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Old Nov 04, 2010, 03:43 PM
Engineer for Christ
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Amherst, VA
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Mini-HowTo
DIY Ground Plane omni directional RX antenna

One of the largest problems I have had with FPV is finding a good omni directional receiver solution. Dipoles work ok, but almost half of the radiation pattern is into the ground and there is a huge hole above it. One solution to this is the ground plane antenna.

The ground plane antenna has multiple grounded elements that effectively drive the propagation pattern back upward. This actually increases the effective gain of the antenna. Range will increase slightly, but the real advantage is signal strength improvement at altitude.

While I find it more useful on the RX side (preferring the Vee on the TX), some may find this an excellent antenna for ground vehicles. Thomas Scherrer flies this antenna on his FPV plane mounted upside down. This is a great solution so long as you don’t need to bank hard.

The construction is similar to a Dipole. It has a single ¼ wave driven element and a counterpoise. However the counterpoise is very different this time. It can have anywhere from 3 to 8 ¼ wave elements. For this tutorial, I will use 4. Thomas Scherrer’s has 3. For this, I am using 14AWG wire to make an antenna for 910 MHz. You can use just about any gauge wire you like. I prefer 12 and 14 AWG because it is stiff.


With that, I will begin the tutorial.

-Alex
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Old Nov 04, 2010, 03:45 PM
Engineer for Christ
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Amherst, VA
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The calculations: Your 1/4 wavelength

All you need is one calculation:
Length in inches = 2808/f in MHz
Or for you metric folks:
Length in mm = 71323/f in MHz

So for 910 MHz, this is 3.086” or 78.3mm
For 1280MHz – 2.19” or 55.7mm
Easy as that!

Now we move on to the construction.

-Alex
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Old Nov 04, 2010, 03:51 PM
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Construction: Substrate type design

There are two methods to building this. One uses a protoboard as a substrate, the other simply uses the coaxial cable. I will make both types.

Substrate type:


I find this a little less time consuming and a little easier to get the geometry correct.

Start by cutting 5 pieces of wire to your calculated ¼ wavelength and strip and tin the ends.

Now strip about 1” of the jacket from the coaxial cable and fold back the shield.

Solder 4 of your ¼ wave wires in an “X” pattern to your protoboard. I actually made one element by measuring it to twice the ¼ wavelength and removed the insulation in the middle of the wire.

Drill a hole in the substrate as close to the “X” center as possible just barely large enough to pass the coaxial line’s center conductor through. Now solder the shield to the “X”

Strip the insulation off of the center conductor as close to the boards as possible. Solder the last ¼ wavelength element to the side of the protruding coaxial line so the wire rests directly over top of the center of the grounded “X”

Finally, measure all of the elements to be sure they are exactly your ¼ wave measurement from the center of the antenna and trim if necessary.

-Alex
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Old Nov 04, 2010, 03:55 PM
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Construction: Coaxial cable type

Coaxial type:

For this type you need to cut 3 or 4 pieces (depending on how many grounded elements you want) of wire to your ¼ wavelength. Thomas uses 3. I will use 4.

Strip ¼ wavelength of the jacket from your coaxial cable and fold back the shield. You should have exactly ¼ wavelength worth of center conductor exposed.

Trim your folded over shield to about 1” (25mm) or so and twist it up into 3 or 4 strands 90 degrees (or 120 degrees for 3 conductors) apart. Tin those strands with solder to keep them stiff.

Now solder your ¼ wavelength wires to these strands so that the base of the wire butts into the coaxial cable.

Measure all of your elements being sure each is exactly ¼ wavelength from the center of the antenna. Trim if necessary.

-Alex
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Old Nov 04, 2010, 03:56 PM
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Setting up the antenna

Adjusting the antenna:

While many of these usually have the radiator 90 degrees from the ground plane, in many cases a slightly greater angle is desirable. As the angle opens up, the gain drops and directs the pattern more outward. I find that 100-120 degrees is most desirable. This allows me to have the antenna mounted on a not flat surface as well as gives me a bit better low altitude resolution.

If you have a Quadcopter, this antenna will do very well if mounted upsidedown with a 120 degree angle on the elements.

-Alex
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Old Nov 04, 2010, 03:57 PM
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Q&a

How High should I mount it? - 1 to 2 meters off the ground is perfect.
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Old Nov 04, 2010, 04:02 PM
When in doubt, throttle out...
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Wow - something else to build - im liking the strength improvement at altitude bit.

n1 again Alex.
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Old Nov 04, 2010, 04:12 PM
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Thanks Alex, your tutorials are great !
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Old Nov 04, 2010, 05:45 PM
fly it high as possible
Falls Church, VA
Joined Jun 2009
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Quote:
Originally Posted by IBCrazy View Post
All you need is one calculation:
Length in inches = 2808/f in MHz
Or for you metric folks:
Length in mm = 71323/f in MHz

So for 910 MHz, this is 3.086” or 78.3mm
For 1280MHz – 2.19” or 55.7mm
Easy as that!

Now we move on to the construction.

-Alex
Hi Alex,

just a couple of questions:

- How high you gonna lift this antenna?

- What kind of coaxsial cable do you use - 50 or 75?

Thanks.
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Old Nov 04, 2010, 05:51 PM
Engineer for Christ
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I use RG58/U 50 Ohm coaxial cable. Any microwave rated cable will do. 50 Ohm is a little better than 75 Ohm.

Put this antenna up in the air about 1-2 meters. I mount it right on my head

-Alex
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Old Nov 04, 2010, 06:10 PM
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Nice work again Alex top man
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Old Nov 11, 2010, 11:09 AM
My name is Tom
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Italy, Emilia-Romagna, Misano Adriatico
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Alex,
When you say the antenna would be ideal for a Quad when turned on it's head, you mean exactly as is, no change to the construction, but with the coax uppermost?

How does flipping it benefit a Quad exactly?

Tom.
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Old Nov 11, 2010, 11:26 AM
Engineer for Christ
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Quote:
Originally Posted by Tommygunn View Post
Alex,
When you say the antenna would be ideal for a Quad when turned on it's head, you mean exactly as is, no change to the construction, but with the coax uppermost?

How does flipping it benefit a Quad exactly?

Tom.
Having the grounded elements up in the air directs the radiation pattern downward. This sends more of the signal where you need it, on the ground. The construction would be identical.

-Alex
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Old Nov 11, 2010, 11:47 AM
My name is Tom
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When you say that more of the signal is directed to the ground your simply assuming that I won't be flying anywhere near as high as you fixed wings fellas, which I won't be? Not that your thinking that I might only be flying a few metres off the ground

Tom.
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Old Nov 11, 2010, 01:12 PM
From trucks to planes
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Joined Sep 2005
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Very interesting. I will definitely experiment at home with this for my 1.2Ghz receiver. I use my FPV for now with my ground R/C (Summit). I didn't get as good results with stock rubber duck antenna as I expected combined with the 800 mw Vtx. I am assuming that this can be used to receive ground level transmissions if pointed toward or slightly angled toward the R/C truck?

Would it be possible to get a visual representation of the radiation pattern like in your other examples? That would be great, if just to ad to my knowledge and others.
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