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> 433.92 Mhz Antenna
Satiagraha
Posted: May 23, 2008 03:34 am
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So for my motorcycle brake light, I'm transmitting over 433.92MHz ( λ = 2ft 3.22in ). After learning a little bit about basic antennas, I realized that even a λ/4 antenna is still kinda long (6.805 inches) to fit inside a motorcycle helmet, although it's probably do-able for the transmitter in the bike.

How can I make the antenna shorter or smaller to fit it more easily into a helmet? Luckily for me, the transmission distance only has to be reliable a couple of feet, maybe 2 wavelengths at most. Maybe if nothing else, I can stick a straight PCB trace or wire at λ/8=3.402 inches from the antenna pin on the chip to the ground pin on the chip (pin-to-pin distance is 3.402 inches, right? not pin to battery or anything). With any luck, the +10dBm transmitter can do that distance even with crappy radiative coupling and crappy reception on the other end. And it's not a very high-powered transmitter, so I don't think I have to worry about the loss component burning out the PA or anything.

Hints? Suggestions? Antenna shapes?



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tekwiz
Posted: May 23, 2008 03:41 am
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Helical loading of a vertical antenna. Or full length, made from self adhesive copper tape, stuck to the helmet shell. The recieve antenna length is not as critical as the transmitting antenna, & both could be made from loops, too. cool.gif


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Satiagraha
Posted: May 23, 2008 03:51 am
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QUOTE (tekwiz @ May 22, 2008 10:41 pm)
Helical loading of a vertical antenna. Or full length, made from self adhesive copper tape, stuck to the helmet shell. The recieve antenna length is not as critical as the transmitting antenna, & both could be made from loops, too. cool.gif

So if I coil up λ/4 length of wire into a helix, it'll receive the proper frequency? Won't that seriously screw up the antenna's impedance and it'll be mis-matched for the receiver? I don't have a clue how to transmatch... (yay for typical apps wink.gif )


Also, how do I design a "loop antenna"? And isn't it a wire from the antenna pin (after transmatching circuit) to PA ground?
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CWB
Posted: May 23, 2008 04:51 am
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when you wind up the wire into a coil , it starts to act as an inductor , effectively lowering the resonant point of the antenna ... trim a little bit off . you can also get one of those ultra short "ducky" antennas for use on the transmitter (pretty much all coil) ... at the freq you are running at , it will be fairly broad (relatively speaking) .

it is not so bad on uhf , but on hf (say the 80 meter amateur band) with a dipole antenna made with 10 gauge copper wire , i have seen one inch make quite a difference in the resonant frequency .

i like the loop antenna idea ... run the wire around the outer edge of the helmet .
a zig-zag pattern up through the center would look cool ... copper foil or conductive paint ?


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Satiagraha
Posted: May 23, 2008 06:45 am
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I found this PDF about 433.92MHz antennas. They mention both loop and helical (ducky) on page 9.

It gives a good description of the shape of the of the PCB loop, but it doesn't tell me how to connect the antenna. It mentions a serial tuning cap... what goes to the other end? One end is the ground plane, one is the antenna pin ("feed" they call it?)? I suppose it wouldn't matter which is which as long as one is one and the other is the other. Also, this says that with 10pF, the antenna impedance is 75 ohms but my receiver circuit is designed for a 50 ohm antenna... How do I transform this? Otherwise it'd result in a high VSWR, right?
Should you leave the space inside the loop on the board blank or can it be filled with parts? Can you have a ground plane inside? If not then I assume you can't have a ground plane on the other side of the PCB behind it either.


As for Helical... they're pretty good about the size and shape, but once again lacking in connection. I guess that's supposed to be obvious and already known? They say 15 turns of approx 28AWG wire wrapped in a 5.5mm diameter coil, then you adjust the length to tune. I assume you connect to one end of the helix and the other hangs out in the air, and you also have a perpendicular ground plane around the base of the antenna just like a normal monopole?


So many questions!

Oh, and I'm afraid painting or attaching it to the outside of the helmet wouldn't work that well as the loop is only a could of inches in circumference. Although I suppose you could make one of those jewish hats... laugh.gif
user posted image



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CWB
Posted: May 23, 2008 02:08 pm
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that beanie is known as a yarmulke (yahr-muhl-kuh or these variations : -muh-, yah-) .

transmit antenna :
those helical (ducky) antennas will work ok without much of a ground plane ... look at the cops that have the speaker/mic/antenna combo unit clipped by their shoulder .
the coax that connects the transmitter to the antenna is connected thusly : the center of the coax is connected to the center (feed point) of the ducky , the shield is connected to the ground plane (if you have one) , else wise just hook the shield to/under the nut that is used on the bnc connector (bnc is pretty standard in this type of work) mounted in the transmitter case .

receiver :
i wouldn't worry too much about the slight mismatch of the receiver antenna .
don't fill the loop and no copper behind it or too close to it ... the copper behind it will make it directional as hell and detune it . a half inch clearance around it will be good enough .
it doesn't have to be a loop ... a straight line would work with a couple of horizontal lines at the bottom of the line for the ground plane (i am thinking up the back of the helmet or from the front over the top) . you could make a dipole antenna as well (look at a drawing of one) ... the feed lines (instead of coax) would be about 3/16" apart .


ps ... why the "fear" of building one right on the helmet ? the diameter/lengths/physical requirements are fairly trivial .
i suppose you could mount a ducky on top and put a propeller on it ... laugh.gif


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tekwiz
Posted: May 23, 2008 06:32 pm
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A helically loaded antenna will have basically the same impedance as a 1/4 wave vertical. But, for your purposes, an on board loop antenna would do just fine for a transmitting antenna. One way to tune it would be to incorporate a parallel tuning cap & use the resulting tuned circuit in the oscillator for your transmitter. You can then tune it with a frequency counter. If this is not possible, a simple coaxial dipole might be better. cool.gif


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Satiagraha
Posted: May 23, 2008 06:43 pm
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Wait, aren't helical (ducky) antennas highly directional and radiate along the axis of the antenna like a gun? Or are duckies omni-directional and have nulls along the axis?
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tekwiz
Posted: May 23, 2008 06:46 pm
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QUOTE (Satiagraha @ May 23, 2008 09:43 am)
Or are duckies omni-directional and have nulls along the axis?

Yes, exactly the same as a 1/4 wave vertical.. cool.gif


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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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Satiagraha
Posted: May 23, 2008 06:53 pm
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QUOTE (tekwiz @ May 23, 2008 01:46 pm)
QUOTE (Satiagraha @ May 23, 2008 09:43 am)
Or are duckies omni-directional and have nulls along the axis?

Yes, exactly the same as a 1/4 wave vertical.. cool.gif

ah, good. In that case I guess I'll go with a small helical antenna on the transmitter in the bike. No need for BNC connectors or coax cable, as the antenna will be soldered directly onto the circuit board. I'll also cut a piece of wire to λ/4 and swap out the helical for it just to see if there's much performance difference. Size is less of a problem for the transmitter than it is for the receiver.


Now how the hell do I know how big to make the ducky????


Also, if they're omni-directional like monopoles... why do they have helical antennas like this that point at the source?



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tekwiz
Posted: May 23, 2008 06:55 pm
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That's up to you. Basically, all a helical antenna is a 1/4 wave vertical, coiled up to save space. The coiling size isn't all that critical, but the bigger it is, the more range it will have. cool.gif


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For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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Satiagraha
Posted: May 23, 2008 06:58 pm
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So I just take a λ/4 = 6.8 inch piece of wire and wrap it up around a random form and call it a day?
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tekwiz
Posted: May 23, 2008 07:01 pm
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QUOTE (Satiagraha @ May 23, 2008 09:58 am)
So I just take a λ/4 = 6.8 inch piece of wire and wrap it up around a random form and call it a day?

Yep. Non conductive form, of course. cool.gif


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For to you, kings & armies are things mighty & enduring.
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Satiagraha
Posted: May 23, 2008 07:02 pm
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Sweet


Ah, the directional helix is due to "axial-mode" operation. Normal mode shape is described in this article I found online:
"A helix will work in the normal mode when the diameter and pitch of the helix is less than 0.1 Lambda When working in this mode the radiation is from the side of the helix, and when the diameter is considerably less than 0.1? the resultant 'spring' has a radiation pattern similar to a short vertical monopole or whip."

Looks like any helix smaller than 2.7 inches in diameter and pitch will be good.



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Satiagraha
Posted: May 23, 2008 07:48 pm
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Do you suppose I could make one with a BNC to plug into an oscilloscope? Then I could FFT it and trim the transmitter antenna so that it produces the best results.
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CWB
Posted: May 23, 2008 10:29 pm
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if you get in the ballpark for resonance it will be ok . at the freq it is at the Q will be low enough to provide decent bandwidth .

if you have access to a spectrum analyzer it would be better .
you could sweep it and see how wide it is .


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Satiagraha
Posted: May 24, 2008 03:26 am
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Does the wire type matter much? Can I use standard PVC insulated hookup wire? Maybe solid conductor telephone wire? What about 10AWG solid copper wire?
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CWB
Posted: May 24, 2008 04:18 am
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10 Gauge would be bullet proof . cumbersome as hell though .
16 gauge would be fine .
18 would work too .
22 is about as small as i would go .

stick with solid ... it really doesn't matter what type of insulation is on it , although enameled wire is friendlier to work with .


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GPG
Posted: May 24, 2008 04:41 am
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Satiagraha
Posted: May 24, 2008 04:58 am
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Sounds good. I have some 2AWG solid wire, although it's not insulated. It's obviously stiff enough to space it out in the form and not need much of an insulation. I can always spray an acrylic sealant on it if I have to.


GPG, that datasheet has a great description of antennas! All the ones I've looked at didn't go into that much details. Although for the helix antenna, it says 0.5mm diameter wire, whereas the copper wire mentioned above is 10 times that diameter! Why is it ok to use 0.5mm wire? (for that matter, what effect does the guage have on the antenna?)



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CWB
Posted: May 24, 2008 05:05 am
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in theory , the smaller the gauge , the narrower the bandwidth .
an infinitely skinny wire would have infinitely narrow bandwidth and vice versa .

that is one reason AM broadcast stations use towers ... there is +- 10KC from center freq .
the large effective diameter of the tower allows flat(ter) response .
another factor is "Q" ... basically the more surface area , the lower the "Q" (for a given frequency) ... the wider the bandwidth .
for a given diameter/amount of surface area the "Q" will go up as the frequency goes down .


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tekwiz
Posted: May 24, 2008 06:59 pm
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Just about any helical antenna I have seen has wire around .5mm in diameter. Just about any wire you can see should work for your app. Higher transmitter powers require bigger wires, of course. cool.gif


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Fortuna favet fortibus.
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Satiagraha
Posted: May 25, 2008 12:14 am
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I'll start out trying the ducky, then fall back onto a λ/4 or λ/8 monopole if the ducky doesn't work out. How cool would it be if this thing turns out to reach 20m away? cool.gif


I've got a bunch of multi-conductor telephone cable which has a dozen or two solid 22AWG wires in it. I'll wrap that up about 5mm in diameter


I'll build it (wrap it... in like 5 minutes wink.gif ) soon, but I can't really test it without the transmitter circuit. I don't have an RF sig gen. I just have to wait until I get the parts in. I suppose I can hook it up to an oscilloscope and look for any 433.92MHz signals out there from other people.



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Satiagraha
Posted: May 25, 2008 05:47 am
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Something like this:
user posted image



The receiver antenna will probably be a λ/4 piece of wire that's bent into an M shape and curved around the back of my head. Is it ok to not really have any ground plane?



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tekwiz
Posted: May 25, 2008 06:45 pm
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Sort of. The rest of the circuitry will act as a ground plane, & you will get increased performance if your total circuitry is at least 1/4 wave long in any dimension. cool.gif


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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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