to antenna theory so please excuse any dumb questions.

I have been playing with a couple of 2.4GHz tranceivers module (Nordics semiconductors 24L01). The modules are complete i.e. they are ready to use with an antenna made onto the PBC.

I have hooked this to a PIC microcontroller and managed to develop a program that transfers data between the modules, all that is fine and OK.

However the range is short, i.e around <5m. What I want to do is to increase the range, the only thing I can change is the antenna and the matching.

The one one board is etched out of copper and looks like a stubbed 1/4 wave. Looking from the top I can see a short track ( ~4mm) going to the left of the fed and a longer winding track the goes to the right.

I am hoping modifying the antenna by cutting the track at the fed and soldering a different antanna onto it and that is where I got stuck.

What antenna would give a better gain, mono pole or full straight dipole ?

The device has a balanced fed output but the circuit uses a balum to convert this to a single output, would I get better gain by fedding a dipole directly and getting rid of the balum?

From what I've read so far the output of a transmitter needs to be matched to the antennas, and that makes sense. But a lot of what I have read appears to suggests matching to 50ohms, any idea why?

How do I know what the impedance of the anntenna would be ? I don't have a network analyser.


Any suggestions would be appreciated.

Cheers
Garry
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Schubert
Antenna Wizard


Joined: 08 Apr 2009
Posts: 161

PostPosted: Wed Jul 20, 2011 7:09 pm    Post subject: Reply with quote

Do you have a picture of the setup? It might help to clarify exactly what we're talking about.


You match to 50 Ohms because that is generally the impedance of the radio and transmission line. Not always, but the majority of the time.

As for optimizing your antenna without a network analyzer, you'll just have to be experimental about it. That is, fiddle with it and see if your range improves. It's pretty difficult to accurately measure the impedance of the antenna if you don't know exactly what is happening (i.e., if you solder a coaxial cable to the antenna feed and plug that into the network analyzer, if you don't properly ground the coaxial cable it will become part of the antenna itself, and you will be measuring the cable as well as the antenna, which won't be accurate).

On the question of dipole versus monopole, I'd recommend a dipole, as it will be easier to make (just isolate it from the ground plane, make it about a half-wavelength long and place the feed at the center).
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garry
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Joined: 19 Jul 2011
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PostPosted: Wed Jul 20, 2011 8:16 pm    Post subject: Reply with quote

Thanks for the reply, the best I can do for a picture is to give the ebay page which is http:

[url]//www.ebay.co.uk/itm/NRF24L01-MultiCeiver-Wireless-Module-/170576092816?pt=LH_DefaultDomain_0&hash=item27b7209e90[/url]


I don't know how to insert a picture, the length of the short stub is about 7mm and the coiled part is about 32mm.

Seems as this device has a balanced output, is it simply a matter of connection one half of the dipole to one side of the transmitter and the other half to the other side or is it more complex? In this case I assume the dipole to be 2 pieces of wire soldered to to board.

As you can see there is no transmission line involved so matching should only be between the transmitter and the antenna, for which I have no idea what the impedence will be.

Cheers
Garry
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Schubert
Antenna Wizard


Joined: 08 Apr 2009
Posts: 161

PostPosted: Wed Jul 20, 2011 10:58 pm    Post subject: Reply with quote

Ok, yeah that antenna is horrid. By meandering the trace back and forth the currents that should be adding in phase to produce radiation will be canceling instead. They were obviously constrained by space but did a pretty poor job in my opinion.

For your dipole, if you keep the transmission line between the radio and the antenna short (0.5" for 2.4 GHz wifi is fine) you don't need to worry about transmission line effects. So yes you can just solder from the radio or pcb to two arms of the dipole. Now, the dipole antenna won't want the ground plane or much metal around it, so try to keep the antenna as far as possible from the circuitry. It won't be easy given your transmission line constraint, so it is ok to make a "V shape" for the dipole, so that the arms travel away from the radio circuitry.

ANyway, just experiment and you should find something that works.
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garry
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Joined: 19 Jul 2011
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PostPosted: Thu Jul 21, 2011 6:29 pm    Post subject: Reply with quote

So by just making the antenna straight should improve the gain. Making rough measurements the meandering antenna is about 32 mm, 2.4Ghz has a wave length of about 125mm so the meandering part is 1/4 wavelegnth.
If I cut the track just after the short arm and solder a straight wire that is about 32mm I should improve the gain.

Thats an easy experiment that I will try this weekend.

Now the dipole, I've just watched the video on this web site and that makes some sense but I am a little confused about the feed. The drawing shows 2 straight lines that represents the 2 halves of the dipole each halve being 1/4 wavelegth long. Does this mean the dipole is 2 actual pieces of wire fed by antiphase signals or is it one piece of wire fed in the center by a single fed.

To be a bit more clear (or confused) could I take a single piece of wire that is 1/2 wavelength bend it into a V and solder the point of the V in place of my current antenna.

Secondly could I take 2 1/4 wave pieces of wire and connect them directly to the output of my transmitter. That assuming I can get a solder bit small enough.

University was OK for the maths side of stuff but not for the more interesting pratical side.

Thanks for the help.

Cheers
Garry
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Schubert
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Joined: 08 Apr 2009
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PostPosted: Sat Jul 23, 2011 2:01 am    Post subject: Reply with quote

Think of the feed as a battery - it has a positive and a negative side. On your radio, the negative side is probably connected to the general ground plane, and that can be extended to produce one arm of the dipole. The positive side of the feed (that goes to the meandered arm in your picture) would be the positive side, or the other arm of the dipole.
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