Getting a 433MHz antenna going

[Nico Coesel]

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?

 

[Joerg]

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.

 

[Baron]

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.

 

[Nico Coesel]

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

 

[Joerg]

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.

A resistor will burn off power but if you have excess you can afford
that. You can use the directional coupler but since you seem to have no
cable or significant length of trace from xmit chip to antenna why
bother? As long as you don't load the xmit chip too much. Mostly they
tell you, by leaving a blister on your finger ;-)

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

Look for roofs with "strange" antennas on there

 

[oopere]

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?

Maximizing power to a fixed load is the same as maximizing voltage to
the load. If you have a spectrum analyzer you may easily probe the
signal at the antenna feed with a "high valued" series smd resistor (you
can even calculate the associated loss). Next, you can try different
matching networks and see if you can get near your transmitter
specifications. Of course, this is... time consuming.

Otoh,the poor man's vector network analyzer is the "slotted line". If
you are able to set up a "long" (>lambda/2) transmission line of known
Zo, you may record the level at some (say 10) points and compute the
reflection coefficient ro: abs(ro) from the vswr and angle (ro) from the
positions of the minimum (or minima). A matching network is then
straightforward. Comment: there are (better) alternatives to resonate
(first) and match (later), such as matching the real part of R o G
(first) and the cancelling jX or jB.

An alternative to the kit would be to try to reproduce this
http://saturn.uni-mb.si/~dogsa/ne_old/gradivo/Low Frequency Circulator-Isolator.pdf
with "better" devices.

Pere

 

[Joerg]

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

 

[Nico Coesel]

Yes, that will work.


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.


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.

 

[Joerg]

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

Yep, I was going to say. If you do RF stuff the ARRL Handbook and the
ARRL Antenna Handbook are must-haves. They aren't nearly as expensive as
scientific books but contain serious hands-on information. That's how I
really learned RF stuff, not at the university.

[...]

 

[Joerg]

Hello Joerg,

The design can be done with less means (you can build a 433 MHz
oscillator yourself, measure frequency with a analog frequency meter
and build a good diode detector for measuring HF power). However, when
you have the equipment, why not use it?

That's like using a gas grill just because you have it, while the manly
thing would be to make a wood fire in the open pit grill ...

Juist kidding

I give some courses on antennas. Many people don’t have a spectrum
analyzer or measuring receiver, so in that case I do the practical
exercises with a diode detector that they can build themselves for
just some Euros. Of course I discuss the quadratic to linear output
transition also. Up to some GHz a return loss bridge can also be build
with SM devices (with more then reasonable directivity).

I like to show "old methods" to show that you can also do good
measurements with primitive means.

Good man. I wish they'd do the same at universities. I remember coming
to a client with my laptop and some kludgy homebrew wideband probes.
Boss wasn't in yet but I got started. By the time he came out of a
production meeting I had the EMI problem unearthed. "We rented an
Agilent analyzer for you, top of the line, it's in my office" ... "Ahm,
I don't think we'll need it anymore, found the problem with this
detector jig here". He was almost in tears because it must have cost
them almost $1000 for a week's rental.

 

[Nico Coesel]

Hello Joerg,

Painfull for your Boss.

Such situations sound very familiar to me. Sometimes they don=92t
believe an opinion or don't take you seriously when you don't show an
(expensive) Agilent or Rohde & Schwarz cabinet with many push buttons.
It doesn't matter what it is, it just has to look impressive....

If it makes all of you feel better: the SA is a cheap Atten model
which we bought to hunt and solve EMC problems. Now it comes in handy
again. The HF generator is a stone age R&S SMDA and the scope is an
analog 470MHz Lecroy-Iwatsu.

 

[Joerg]

Wimpie wrote:

[...]

Good man. I wish they'd do the same at universities. I remember coming
to a client with my laptop and some kludgy homebrew wideband probes.
Boss wasn't in yet but I got started. By the time he came out of a
production meeting I had the EMI problem unearthed. "We rented an
Agilent analyzer for you, top of the line, it's in my office" ... "Ahm,
I don't think we'll need it anymore, found the problem with this
detector jig here". He was almost in tears because it must have cost
them almost $1000 for a week's rental.

Hello Joerg,

Painfull for your Boss.

Such situations sound very familiar to me. Sometimes they don’t
believe an opinion or don't take you seriously when you don't show an
(expensive) Agilent or Rohde & Schwarz cabinet with many push buttons.
It doesn't matter what it is, it just has to look impressive....

It was a client so he was my boss only for one day

With some clients the faith in expensive name-brand lab equipment has
been shaken. For example at one where I waltzed in armed with a laptop
and a piece of wire with a 3.5mm phono jack. We used one of their safety
transformers for audio line isolation and via the sound card found a
noise peak that a $10k Stanfrod Research analyzer was unable to see. At
first they wanted to discourage me. "But we've already looked with a
high-end audio analyzer". After unexpectedly seeing the peak on my
laptop screen some expletives slipped out ...

Finding the source of it was then only a matter of minutes.

 

[Joerg]

So let's see... I can go into business selling $10k test equipment with
so-so performance or become a consultant and have to be successful using
$10 improvised sensors and a laptop.

I know which business I'm going into!

Correction: It was a snipped off cable with molded phono jack on there,
salvaged from an old stereo tower. So it was $0

Ok, the laptop is a ruggedized one, not so cheap.