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Thread: DIY antennas for your UHF system

  1. #1
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    DIY antennas for your UHF system

    In this tutorial SecretSpy711 and I will be showing you how to make a few simple DIY antennas for your UHF control as well as discussing the benefits and downsides to their use. These antennas can be made with minimal soldering skill and most FPV pilots will find building their own antennas very rewarding. In most cases, a DIY antenna will outperform most commercially available antennas and they generally cost less than $4 to make.

    The antennas we will be explaining are:

    Half-wave dipole
    1/4 wave monopole
    Shark Tail
    Moxon rectangle

    We will be happy to answer any questions you may have. Don't be afraid to try your own. You will be surprised how easy it can be.

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    [url]videoaerialsystems.com[/url] - Performance video piloting

  2. #2
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    The basic half wave dipole

    The half wave dipole is a very basic antenna that is the base for which every linear antenna is compared. They are cheap to build, easy to use, and very efficient.

    Time to build: 15-20 minutes.

    The benefits of a dipole:

    Nearly 100% efficient
    Gain - 2.15dbi
    Cheap and easy to build

    Downside of a dipole:
    Antenna is 12.5" long (310mm)

    Parts needed:
    Coaxial cable jumper
    2 lengths of 18-24AWG wire ~6.25" long
    Scrap cardboard 13" long or more

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    Start by prepping your coaxial cable. In this tutorial I am using a cut RG316 jumper I got from EBAY. I just type "RG316 SMA male jumper" into the search engine and hundereds of jumpers of various lengths came up. Choose whatever length suits you. Any length less than 6 feet long will do.

    To prep the cable, remove ~1" of the outer jacket from the cable. The 16/18AWG setting on my wire strippers worked perfectly.
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    Lightly tin the exposed shield with solder. Once cooled off, use a set of wire strippers to break the exposed shield in the middle without removing the insulation around the center conductor. This keeps the shield from fraying when cutting it. I found the 18.20AWG setting made a nice cut here.

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    Remove the insulation from the center conductor leaving between 1/16"-1/8" center conductor exposed. Finally, tin the center conductor and bend it 90 degrees.

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    Next, prepare your wires. In this I am using .035" Mig welding wire for the active element and 24AWG copper for the counterpoise. It really doesn't matter which wire is which. I am using the welding wire because it is stiff and using the copper wire on the other end because it is soft (so I can land on it and it will fold up). Solder your active element (in my case MIG welding wire) to the exposed center conductor.
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    Strip ~1/2" of insulation from the wire you intend to use as a counterpoise and bend the exposed part 90 degrees. Solder this wire to the shield right at the top of the cable.

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    The next step is to tune the dipole. I find it is easiest to find a scrap piece of cardboard or other material to make an easy measurement jig. Make 3 parallel lines on the board each 6.25" (158mm) apart. Place the center of the antenna on the center line. Using a pair of diagonals, simply trim the elements at the lines on the jig.

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    Finally you will want to protect the feed point. You may use epoxy, hot glue, or heatshrink tubing. I am using black hot glue as it is simple to use. If using hot glue, do not use cheap hot glue as it will crack. Use a good industrial grade glue that is non-conductive. Lightly coat the antenna feedpoint for strength.

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    Last edited by IBCrazy; 12th March 2014 at 11:11 AM.
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    [url]videoaerialsystems.com[/url] - Performance video piloting

  3. #3
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    1/4 wave monopole

    The 1/4 wave monopole was made popular by Sander from ImmersionRC. While not nearly as efficient as a dipole, the compactness of the antenna makes it the go to for many FPV pilots:

    Benefits of a monopole:
    Compact
    Very simple

    downsides of a monopole:
    Low efficiency (~60% or so)
    Can introduce noise into your system as it uses system ground for the counterpoise.

    I make my monopole different from Sander's style as I find it very difficult to get the soldering iron tip inside the right angle connector to make a neat joint. While his method is stronger, my method is easier. Like the dipole I use a jumper from EBAY. This time I am using a jumper with 90 degree SMA connectors on it. I found the shortest jumpers available (15cm long) and simply cut them about 1" from the end of the SMA.

    Preparation is simple: remove the outer jacket of the exposed cable. Tin up the shield. Using a pair of wire strippers, cut the shield and remove it. Then cut the insulation from the center conductor leaving 1/16-1/8" of insulation remaining.

    Solder your wire to the end of the exposed cable. you will want to somehow protect this joint with an antenna tube, hot glue, or heat shrink tubing. To tune it, simply use a tape measure. Measure from the bottom of the SMA and trim the antenna between 6.125 and 6.25".

    You can turn this into a dipole by soldering a 6.25" length of wire to the side of the SMA as well

    -Alex
    Last edited by IBCrazy; 11th March 2014 at 09:22 PM.
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    [url]videoaerialsystems.com[/url] - Performance video piloting

  4. #4
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    SharkTail

    You could think of the SharkTail as a coil-loaded dipole taken to extremes. It has a radiation pattern identical to a dipole, but is much more compact. I designed it specifically for micro-quads, but it may find other uses.



    Benefits of a SharkTail
    • 1/3rd the size of a half-wave dipole.
    • Roughly 1/2 the size of a monopole
    • Less susceptible to detuning compared to a monopole, and not dependent on the groundplane of whatever it's connected to.
    • Places the elements further away off the back of your micro-quad.


    Downsides of a SharkTail
    • Slightly less range than a half-wave dipole
    • More complex to make than a dipole or monopole.


    It is somewhat narrow-band, so be careful when measuring and cutting. This fact helps it reject interference, but also means you need to be accurate when making it.

    Materials:

    1. A 50-ohm SMA coax pigtail
    2. A length of 50-ohm coax for the tuning stub. You’ll need about 55mm so you can strip it and trim it accurately later.
    3. .035” carbon-steel welding wire. You’ll need about 28 inches (~700mm). Again, we will be trimming this accurately later. Using a wire with a different diameter will affect the number of turns you need.
    4. Something to use as a mandrel to coil the wire. A hobby-knife handle chucked into your drill works great. My two hobby knife handles are 7.88 and 8.04 mm diameter. A 5/16-inch or 8.0mm dowel from the hardware store would work too.
    5. Epoxy for encapsulating and reinforcing the feedpoint.




    Making the antenna:

    Tightly coil the welding wire around the mandrel. It doesn’t matter which direction, this is a linearly-polarized antenna.



    After coiling, gently let it spring out (careful, the end of the wire is sharp.)



    Pull the coils out to 6mm spacing between turns. Getting the spacing exactly right is not critical, except the spacing very close to the feedpoint can change the center frequency. You can cut it to 17 turns now, or leave it at about 20 turns and trim it accurately later. I prefer to trim later. At this point the coil diameter should be 11.2 – 11.5 mm measured with calipers.



    Strip the coax to expose ~6mm shield, ~3mm dielectric, and 2-3mm center conductor. Tin the shield. (You may get better results by tinning the shield first, then scoring it and breaking off the end to expose the dielectric.)



    Stick the coax into the middle of the coil so that the shield and center conductor contact the welding wire, 180 degrees apart. Solder both points.



    Cut off the turn of wire connecting the top coil to the bottom coil. The bottom coil will now be entirely supported by the solder attaching it to the center conductor until we reinforce the feedpoint later.




    The matching stub:

    Using a length of 50-ohm coax, strip one end, leaving about 2 mm of dielectric and about 2 mm center conductor. Twist the shield into a tail.



    Position the stub on the antenna, center-to-center and shield-to-shield and solder it on. Careful not to hold the soldering iron on too long near the center conductor, otherwise the coil may fall off.


    Trim the stub to 45mm from the tail. This is an OPEN stub -- do not short the end.



    Trim both coil elements to 8 turns counting from where they are soldered. If you want to cover the elements with shrink tube, trim to 8 1/4 turns. Encapsulate the feed point with epoxy, ensuring that the first half-turn or so is supported.



    If you have an SWR meter, or if you want to tune by range testing and you find that you cut your elements slightly too short (center frequency is slightly too high), you can squish together the first couple of turns near the feedpoint a little bit to shift the center frequency lower. Pulling them apart will shift the center frequency slightly higher.

    Try it and let me know what you think! I just ask that you not produce these for commercial sale.
    Last edited by SecretSpy711; 14th March 2014 at 02:22 PM.
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  5. #5
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    Moxon rectangle

    Reserved for Moxon Rectangle UHF.
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    [url]videoaerialsystems.com[/url] - Performance video piloting

  6. #6
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    alright, now I need the swr meter for UHF, can anyone recommend the good low-cost swr meter?, brand and model please, Thanks

  7. #7
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    ^ I never found an SWR meter for the 433 band that I liked. They all were really terrible. However, I find that if you follow the tutorial closely there is no need for an SWR meter. These are really easy to build correctly and tune.

    You can always make your own DIY SWR meter. There's a thread on it here somewhere.

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    [url]videoaerialsystems.com[/url] - Performance video piloting

  8. #8
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    How do you guys come up with your measurements? At 433 MHz, 1/4 wavelength is 173mm but you guys have 158mm and Sander has 164mm for the length of each element. Is it trial and error or are you compensating for something?

  9. #9
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    You have to account for the speed of radio waves through a wire, which is different than in a vacuum. Sander's measurement was determined by trial and error and coming up with a single number that is best for a wide range of plastic tubes that are used to keep the wire straight. Some plastics detune the antenna more than others and so you account for it by changing the length slightly.
    Last edited by SecretSpy711; 12th March 2014 at 01:14 PM.
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  10. #10
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    what gauge coax are you using on the sharktail and the tuning stub, and how much does it matter?
    [B]NEF to the P to the V[/B]

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