After many have had huge success with my TX antenna tutorial I have been experimenting with directional antennas. So by request I am finally writing a DIY Yagi antenna tutorial. In this tutorial I will discuss a little bit of theory behind the Yagi and then go into the design and construction. For this tutorial, I built 3 Yagi’s for 1.3 GHz since it is quite difficult to find a 1.3 GHz Yagi off the shelf and 1.3 GHz seems to be gaining popularity.

The three antennas discussed herein are:
4 element - 9 dbi Yag
3 element – 7 dbi Yagi
2 element - 5.5 dbi Moxon Rectangle

To make any of these antennas at a different frequency you multiply ALL of the dimensions by the formula:
Multiplication factor = 1280/frequency needed

-Alex

A Yagi antenna is simply a dipole with added parasitic elements. These elements are known as the reflector (in the rear) and directors (in the front). The dipole is the second element from the rear and is also known as the driven element. There can be anywhere from 2 to an unlimited number of elements.

The parasitic elements are placed in such a location that they reflect the signal between themselves properly in phase. The addition of more elements increases the overall gain and compresses the signal. This also increases the F/B (front to back) ratio which is the difference in reception to the front versus the rear of the antenna.

Adding more elements has two main difficulties: First is that the addition of elements lowers the impedance of the antenna and thus high gain antennas usually need a tuner or balun to operate well. Second is that the addition of elements makes their spacing and size increasingly critical. It is not hard for a Yagi to quickly become a dummy load. Thus it is my recommendation that you limit your Yagi to 5 or 6 elements at the most.

-Alex

The driven element of a Yagi is a basic dipole. To construct this you may follow my basic antenna tutorial. Your dipole total height should be:

Dipole height in inches = 5616/f in MHZ
Or
Dipole height in centimeters = 14250/f in MHz

Both elements should be equal length with feed points not more than Ό” apart. I use 12 AWG solid copper wire as it is plenty stiff and solders well. I feed it with 50 ohm coaxial cable, usually RG174 or RG316.

For 1280 MHz (1.3 GHZ band) this is 4-1/4” total height or 10.81cm

-Alex

I have found the best results for a homebrew Yagi to be using an insulator as the boom and copper or steel wire as the elements. Balsa wood, and epoxy mixers work very well. All elements should be centered on your boom and their location measured from a single point to eliminate error. Thus I will call the Reflector position 0 and all measurements will be taken from there.

For the 4 element (9 dbi) Yagi the measurements (in inches) are as follows:
Reflector = 4.44” @ position = 0
Driven element = 4.41” @ position 1.85”
Director #1 = 4.10” @ position 2.5”
Director 2 = 3.76” @ position 4.5”

The 3 element (7 dbi) Yagi measurements (in inches) are as follows:
Reflector: 4.48” @ position = 0
Driven Element: 4.25” @ position = 2.32”
Director: 3.7” @ position = 4.21”

Notice the elements are spaced differently and are different sizes. Even the driven element is slightly different. This is because Yagis are a dynamic antenna. Adding or removing elements often requires a complete redesign and build for optimal performance.

Alternatively, you can design your own Yagi using 4nec2: http://home.ict.nl/~arivoors/ or The WK5DJ Yagi calculator: http://vk5dj.mountgambier.org/Yagi/Yagi.html The 4NEC2 is more versatile and more accurate for Yagis of 4 elements or less, but I cannot seem to get it to work for some reason so there are no screen shots

-Alex

The Moxon rectangle is essentially a 2 element Yagi with the elements bent to direct the signal forward. However the rectangle has a very high F/B ratio of about 25. Unlike the Yagi which has deep nulls to the sides, the Rectangle has it’s nulls to the rear. This makes for an excellent FPV antenna. It has a good frontal sweep of nearly 180 degrees thus allowing you to fly to the sides of the antenna without excessive degradation of signal.

I recommend building this antenna on an insulating plate such as a scrap of balsa wood or plastic. I built mine using the MOXGen program available here: http://www.moxonantennaproject.com/links.htm

-Alex

Alex,
Thanks for this info! Gonna build one!
Why do you want a Horizontal Polarization on the MOXON antenna?

You don't. You want vertical polarization. It's just a bent Yagi. The feedpoint should be in the front. The program just rotates the picture 90 degrees.

Originally Posted by IBCrazy

You don't. You want vertical polarization. It's just a bent Yagi. The feedpoint should be in the front. The program just rotates the picture 90 degrees.

OK, it was just that the picture of the program has the polrization as horizontal. That's why I asked.

Thanks again.

After some testing I have determined that I like the Moxon the best. Eventhough it's the hardest to construct and has the shortest overall range, the coverage is incredible. Range is about 60% greater than an omni antenna yet isn't blind to the sides like a Yagi. Just don't fly too far behind it.

If you want range, build the 4 element Yagi. Otherwise go with the Moxon.

-Alex

how about directly overhead?

Directly overhead the Moxon performs a little better than the Yagi so long as you don't fly too far behind yourself. If you plan on flying far behind yourself, you want the 3 element Yagi. All three of these antennas have done well for me. I like the Moxon the most, but the 3 element Yagi is a close second. Most people will like those antennas a lot.

-Alex

you are a wizard !

The FPV community owes you so much ! . the idea of sticking the yagi on the trimersion is ACE

I bow in front of your knowledge and ingenuity ! And I thank you !


now... where are those copper wires and balsa ?

I have Les Moxon's (G6XN) book 'HF Antennas For All Locations' and it covers these and many other designs, with a lot of technical and thought provoking theory. Some of it is heavy reading but excellent nonetheless.

Nigel.

IBCrazy, thanks for another great thread!

Would it be possible to make this antennas directly on PCB?
I'm thinking about a 5.8Ghz Moxen style antenna here, it would be very compact and simple.