Would you like to build a simple antenna for 70 MHDiagramma radiazione 3D, antenna Quagi 70 MHz - IK1HGEz band? I sized "on the fly" this 4 elements Quagi for 4 meter band, but I did not make any prototypes, yet; anybody wants to make one?

Use aluminum pipe, diameter 10mm x 1mm thickness. You find it in DIY shops in 2 meters long pieces, or in non-ferrous metals storesin 4 meters long pieces (yeah but, how are you then going to carry it to your home?) so you'll need to make at least one or two joints.

In order to make sharp corners for the loop elements, because bending the aluminum pipe with small curvature is not that easy, I kept into account a curvature radius of 30mm (three times the tube diameter). However, if you can get even a sharper corner, that's good for your antenna!

The antenna boom must be made of wood or other non-conductive material (the boom is not shown in the pictures because, beeing insulating, it's not needed in the simulation model).

Feed your antenna in the middle of an horizzontal side of the "Driven Element" (the one

named "DE"), by creating a gap in the loop, about 1cm wide or less. I would crimp a wire terminal to the hot conductor, and one to the shield of the 50 ohm coax cable, and then I would screw  the wire terminals onto the aluminum terminals of the loop. If you know how to make better reliable connections, let me know...

This Quagi antenna is designed for resonance on 70.2MHz with purely ohmic impedence of 50 ohm "all inculsive" (that is, including radiation resitance and the small loss in the metal). You might use it without any balun, anyway I suggest to make at least a few loops with the coax cable as close as possible to the antenna feed point., or to insert a 1:1 balun with low loss on 70 MHz.

Try to be as precise as you can. Refer to the several tricks and tips about Quagi antenna homebrewing that you can easily find on the net, tipically for VHF and UHF ham bands.

Quagi 4 elementi 70 MHz - Struttura in 3DIf you make  this antenna, feel free to contact me because I would be happy help and assist you. Do it especially in case you need to use different materials than those that I considered, if the Al tube diameter is differnet than 10mm, or if you want to make fillets with a tub of different diameter.

All data and pictures shown in this page are computed with NEC2 by using 4nec2 di Arie Voors.

Enjoy antenna homebrewing!

IK1HGE (also "misknown" as AI4YG)

 

 

 

 

 

    

 

Parameter

Value

Unit (Conditions)

Frequency

70.2

MHz

Gain

10.9

dBi in the free space

SWR 1:

1.01

(70.2 MHz)

SWR (min) 1:

1.01

(70.2 MHz)

Bandwidth

69.8 - 70.55

MHz (SWR less than 1.4)

Tale off angle

10

Degrees above a good ground (Height 5.5 meters, as expected with a 6 meters mast; for better performance, place it at least at 8.5m above ground)

 

 

Quagi 4 el. for 70MHz, construction parameters. All dimensions are in meters and are approximated to 1/10 millimiter (or 100 microns), for those who want to simulate the antenna with NEC. For building the antenna, you can approximate the dimension to one millimiter. Spacing means the distance between an element and the previous one (or, in the case of the reflector RE, it is the distance from the Driven Element DE). The position is the distance always measured from the DE, positive toward the directors, negative toward the reflector RE. In case of any doubts, feel free to drop me an e-mail.

Aluminum tube diameter for building the square loops:

0.01

Aluminum directors diameter

0.01

Boom length (wood) 3.92

Element

Length

Position

 

Spacing

RE

4.5983

-1.3077

DE-RE

1.3077

DE

4.5105

0

DE-DE

0

D1

1.8605

 0.8004

DE-D1

 0.8004

D2

 1.8400

 2.5450

D1-D2

 1.7446

 

 

Diagramma di radiazione piano orizzontale, polarizzazione orizzontale - IK1HGE Quagi 70MHz 4 el.Diagramma di radiazione piano verticale (elevazione), polarizzazione orizzontale - IK1HGE Quagi 70MHz 4 el.

 

At the time when I dimensioned this antenna,  the 4 meter band in Italy is from 70.1 to 70.3 MHz thus, as you can see from the SWR and impedence charts,the antenna is centered on 70.2MHz and it has an acceptable SWR over a more than enough bandwidth. THe simulations are normally affected by a relative error of 1%. In my previous project I measured an error of  0.4%, which also includes the tolerances of the human execution. This means that, in your prototype, you must be able and ready to sligthly shorten or lengthen at least the Driven element...Cry You might make a telescopic junction somewhere, that is 20cm long tube, diameter about 8mm, inserted into the 10mm tube and tightened with a couple of self tapping screws. To calibrate the antenna, slide the tubes one into the other and then tighten the screws.

.

Quagi 70MHz 4 el. Curva di R.O.S. - IK1HGE

 

Quagi 70MHz 4el. Curva di impedenza - IK1HGE

The pictures here above refer to the antenna palced in the free space. In the following diagram, instead, we imagine to place the antenna on a 6 meters mast, so that the antenna height will be about 5.5 meters, measured from ground to the boom (horizontal axis of the antenna).

Diagramma di radiazione sul piano verticale (elevazione) in presenza di buon piano di terra. Antenna  5.5m dal suolo.