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From: Wimpie on 31 Mar 2010 14:37
On 30 mar, 20:11, n...(a)puntnl.niks (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?
>
> --
> Failure does not prove something is impossible, failure simply
> indicates you are not using the right tools...
> nico(a)nctdevpuntnl (punt=.)
> --------------------------------------------------------------

Hello Nico,

As your quarter wave antenna has a L-shape and runs close (that is <
0.25 lambda) to the ground from were it is fed, the resonant impedance
will be probably << 50 Ohms. It will also be narrow band, but that is
no problem for a 433 MHz SRD application. It may also reduce unwanted
emissions.

When you tune it to resonance, it will show too low impedance. So you
need to match also. Matching can be done by making the lambda/4
antenna somewhat longer and add a shunt capacitor, or making it
somewhat shorter and add a parallel inductor.

The gear you have is sufficient. You can feed your construction
(inclusive housing) with a common mode decoupled coaxial cable from a
50 Ohms source. Add a 6 dB attenuator if you have doubts about your
source.

Make a simple resistive probe (with other coaxial cable and add
resistor in series) and connect to SA. Adjust the length to minimum
reading on the SA, and then your antenna is in series (lambda/4)
resonance. From the voltage drop with respect to no antenna connected,
you can assess the resonance impedance.

From here you can change the length to add series reactance and add a
parallel reactance to transform to 50 Ohms. You can also determine the
resonance impedance with the slotted line method, but this requires
more RF experience.

When finished it is good to do some field strength measurements to get
some feeling for EIRP and radiation pattern. You can construct the
measuring antenna yourself also.

In case of not having sufficient time or RF experience, you may
consider to contact a local antenna designer

Best regards,


Wim
PA3DJS
www.tetech.nl
When your remove ABC, PM will reach my mail box.

From: Joerg on 31 Mar 2010 16:38
Wimpie wrote:
> On 30 mar, 20:11, n...(a)puntnl.niks (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?
>>
>> --
>> Failure does not prove something is impossible, failure simply
>> indicates you are not using the right tools...
>> nico(a)nctdevpuntnl (punt=.)
>> --------------------------------------------------------------
>
> Hello Nico,
>
> As your quarter wave antenna has a L-shape and runs close (that is <
> 0.25 lambda) to the ground from were it is fed, the resonant impedance
> will be probably << 50 Ohms. It will also be narrow band, but that is
> no problem for a 433 MHz SRD application. It may also reduce unwanted
> emissions.
>
> When you tune it to resonance, it will show too low impedance. So you
> need to match also. Matching can be done by making the lambda/4
> antenna somewhat longer and add a shunt capacitor, or making it
> somewhat shorter and add a parallel inductor.
>
> The gear you have is sufficient. You can feed your construction
> (inclusive housing) with a common mode decoupled coaxial cable from a
> 50 Ohms source. Add a 6 dB attenuator if you have doubts about your
> source.
>

In fact, that gear is total luxury. When I was a kid we used to tune
that stuff using a slightly modified old black&white TV set, with a
surplus analog panel meter dangling off of the AGC voltage. And that
could not be touched during operation because it was a hot chassis,
since it had to be an old set where the AGC reacted (somewhat) in the
absence of sync pulses :-)

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: Nico Coesel on 31 Mar 2010 17:23
dplatt(a)radagast.org (Dave Platt) wrote:

>In article <4bb251f3.96445937(a)news.planet.nl>,
>Nico Coesel <nico(a)puntnl.niks> wrote:
>
>>I was planning to use the directional coupler and the spectrum
>>analyser (with tracking generator) to measure the reflected power
>>(sort of SWR).
>
>Yes, that will work.
>
>> From what I understand the SWR doesn't say whether to
>>add a capacitor, resistor or inductor.
>
>Correct... a single SWR measurement cannot tell you this. Multiple
>SWR (or reflected-power) measurements at different frequencies *can*
>tell you a lot about what you need to do.
>
>You really *never* want to actually add a resistor... they're lossy.
>It's possible to change impedances without adding substantial loss...
>it's done by creating a reactive impedance transformer (e.g. an
>L-match), and that's a much better approach.
>
>> 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.
>
>That's a poor choice... it may be logical, but you won't really like
>the results. It may lower the SWR, it may raise the SWR, and
>in any case it will dissipate RF power as heat (wasting transmit
>power, and reducing receiver sensitivity).
>
>If you can adjust the frequency that you're feeding into the test
>setup, and tweak it a bit on either side of your endpoint frequency,
>you can use something along the lines of the following approach.
>You'll want to trim to resonance, and then match (probably).
>
>- Trim the antenna to something close to the correct resonant
> frequency.
>
>- Tweak the frequency back and forth to find the SWR minimum.
>
>- If the SWR minimum occurs below your design frequency, then your
> antenna is too long. Trim it, or add a few pF of capacitance in
> series with the feedpoint.
>
>- Conversely, if SWR minimum is above your desired frequency, the
> antenna is too short. Lengthen it, or add a bit of inductance in
> series with the feedpoint.
>
>Lather, rinse, repeat, until you've got the SWR minimum very close to
>the design frequency. At this point, the antenna (plus any reactance
>you added) will be resonant at the desired frequency - it will present
>a resistive impedance.
>
>The next step will be to match it. My guess is that the type of
>antenna you've specified (an "L", not far from the ground plane) is
>going to have a feedpoint impedance which is significantly below 50
>ohms. As a result you'll need to raise its effective impedance
>somewhat.
>
>The way to do this will probably be with an L-match circuit, which
>will take only two components... one inductor and one capacitor. What
>you would do is:
>
>(1) Add some reactance (either capacitive or inductive) in series with
> the antenna feedpoint - that is, between the transceiver and the
> antenna system.
>
>(2) Add some reactance of the *opposite* type (e.g. an inductor, if
> you stuck a cap in series with the antenna) between one side of
> the series reactive component, and ground... that is, in shunt.
>
> If you're matching a too-low antenna impedance (as I suspect you
> are), the added shunt reactance would go on the transceiver side
> of the series reactance. If you're matching a too-high antenna
> impedance, stick the shunt between the antenna side of the series
> reactance, and ground.

Thanks for this extensive recipe. Why isn't information like this in
books or appnotes? Its just what I was looking for. I'll try to follow
it. I'll also add a PI matching network like others advised.

>It's often possible to combine the series matching reactance, with the
>reactance that you added to tune the antenna to resonance... the
>impedances add together.
>
>The exact amounts of reactance (positive and negative) which you would
>need to add, could be calculated exactly if you knew the actual
>antenna impedance. Since you don't, you'll probably have to
>cut-and-try. They'll be somewhere in the range of zero to 50 ohms
>(reactive) at your design frequency. SMT components glues to
>toothpicks or matchsticks, and then pressed down onto pads on your
>prototype PC board, can let you iterate through multiple combinations
>of components fairly quickly.

I'll order an inductor and capacitor kit :-) I tried some evaluation
versions of simulation software but as usual simulation is only useful
if you have some idea on what the outcome should be.

>If you do everything right, what you can end up with is a
>two-component network (one reactance in series with the antenna, one
>in shunt to ground) which matches the antenna to a 50-ohm-resistive
>(or whatever you choose) feedline.
>
>If you wish (and if you have PC-board space) you can then replace the
>lumped SMT components with PC-board stripline equivalents. so that the
>matching network is actually built into the PC board itself.

PC board space is very limited. We actually did quite well by creating
a device that uses about 2/3 of the PCB space the competition requires
for the same function. And easier to assemble as well.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico(a)nctdevpuntnl (punt=.)
--------------------------------------------------------------
From: EE123 on 31 Mar 2010 19:36


Hello Nicol

You could buy a MFJ-269 from MFJ Enterprises.

It nominally does 2 - 170 MHz BUT it also does 400 - 450 MHz
(approximately)

The cost is about $400 US,
Maybe a little more for various attachments.

HTH,
Dave
From: Dave Platt on 1 Apr 2010 00:28
In article <4bb3b519.187363265(a)news.planet.nl>,
Nico Coesel <nico(a)puntnl.niks> wrote:

>Thanks for this extensive recipe. Why isn't information like this in
>books or appnotes?

Well, it's certainly in *some* books... various of the ARRL handbooks
and manuals have it.

> Its just what I was looking for. I'll try to follow
>it. I'll also add a PI matching network like others advised.

No need to do both a Pi and an L network. The L is simply a Pi, with
one of the shunt reactances omitted (i.e. infinite). You can do with
a Pi (or a "T") the same things you can do with an L. The Pi and T
are somewhat more flexible, but may be a bit lossier than an L.

If you lay out the traces for a Pi network, you can simply not
populate one of the two shunt components and you'll have an L. That
would give you the best design flexibility for the future. I imagine.

>PC board space is very limited. We actually did quite well by creating
>a device that uses about 2/3 of the PCB space the competition requires
>for the same function. And easier to assemble as well.

In that case you'll probably want to stick with lumped components... a
few cents more in parts, but less space... sounds like the right
tradeoff for your design.

--
Dave Platt <dplatt(a)radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
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