|Oct 25, 2010, 06:16 PM|
DIY BiQuad 11dbi antenna tutorial!
For those who are looking to transition into long rang flights, this just might be the antenna for you. The BiQuad is a fairly simple to build 11dbi antenna that puts 99% of it's beam right where you need it - out front. No loss of flight area due to sides lobes, compact design and many mounting capabilites make this an excellent upgrade to your patch or 9dbi Yagi. The antenna gives a nice 50-55 degree beam width for you to fly in on your long distance ventures.
So, first a little explanation, then on to building it.
What exactly is a BiQuad antenna? - A BiQuad is two full wave bisquare antennas in parallel in front of a reflector 1/8 wavelength away. Without the reflector, the propagation pattern is very much like that of a dipole. The reflector drives the beam outward.
What is the gain/beam width? ~ 11dbi with a 50 degree beam
Why should I fly it? There are several advantages to the BiQuad. First and foremost, unlike the patch, this antenna is NOT ground plane dependent. While being close to the ground will actually increase the gain some, it is not necessary. It also has small side lobes in the far field, and thus you can fly in close without flying into a null. There is, however, still a null directly overhead just like a dipole would have.
How difficult is it to build? – This antenna is fairly good when it comes to tolerances, so precision isn’t a large contributor like many other high gain antennas. However getting the “bow tie” straight is a bit tedious. Also, unbraiding and soldering the coaxial shield is tedious, but not difficult. If you are somewhat handy, this antenna shouldn’t be a problem.
Below are the radiation patterns and pictures of the BiQuad antenna.
Ready to build? Read on
|Oct 25, 2010, 06:18 PM|
Building the antenna - Materials needed
You need the following materials:
1 sheet of sheet metal (thickness doesn’t matter much. I used 26 gauge galvanized steel)
2 nylon bolts (I used Ľ”bolts)
1 coaxial cable (I used RG58/U)
1 length of solid copper wire (I used 14 AWG)
That's it. There's not much to this one!
|Oct 25, 2010, 06:21 PM|
Building the antenna - Making the calculations
Since this is a closed loop resonator and I'm going to assume that the pilot is not flying in the rain, there will be no degradation factor. I use a direct speed of light calculation. There are 3 calculations you need:
Full wave length:
Full wavelength in inches = 11732/f in MHz
Full wavelength in cm = 29800/f in MHz
For the following frequencies you have:
910 MHz = 12.89”/32.75cm
1280MHz = 9.17”/23.28cm
2450MHz (2.4GHz) = 4.79”/12.16cm
Section length (this is simply your full wave length divided by 4):
Section length in inches = 2933/f in MHz
Section length in cm = 7450/f in MHz
For the following frequencies, the section lengths are as follows:
910 MHz = 3.22”/8.18cm
1280 MHz = 2.29”/5.82cm
2.4GHz = 1.197”/3.041cm
Element spacing (this is your full wavelength/8):
Element spacing in inches = 1467/f in MHz
Element spacing in cm = 3725/f in MHz
For the following frequencies, your element spacing is as follows:
910 MHz = 1.61”/4.09cm
1280 MHz = 1.15”/2.91cm
2.4GHz = .6”/1.52cm
That wasn't so hard was it? Now write down those numbers and on to the next step.
|Oct 25, 2010, 06:26 PM|
Building the antenna - The driven element and reflector
The resonator (which I will call the bowtie) is made out of a single wire cut exactly twice your full wavelength. Since I am making mine for 910 MHz, my wire is 25.79” long. I am using 14AWG wire for this. 12 and 16 AWG also work well.
The first thing I did was mark the element at every section length for a total of 7 marks. These will be the bending points. Your bends should be sharp and 90 degrees, but being off a small margin won’t hurt. I first bent mine in a “W” shape. Then I bent the remaining points into the “bowtie”.
The reflector should be square cut with sides of your full wavelength measurement. The reflector size is not critical, so do not worry about being off by a small margin on its size. Since mine is made for 910 MHz, my element is 12 – 7/8” square
Now you need to drill a hole in the middle of it to feed your coaxial line through. You want to keep this hole as close to the outer diameter of your coaxial cable as possible.
Now would be a good time to straighten your bowtie. Once done, place your active element with the center of the bowtie over the hole making sure it is square with the plate. Mark the edges of the bowtie and drill holes for your nylon bolts. Place one nut on each nylon bolt as a stopper. The top of the bolt to the bottom of the nut should be your element spacing measurement. For mine at 910MHz this comes out to be 1.61”. Insert the bolts and secure them with a nut on the back side of the plate.
|Oct 25, 2010, 06:33 PM|
Building the antenna - Assembly
Now you are ready for the aggravating part, assembling the antenna.
First, strip a good length off of the coaxial cable jacket. You need to strip off at least as much as your element spacing. Feed this through your reflector hole. Measure down from the base of the remaining jacket your element spacing and wrap some electrical or other tap around it there so exactly your element spacing measurement worth of jacket is protruding. If you re more skilled than I am, you might be able to fold the shield over, but if you are like me, your best bet is to fray it apart. Fold the shield ends over the jacket down to the reflector plate and solder them to the plate. I also would add a little solder to the wires to stiffen them.
Now cut all by ˝” off of the protruding active element in your cable. Strip the insulation off within 1/8” (3mm) of where the shield is folded over. Solder the center of your bow tie to this element as close to the shield as possible without making contact with it. Now solder the two wire ends to the top of the shield without letting it touch the active element of the coaxial cable.
Finally, glue the edges of the bowtie to your protruding bolts. You now have an 11dbi long range antenna!
|Oct 25, 2010, 06:35 PM|
Questions and answers
Q: How is this antenna polarized?
A: If the bowtie is across (horizontal), the polarization is vertical. ie mount it with the bowtie horizontal, if it looks like an "8", you are out of polarization.
Q: I have seen others use a copper tube to feed the coaxial line through. Why don't you do that?
A: This was meant to be as simple as possible with the lowest parts count. The last thing you want to do is have to buy a 10 foot roll of copper tubing so you can make a 1" part. If you have a small copper/brass tube available, by all means use it. It might be easier than folding over the shield.
Q: Most Biquads I have seen have the reflector sides folded in. Why doesn’t yours?
A: Folding the edges over decreases the size of the vertical side lobes and increases the forward gain very slightly(~.2db). For FPV, these side lobes are a good thing to have when coming in for a landing so the edges remain straight.
Q: Aren’t you shorting out your coaxial cable with this antenna?
A: Yes… sort of. While to DC current the element might look like a short, to microwave frequencies it is actually a load.
Q: How did you verify/optimize the antenna for 50 Ohms?
A: I finally got 4NEC2’s optimizer to work (yay!)
Q: This antenna is a bit too large for me, can I use a smaller reflector?
A: Yes. You will lose some gain by doing so, but not much. Just be sure the reflector is at least as wide as your element.
Q: The copper wire seems to get bent really easily, can I use steel wire?
A: Absolutely. In fact, I recommend it if you can solder it. Metal coat hangers work well too
Q: How should I mount this antenna? Near the ground like a patch or up in the air like a Yagi or dipole?
A: It’s up to you. Experiment with it. The biquad behaves very much like a dipole, so many people will have success with it up in the air. However having reflective ground nearby will increase vertical gain slightly and limit multipath problems at the expense of the Fresnel zone.
Q: Can I feed this antenna through the side?
A: Yes. Performance will be slightly less, but it will likely not be noticed. Just be sure not to crush the center foam core insulator in the cable with the sharp 90 degree bend in the cable.
|Oct 25, 2010, 08:52 PM|
Looks great Alex.
How would this perform just placed flat on the ground? (with the active element "up" of course)
I know.... experiment.... but do you think it would significantly alter gain or skew the pattern as compared to say... pole mount 5ft high.
Another question if I may.
What would be a good antenna design to start experimenting with for mounting on a vehicle roof? (think Dodge mini van)
Plan to build a "Flight Station" inside my van during the winter.
All external roof mounted gear (ant's and Rx) will be removable magnet mount, so no concern about wind loads.
|Oct 25, 2010, 08:58 PM|
For the van project it depends on what equipment you have. My fast patch works great if you power the RX with the power from the car battery and let the back panel rest on the roof of the car. That will actually work as an artificial ground plane and result in a wide coverage antenna with a decent near field pattern.
I would not recommend a Yagi or a Moxon rectangle. The biquad would be my choice if you have an antenna tracker. Again, place it with the reflector touching the roof of the van and power it from the van's battery.
|Oct 25, 2010, 09:11 PM|
Thanks for the quick reply.
Current video gear is 900mhz using ch.1 910mhz.
I have 2 commercially made patch antennas (DPCAV) and the Oracle. No tracker.
Plus various commercial 900mhz "ducky" antennas and di-poles. (never use 'em)
I mainly (so-far) just use the 2 patch's with the Oracle, but have not tried any great distance with that setup. (3500 ft. so-far, signal good)
Would you recommend the patch (or the Bi-Quad) to be flat on the van roof, or angled in the direction you plan to fly?
I'm guessing flat for close in flying, and angled when you want to possibly go farther out.
Edit; all GS gear powered from the van battery.
|Oct 25, 2010, 09:16 PM|
If you want longer range, go with two BiQuads. Place them 45 degrees apart for a beam of about 90 degrees. Use two patches aimed 60-70 degrees apart for a beamwith of 150 degrees. The BiQuads will have 60% greater range, but not nearly the coverage of the patches. I went two miles on my dpcav patches and had plenty of signal left.
Always aim the antenna upright with a slight tilt (about 20 degrees upward is perfect).
|Oct 25, 2010, 09:17 PM|
|Oct 25, 2010, 09:23 PM|
Looks like I will have to get busy building BiQuads....
|Oct 25, 2010, 09:46 PM|
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