LONG YAGI 4 ELEMENTS ANTENNA

 Designed by 1 CM 062 op. Gianni

Made by  1 CM 026 op. PAT

 

The antenna shown in Fig. 1 is a four elements Long Yagi, installed since last year at 12 meters above the ground.
The boom is 5.90 meters long and the feeding system is a common Gamma Match.
After many trials on my computer with Y.O. (Yagi Optimizer), I found that the best compromise between Gain, F/B, Impedance and Efficiency, is a long boom with few elements.
Moreover , after many years of operations on 27 Mhz, I recognize that the Gain is very important but also the horizontal and vertical beamwidth are essential for an extra performance. To achieve maximum gain, I tuned my antenna at 27.555 Mhz with a passband of only 300 Khz. from the centre band. Also this feature (very sharp response) help me very much to reduce drasticly the splatters  from local stations  on AM or FM .
Many operators contacted in the last months ask me to give more details of my antenna system: I think Internet is the best way to reach all of them and others antenna's fans.  

Fig.1

 

 

Antenna measurements in mm.

Boom	5900    long (0.54 l.w.)
Reflector	2708.8        "
Radiator	2600.0        "
Director 1	2504.7        "
Director 2	2424.2        "

 

The reference point (zero) for all spacings is the reflector

Reflector spacing	0    mm
Refl./Radiator	1044.5  mm
Refl./Direcetor 1	3126.0 mm
Refl./director 2	5900.0 mm.

 

Since the element lenght is depending of the diameter of pipe used, I will list for  each half element  the lenght and the diameter for all sections; the first 2 sections are the same for all the four elements:  

Section 1	1000 mm long by 20 mm. diam.
Section 2	900 mm long by 16 mm. diam

The last section is variable as below:

Reflector:	887  mm. by 12 mm. diam
Radiator:	774    “     “    “   “      “
Director 1	674    "     "    "    "     "
Director 2	589    "     "    "    "     "

 

Results obtained by Y.O.

GAIN	15.66  Dbi
F/B	22.01 Db.
Imp.	15.8 -J0.5
SWR	< 1:1.5  from 27.3 to 27.8 MHz
Vert.beamwidth	< 14°

 

 

 

 

 

At 27.555 (band centre) the   SWR is 1:1.0  

  The antenna was tuned at about 3 meters above the ground  and after a couple of adjustments  of Gamma Matching, the SWR was very close to 1.The  measured resonance frequency was at 27.450 Mhz.This was espected since the computer doesn't apply any correction for the boom diameter. Cutting off the tip by 10 millimeters on both sections of the radiator, I  brought the resonance frequency at 27.555.
Rising the antenna at 12 meters, the SWR was the same. And also rotating the antenna to different directions no variation was noticed.

 

CONCLUSIONS 

Once again, computer antenna modeling has played an important role in optimizing an antenna design.
Being able to make actual field antenna pattern measurements allows me to check if my assumptions are true or not.
The reference antenna was a dipole at the same height but about 15 meters away from the antenna under test.
The Long Yagi really does have a very respectable gain and a high front-to-back ratio.
When listening to DX station arriving at very low angles, an S6 signal disappears into the noise using the dipole, but using to the Long Yagi, not only the signal reach S8/9 but also the amount of QRM is lower.
The modeling software is very accurate and the results obtained are very close to that predicted; and by viewing the current distribution, the associated radiation patterns and impedance, it's became much easier to understand which way to modify a design to achieve a desidered result.  

 

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For further information and to communicate your experience with Long Yagi 4 Elements Antenna write to:

clubmarconi@libero.it

Or to:

Club G.Marconi, Fermo posta -16161 Genova/57 - Italy

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