Bi-directional Yagi for 144 MHz

ZS6BTE 4 Jan 2016

 

This antenna addresses a special purpose, where a rotator is not available or required, yet operation from two directions (off the front and back of the antenna) is needed.

 

The antenna is driven at the centre-most element with 50 ohm coax feeding a 4:1 balun to provide an impedance of 200 ohms. The predicted SWR is 1.1

To keep the overall length down to less than 5m (4.8m in fact), the number of elements was restricted to 9 including the driven element.

 

Tuning

During construction, make the driven element somewhat longer than specified, then progressively shorten until best SWR is achieved. All cables, etc, should be attached at the time of setting the best SWR.  The impedance is mostly pure resistance at 183 ohms, with low reactance, Fig 1.

The 4:1 balun will keep RF off the outside of the coax as well as maintaining the necessary high impedance of ~200 ohms at the feed point of the antenna.

 

Best Gain

Gain is good at 11.9 dBi for what is essentially a 5-el Yagi and of course there is no back-to-front ratio as the antenna's pick-up from the "front" is mirror-imaged at the "rear", with good symmetry.

During installation adjust the height above ground in the order of  half wavelength or so to peak up a DX signal. This will provide some ground gain providing total gain in excess of the modeled prediction, by as much as 3 dB typically, and also prevent anti-phase pickup from ground reflections in the wanted directions which could nullify much of the gain.

 

Noise Performance

With no back-to-front ratio this antenna will collect noise pickup from two directions and appear very noisy. Even so, the lobe performance in the horizontal plane, Fig 5, is good at more than -15 dB down, which will match the performance of most uni-directional Yagis, but in the vertical plane, Fig 6, the worst lobes are only ~10.3 dB down, which is not all that bad on a special-purpose antenna such as this.

 

 

Fig 1: Essential parameters

 

 

 

 

Fig 2: Geometry. Driven element plus 8 directors

 

 

Fig 3: Bi-directional Gain

 

 

Fig 4: SWR at 200 ohms (use a 4:1 balun)

 

Fig 5: Horizontal pattern at 144 MHz

 

 

Fig 6: Vertical pattern at 144 MHz

 

 

 

Construction

Element lengths in the NEC file below are half lengths (and free space dimensions, where the elements are mounted on insulators). The overall length to cut an element is twice this. If they are to be grounded to the metal boom, add 66% of the boom's diameter to the overall length:

so the length for director 1 grounded on or in a 25mm sq boom is (0.4507 x 2)m + (25 x 0.66)mm = (0.9014m) + (16.5mm) = 0.9179m.

 

Use 25mm sq alu tube, for grounded elements drill holes through this tube to locate the element spacings. Correct the element lengths according to the diameter of the boom. Polish all joints to a shine with a stainless steel brush. Then aluminium solder (NOT braise - too much heat) these elements in place both sides of the boom using a LPG torch or a soft oxy-acetylene flame. The driven element is a simple dipole, mounted on some plastic to insulate it from the boom. The mechanical balance point is at the driven element, it will be a good idea to offset the mounting point to the mast a bit the keep the feed point away from the mast boom. Ground the boom to the mast, which should also be grounded for safety.

 

Notes 

This antenna has not been constructed by me.

 

NEC File

CM NEC Input File of "bi directional yagi.nec"

CM 11.9 dBi gain, b/f ratio 0 dB. SWR in 200 ohms 1.1. Elements 5 mm ungrounded on the boom. Length 4.92m.

CM Use 5 mm alu rod for the elements.

CM Free space dimensions, add the element length correction for the metal boom (~66%) if elements are grounded on the boom.

CE Eight directors needed. Overall length 2.4035 x 2 meters

 

SY drlen=0.489531       'driven element half length meters

SY dir1len=0.450701     'director 1 half-length meters

SY dir1sp=0.391029      'director 1 spacing from driven element meters

SY dir2len=0.455359     'director 2 half-length meters

SY dir2sp=0.963761      'director 2 spacing from driven element meters

SY dir3len=-0.46407     'director 3 half length meters

SY dir3sp=1.66629       'director 3 spacing from driven element meters

SY dir4len=0.483938     'director 4 half length meters

SY dir4sp=2.4035        'director 4 spacing from driven element meters

 

GW  2  11   0.000     -drlen       0.000   0.000      drlen       0.000   0.0025

GW  6  11   dir1sp    -dir1len     0.000   dir1sp     dir1len     0.000   0.0025

GW  3  11   dir2sp    -dir2len     0.000   dir2sp     dir2len     0.000   0.0025

GW  4  11   dir3sp    -dir3len     0.000   dir3sp     dir3len     0.000   0.0025

GW  5  11   dir4sp    -dir4len     0.000   dir4sp     dir4len     0.000   0.0025

 

GW  8  11   -dir1sp    -dir1len    -0.000   -dir1sp    dir1len    -0.000   0.0025

GW  9  11   -dir2sp    -dir2len     0.000   -dir2sp    dir2len     0.000   0.0025

GW  10 11   -dir3sp    -dir3len     0.000   -dir3sp    dir3len     0.000   0.0025

GW  11 11   -dir4sp    -dir4len     0.000   -dir4sp    dir4len     0.000   0.0025

 

GE  0

FR  0  1    0    0  144.3    0.00E+00  0.00E+00  0.00E+00  0.00E+00  0.00E+00

EX  0  2    6    0  1.00E+00  0.00E+00  0.00E+00  0.00E+00  0.00E+00  0.00E+00

RP 0 1 360 1510 90. 0. 0. 1. 0. 0.

EN