Quagi Antenna for 430 MHz
Quagi antenna, as the name suggests, is the union of two types of directional antennas: Yagi and the cubical quad. As you might guess from the pictures antenna has two quad elements, the reflector and the radiator, while the other elements, all the directors, are yagi style.
The union of these two type of antennas comes into this third antenna that combines their best technical characteristics. Treating briefly the main features of the two types we can say that the quad advantage, especially at higher frequencies as in this case, is an easier supply system: the antenna is directly fed from the coaxial cable; We counter that by increasing the number of the elements we no longer have a proportionate increase in gain, also referred to the difficult to build these quad elements. As directors, therefore, we will use the Yagi configuration.
Construction notes
What follow is how I made and the material used to build my Quagi, of course, it’s only a little hint where to start for building a more personalized antenna. I usually build antennas with materials that are easy to find or already I’ve have at home.
For the boom I used a PVC pipe 40 mm diameter 1500 mm long; all the elements, including the two loops were made of aluminum rod Ø 5 mm, although the “sacred texts” indicate a lesser diameter, I had to recalibrate slightly the radiator. To isolate the loops I used bakelite tubes Ø 12 mm.
The construction is very simple: once cut the elements to co
rrect size I have inserted them in the holes made on the boom; I did the same thing for the loop: but first I’ve inserted the bakelite tubes, then I shaped loops between the holes in the bakelite tube. The reflector, once calibrated to the maximum F / B ratio, has been permanently short-circuited with a terminal for electrical connections. I did not put any insulator between the feed points of the radiator due to the intrinsic mechanical rigidity given by aliminium rod. I threaded the ends of loop and with four inox nuts I’ve blocked the two lugs used to connect the coaxial cable. The fixing of the elements was done with hot glue, you can do much better! The electrical connections were then insulated with silicone.
Measures
The measures (in cm) are about what you can find in relevant publications:
Elements length :
Reflector: 71 – Radiator: 67.3
Directors from 1st to 6th: 29.8; 29.7; 29.5; 29.4; 29.3; 29.1
Spacing:
ref-rad: 17.8; rad-1 dir: 13.3; 1st-2nd dir: 27.9; 2nd-3rd dir: 14.8; 3rd -4th dir: 22.2; 4th-5th dir: 22.2; 5th-6th dir: 22.2.
The gain of this 8 elements should be about 12 dB. The F / B is very good. The photos show my “old” Quagi, indeed this antenna has about eight years of service on the roof and has never given problems thanks to its simple mechanics.
