Getting a 433MHz antenna going

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.

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Regards, Joerg

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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
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Nico Coesel Inscribed thus:

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.

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.

Sounds like you have most of the gear needed.

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

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                     Baron.

Yes.

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.

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

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--------------------------------------------------------------

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 ;-)

Look for roofs with "strange" antennas on there :-)

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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. 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.

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.

--
Dave Platt                                    AE6EO
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[...]

I own one of these:

- they're a neat little tool for this kind of work. Personally I'd just run the trace and leave room for 3 surface-mount components in a pi- configuration for tuning - as well as an RF connector for prototyping. Then you can plug the T100 and come up with a good matching network.

Or, just try one of these:

/SP-Series-The-Splatch

Cheers,

Al

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

with "better" devices.

Pere

Try to keep the gap between the trace and the ground plane as wide as you can, especially at the end furthest away from the feed point, as this will make matching easier. As already mentioned, first trim the length and only then use matching components. Again, as already suggested, put the footprints for a pi network on the pcb so that all options are available to you.

I have used this combination many times (using a tracking generator) and it will certainly tell you when you have got it right. Knowing the direction in which to make changes is harder though without a VNA. (I now have a second hand 8753A and using this does save a lot of time.) Aim to get a return loss of better than -10dB, but if you can only achieve around -6dB you will still probably be happy with the system performance.

Don't forget that the housing, even if plastic, may affect the tuning.

A useful check after you have matched the antenna is to wave your hand around nearby while watching the reflected signal. You should see significant variations in return loss as the signal reflected from your hand is received by the antenna. If this doesn't happen, a lot of the output power may be being absorbed by matching network losses rather than being radiated usefully.

John

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

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

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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.

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.

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
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nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

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

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

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.

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                                    AE6EO
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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?

I give some courses on antennas. Many people don=92t 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.

Best regards,

Wim PA3DJS

Please don't forget to remove abc before hitting the "send" button.

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.

[...]

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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 :-)

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.

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s d

ed

t.

e

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....

Best regards,

Wim PA3DJS

Without abc, the mail address is valid.

[rest deleted]

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.

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