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hi, could any1 give me some advice about directional antennas. What type of antenna would give the narrowest beam? Im using 433mhz rf at a distance of about 8meters. Would it be possible to make my own by putting an antenna in a reflective box with a slot in it? Thanks
Directional antennas typically tradeoff size for directionality. In addition, most good antenna's sizes are scaled as a function of frequency. In other words, the lower the frequency the larger the antenna.
There are several ways to get directionality at 433 MHz. One way is to make a Yagi-Uda beam antenna (usually just called a Yagi) which is a common type. Here is a link to manufacturer of several types. **broken link removed**
In this chart you will find some Yagi models and you will see that their directionality gets better as you add more elements. For example, a 3 element yagi provides about 6.5 dB of directionality. This is still a fairly broad pattern. Even though its performance is modest, its size is still 16 x 14 x 5 inches. For better performance in a yagi, you have to add elements and you can only take it so far. For example, they offer an antenna that gives 15 dB of gain, but to get this, they put four antennas together in an array with a total size of 57 x 47 x 39 inches.
Another type worth considering is the corner reflector. They can build one that gives you 12 dB gain with a size of 30 x 23 x 50 inches. Another type is a semi-parabolic reflector which gives 15 dB of gain but its 46 x 25 x 88 inches. Such an antenna has a very narrow beam, about 18 degrees wide.
These commercial designs will give you an idea of what is possible. Since your range is only 8 meters, which is very little by the way, it will be hard to justify such a large antenna. We usually don't try to get directionality when sending RF such short distances. Why do you want directionality?
Making antennas is something of a precise art. Without the instruments necessary to fine tune a design, you pretty much have to stick to either simple conventional antennas using cookbook formulas, or specific recipes for more complex antennas using dimensions published in a magazine article or an online article. Often it is possible to scale a design for another frequency by multiplying all dimensions by the inverse of the frequency (lower frequency means larger dimensions).
Unfortunately, I do not have a specific site to point to that might offer some hope for a simple reflector box design, and certainely not something that is really small. Such a reflector box will likely work if it is, say, 60 to 100 cm on a side (thats about 24 to 39 inches roughly). It would be easiest to make it a lot like the so-called corner reflector, where you really only need two flat surfaces behind a dipole, the surfaces being joined along one edge at right angles.
Thanks RadioRon, that is exactly the type of help i was hoping for. Im building a short range position sensor so i need to have a directional antenna rotating to work out a bearing for a transmitter. Thanks again
I believe I'm somehow familiar with that threat. I'd prefer 915MHz instead of 433 though. Makes the antenna smaller and easier to rotate.
Let me guess. Isn't it planned for a basket ball field?
Anyway, here are some numbers to figure the correct values of the antenna. SL(mm)=75400/f(MHz), RL(mm)=152250/f(MHz), RA(mm)=40000/f(MHz) and SA(mm)=75400/f(MHz)
For a better directional pattern stack more reflectors (green) at a distance of RA-3%
You might take a look at amateur radio publications as one possible source of information. Parabolic reflectors and similar antennas are likely to provide the narrowest beam.
Unfortunately much of the best information on many subjects is in a form that must be purchased (or borrowed from a library).
I was going to mention a yagi-uda with N elements or a log perioidic antenna. What type of gain do you have in mind? also is there any specific HPBW you'd have in mind? You can control and fine tune your directionality with multiple parasitic elements, up to six would provide a sharp enough beam. more would only change the front-back ratio a little.
Your wavelength will be .69 meters approximately. Usually, the reflector is the longest part of the dipole array or the yagi. It can vary between .5 to .6 lambda.
The near field field range is about 2*D^2/lambda. where D is the antennas largest aperture diameter, in this case the reflector. You near field range would then be .42 m. 8 meters is clearly outside the near field range. I see no reason why he couldnt use the antenna, so long as he sets up his measurements properly.
I used 144 mhz, 216 mhz and 900 mhz yagis perfectly fine.
OP, you can buy most of the parts at menards or so. you probably can find some connectors( n types or bnc) at radio hamfests really cheap. you can also find VSWR meters, spectrum analyzers there too, maybe an rf signal generator.
The ARRL handbook has great texts and recipes for homebrew directional antennas. i'd start there.
YUP. Great Rf engineering textbook that anyone can handle.
But for professional antennas, I use antenna theory: design and analysis by constantin balanis 3rd edition and Antenna Engineering handbook by john volakis 4th edition.
If its a specific antenna, i'll use a book just on that type.
The OP is looking for a narrow beamwidth, how narrow is not specified.
However the narrower the beamwidth, the larger the antenna.
For a compact solution, why not just use a simple antenna, like a dipole and use the null response off the end of the antenna?
The null is far sharper than the peak response, which is what the OP was looking for.
On second thoughts...
I had made the assumption that this was for a position location system, but that is not stated in the original problem. If the idea is to select one of several transmitters using a beam antenna, the null idea wont work.
Need more information.
what impedance does it have to have? i'm assuming 50? if so i made a nice little yagi-uda with a j-pole for the driven element. it worked very well. it was made for a classroom experiment , it did the best out of the class.
I believe that the best way to do this is with a pseudo-Doppler antenna array. For an all-band example, see https://www.highfrequencyelectronics.com/Archives/Apr07/HFE0407_Jahoda.pdf. You can buy a unit (not practical for a robot) at **broken link removed** Get more useful results from Google: phased array direction finding antenna
Basically the idea is if you spin a short vertical antenna in a circle, its movement will induce a Doppler shift in the received signal. This will tell you where the target is (when the frequency is maximum, the antenna is heading straight towards the transmitter. ) But it's not necessary to physically spin an antenna fast enough. Fortunately you can switch an array of 4 or more antennas and get the same result. (Actually you 'switch' them with a sine wave).
Practical construction: Four short (less than 1/4 wave) antennas placed on the circumference of a small circle. Bring the 4 signals to the center, with each one having a MOSFET (2N7000?) in series to the receiver feed line. Generate a 1kHz sine wave, in 0, 90, 180, 270 degrees (op amp circuits are available, or use your MCU.) Apply each phase to one transistor.
You should be able to use a simple FM receiver. At maximum positive swing of the 1kHz audio waveform, the tangent at the antenna which is active is pointing to the transmitter.
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