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From: Nico Coesel on 30 Mar 2010 14:11
Every now and then you find something new on your path...

I'm working on a wireless device which is going to use a PCB trace as
an antenna. The bitrate is quite high (approx 250kbit) and we decided
to use the 433MHz band because it has little restrictions. The main
problem is the antenna. I managed to put a 115mm (4530 mil) long
trace (1/4 labda monopole) onto the 50mmx80mm (2" x 3.15") board to
form an L shaped antenna. Its fed from a 50 Ohm transmission line
which runs over a reasonable big ground plane (top and bottom stitched
). So far I was able to gather some info from applications notes and
so on.

Ofcourse there is more on the board than just the antenna although I
made sure the antenna runs far away from the dense populated areas.

Now the real problem is going to get the antenna tuned. As far as I
can see that takes two steps: getting the antenna to resonate at the
desired frequency and matching the impedance. The gear I have
available is a spectrum analyzer, an oscilloscope, a directional
coupler and an HF generator.

I know a vector network analyzer would be the right tool especially
for determining the mismatch, but I'm wondering if I could do without.
If not, I've found this kit.

http://www.sdr-kits.net/

Looks nice and affordable any comments?

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico(a)nctdevpuntnl (punt=.)
--------------------------------------------------------------
From: Joerg on 30 Mar 2010 14:21
Nico Coesel wrote:
> Every now and then you find something new on your path...
>
> I'm working on a wireless device which is going to use a PCB trace as
> an antenna. The bitrate is quite high (approx 250kbit) and we decided
> to use the 433MHz band because it has little restrictions. The main
> problem is the antenna. I managed to put a 115mm (4530 mil) long
> trace (1/4 labda monopole) onto the 50mmx80mm (2" x 3.15") board to
> form an L shaped antenna. Its fed from a 50 Ohm transmission line
> which runs over a reasonable big ground plane (top and bottom stitched
> ). So far I was able to gather some info from applications notes and
> so on.
>
> Ofcourse there is more on the board than just the antenna although I
> made sure the antenna runs far away from the dense populated areas.
>
> Now the real problem is going to get the antenna tuned. As far as I
> can see that takes two steps: getting the antenna to resonate at the
> desired frequency and matching the impedance. The gear I have
> available is a spectrum analyzer, an oscilloscope, a directional
> coupler and an HF generator.
>
> I know a vector network analyzer would be the right tool especially
> for determining the mismatch, but I'm wondering if I could do without.
> If not, I've found this kit.
>
> http://www.sdr-kits.net/
>
> Looks nice and affordable any comments?
>

Since it was designed by a ham it's probably alright. But you don't
necessarily need one for this job. Resonance can be achieved by trial
and error, probably you are going to add inductors until the antenna has
the correct electrical length. For matching you might want to borrow a
UHF-wattmeter or SWR-bridge from a ham operator. But it must work on the
70cm band which not all of them do. If he has a dipmeter that would be
nice, too, makes finding resonance a breeze.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: Tim Wescott on 30 Mar 2010 14:28
Joerg wrote:
> Nico Coesel wrote:
>> Every now and then you find something new on your path...
>>
>> I'm working on a wireless device which is going to use a PCB trace as
>> an antenna. The bitrate is quite high (approx 250kbit) and we decided
>> to use the 433MHz band because it has little restrictions. The main
>> problem is the antenna. I managed to put a 115mm (4530 mil) long
>> trace (1/4 labda monopole) onto the 50mmx80mm (2" x 3.15") board to
>> form an L shaped antenna. Its fed from a 50 Ohm transmission line
>> which runs over a reasonable big ground plane (top and bottom stitched
>> ). So far I was able to gather some info from applications notes and
>> so on.
>> Ofcourse there is more on the board than just the antenna although I
>> made sure the antenna runs far away from the dense populated areas.
>>
>> Now the real problem is going to get the antenna tuned. As far as I
>> can see that takes two steps: getting the antenna to resonate at the
>> desired frequency and matching the impedance. The gear I have
>> available is a spectrum analyzer, an oscilloscope, a directional
>> coupler and an HF generator.
>> I know a vector network analyzer would be the right tool especially
>> for determining the mismatch, but I'm wondering if I could do without.
>> If not, I've found this kit.
>> http://www.sdr-kits.net/
>>
>> Looks nice and affordable any comments?
>>
>
> Since it was designed by a ham it's probably alright. But you don't
> necessarily need one for this job. Resonance can be achieved by trial
> and error, probably you are going to add inductors until the antenna has
> the correct electrical length. For matching you might want to borrow a
> UHF-wattmeter or SWR-bridge from a ham operator. But it must work on the
> 70cm band which not all of them do. If he has a dipmeter that would be
> nice, too, makes finding resonance a breeze.
>
Or build a Monimatch into the board, with the various passives off-board
if he wants to be cheap.

That way when Rev B rolls around and the antenna loses tuning because of
the different stuff close to it, he can tune it again.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: Baron on 30 Mar 2010 16:57
Nico Coesel Inscribed thus:

> Every now and then you find something new on your path...
>
> I'm working on a wireless device which is going to use a PCB trace as
> an antenna. The bitrate is quite high (approx 250kbit) and we decided
> to use the 433MHz band because it has little restrictions. The main
> problem is the antenna. I managed to put a 115mm (4530 mil) long
> trace (1/4 lambda monopole) onto the 50mmx80mm (2" x 3.15") board to
> form an L shaped antenna. Its fed from a 50 Ohm transmission line
> which runs over a reasonable big ground plane (top and bottom stitched
> ). So far I was able to gather some info from applications notes and
> so on.

I don't know what your TX is, but you may not have to stick to 50 Ohm
feed. For instance if the amplifier output impedance is 10 Ohm you
could reduce losses by not having to use matching circuits. A 1/4 wave
monopole has a low natural feed impedance.

> Ofcourse there is more on the board than just the antenna although I
> made sure the antenna runs far away from the dense populated areas.
>
> Now the real problem is going to get the antenna tuned. As far as I
> can see that takes two steps: getting the antenna to resonate at the
> desired frequency and matching the impedance.

Any length of wire will radiate a signal. Tuning a length of wire is
only a method of maximizing efficiency. Making a length of wire
resonant makes calculating the impedance at any particular point
easier.

It might help to pull a 433Mhz key fob apart and have a look at the
antenna in there. Many are just a simple loop.

> The gear I have available is a spectrum analyzer, an oscilloscope, a
> directional coupler and an HF generator.

Sounds like you have most of the gear needed.

> I know a vector network analyzer would be the right tool especially
> for determining the mismatch, but I'm wondering if I could do without.
> If not, I've found this kit.
>
> http://www.sdr-kits.net/
>
> Looks nice and affordable any comments?

As I said, I think you have most of the stuff you need.

--
Best Regards:
Baron.
From: Nico Coesel on 30 Mar 2010 17:16
Joerg <invalid(a)invalid.invalid> wrote:

>Nico Coesel wrote:
>> Every now and then you find something new on your path...
>>
>> I'm working on a wireless device which is going to use a PCB trace as
>> an antenna. The bitrate is quite high (approx 250kbit) and we decided
>> to use the 433MHz band because it has little restrictions. The main
>> problem is the antenna. I managed to put a 115mm (4530 mil) long
>> trace (1/4 labda monopole) onto the 50mmx80mm (2" x 3.15") board to
>> form an L shaped antenna. Its fed from a 50 Ohm transmission line
>> which runs over a reasonable big ground plane (top and bottom stitched
>> ). So far I was able to gather some info from applications notes and
>> so on.
>>
>> Ofcourse there is more on the board than just the antenna although I
>> made sure the antenna runs far away from the dense populated areas.
>>
>> Now the real problem is going to get the antenna tuned. As far as I
>> can see that takes two steps: getting the antenna to resonate at the
>> desired frequency and matching the impedance. The gear I have
>> available is a spectrum analyzer, an oscilloscope, a directional
>> coupler and an HF generator.
>>
>> I know a vector network analyzer would be the right tool especially
>> for determining the mismatch, but I'm wondering if I could do without.
>> If not, I've found this kit.
>>
>> http://www.sdr-kits.net/
>>
>> Looks nice and affordable any comments?
>>
>
>Since it was designed by a ham it's probably alright. But you don't
>necessarily need one for this job. Resonance can be achieved by trial
>and error, probably you are going to add inductors until the antenna has
>the correct electrical length.

Yes.

>For matching you might want to borrow a
>UHF-wattmeter or SWR-bridge from a ham operator. But it must work on the

I was planning to use the directional coupler and the spectrum
analyser (with tracking generator) to measure the reflected power
(sort of SWR). From what I understand the SWR doesn't say whether to
add a capacitor, resistor or inductor. OTOH, if the antenna is in
resonance it is supposed to be resistive. The transmission line is
supposed to be resistive as well so the most logical thing to do would
be to add a resistor.

>70cm band which not all of them do. If he has a dipmeter that would be
>nice, too, makes finding resonance a breeze.

Sounds like a good idea. The problem is the 'ham operator' part :-)










































--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico(a)nctdevpuntnl (punt=.)
--------------------------------------------------------------
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