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The Great 21dBi 3G Yagi Antenna Scam

(aka "The Great 24dBi 3G Yagi Antenna Scam" or "The Great 18dBi 3G Yagi Antenna Scam")

Scroll down to see just how long a REAL 21dBi or 24dBi 3G antenna would be !

 


Gain vs Length Mockup

Our REAL 16dBi Yagi

FAKE 21dBi Yagi

A 850Mhz 24dBi 3G antenna
would be this long !
as confirmed by
Mathematical Calculation
Magic ?
how long is a 24dBi 850Mhz Yagi Antenna |
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How long is a 16dBi 850Mhz Yagi Antenna
Our 16dBi 850MHz Yagi is 1510mm long (including the mounting plate and all elements)
This is the length required for an antenna of this gain at this frequency

 
 
 

At 24dBi an 850MHz Yagi
would be 15m long ! 
This is the length required for an antenna of this gain at this centre frequency... but its more complicated than that!

 

 

 

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How long is a 24dBi 850Mhz Yagi Antenna really?
Whats a 24dBi 850Mhz Yagi Antenna
BUYER BEWARE !
Sold as a 22 or 24dBi antenna but
is capable of only 14dBi !

These antennas are only
1m in total length.

The gain is very much overstated and falsely advertized by several online stores and a number of Telstra Shops.
So far, we have found two "manufacturers" of these identical antennas. 
 Following our request of both of them to provide Free-Radiation, Gain vs Frequency and VSWR vs freq. test results, to our surprise one of them did actually provide information.
The other just ignored us.
The Australian re-sellers we contacted were unwilling to supply any information.

Here's the manufacturer's test results:
Model: Yagi-02
Target Bands: 806-960/1800/1900MHz

Advertised Gain: 24dB, 24dBi, 22dBi, 18dBi
or 17dBi (brochure dependent
)
 Freq Forward Gain FBR
824MHz  - 7.7dBi -26dB
864MHz  - 11.8dBi - 19dB
884MHz - 12.5dBi - 18dB
924MHz - 13.4dBi - 25dB
960MHz - 14.0dBi - 26dB 

As you can see, the modeling of this antenna produces the same maximum as the manufacturers own test results. That is about 11.5dBi for the telstra 850Mhz band and about 13.5dBi for the Optus 900MHz band.

 

 

 

But the REAL limitation is more to do with bandwidth.

    

The graph below is show a hypothetical comparison of a very long hi-gain Yagi and a shorter low gain Yagi. The values are purly hypothetical for ilustration purposes only.

All Yagi Antennas have a "Bandwidth" with a maximum gain centered about a particular frequency.
As more elements are added to the Yagi it becomes longer and higher in gain at its optimum centre frequency. The Bandwidth however has the opposite reaction and becomes narrower.

    
 
From the chart above we can see that as the antenna gets longer, the bandwidth becomes unusable
 
    Gain at operating frequency
  Frequency Long antenna medium antenna Short antenna
Receive:  830MHz 16 dBi
15 dbi 13.5 dBi
 Transmit: 880Mhz 21 dBi 16.5 dBi
13dBi
Useful bandwidth within 1dB of max gain:
50MHz
90MHz >100MHz
         
         
 
 
 
 
 
 

 111  222  333
     
VK5DJ's YAGI CALCULATOR  










Yagi design frequency = 860.00 MHz



Wavelength =349 mm          
Parasitic elements contacting a square section metal boom 22.00 mm across.
Folded dipole mounted same as directors and reflector      
Director/reflector diam =10 mm          
Radiator diam =10 mm          
               
REFLECTOR            
184 mm long at boom position = 30 mm (IT = 81.0 mm)    
               
RADIATOR            
Single dipole 158 mm tip to tip, spaced 70 mm from reflector at boom posn 100 mm (IT = 68.0 mm)
Folded dipole 178 mm tip to tip, spaced 70 mm from reflector at boom posn 100 mm (IT = 78.0 mm)
               
DIRECTORS            

Dir

Length

Spaced

Boom position

IT

Gain

Gain

 

(no.)

(mm)

(mm)

(mm)

 

(mm)

(dBd)

(dBi)

1 157 26 126   67.5 4.8 6.9
2 154 63 189   66 6.5 8.6
3 152 75 264   65 7.8 9.9
4 150 87 351   64 8.9 11
5 149 98 448   63.5 9.8 11.9
6 147 105 553   62.5 10.5 12.7
7 145 110 663   61.5 11.2 13.3
8 144 115 778   61 11.7 13.9
9 143 120 898   60.5 12.2 14.4
10 141 125 1023   59.5 12.7 14.9
11 140 131 1154   59 13.1 15.3
12 139 134 1288   58.5 13.5 15.7
13 138 136 1424   58 13.8 16
14 137 138 1562   57.5 14.2 16.3
15 136 139 1702   57 14.5 16.6
16 136 139 1841   57 14.7 16.9
17 135 139 1980   56.5 15 17.1
18 134 139 2120   56 15.2 17.4
19 134 139 2259   56 15.4 17.6
20 133 139 2399   55.5 15.6 17.8
21 132 139 2538   55 15.8 18
22 132 139 2678   55 16 18.2
23 132 139 2817   55 16.2 18.3
24 131 139 2956   54.5 16.4 18.5
25 131 139 3096   54.5 16.5 18.7
26 130 139 3235   54 16.7 18.8
27 130 139 3375   54 16.8 19
28 130 139 3514   54 17 19.1
29 129 139 3654   53.5 17.1 19.2
30 129 139 3793   53.5 17.2 19.4
31 129 139 3933   53.5 17.3 19.5
32 129 139 4072   53.5 17.5 19.6
33 128 139 4211   53 17.6 19.7
34 128 139 4351   53 17.7 19.8
35 128 139 4490   53 17.8 19.9
36 128 139 4630   53 17.9 20
37 128 139 4769   53 18 20.1
38 128 139 4909   53 18.1 20.2
39 127 139 5048   52.5 18.2 20.3
40 127 139 5187   52.5 18.3 20.4
41 127 139 5327   52.5 18.4 20.5
42 127 139 5466   52.5 18.5 20.6
43 127 139 5606   52.5 18.5 20.7
44 127 139 5745   52.5 18.6 20.8
45 127 139 5885   52.5 18.7 20.9
46 127 139 6024   52.5 18.8 20.9
47 127 139 6164   52.5 18.9 21
48 127 139 6303   52.5 18.9 21.1
49 127 139 6442   52.5 19 21.2
50 126 139 6582   52 19.1 21.2
51 126 139 6721   52 19.2 21.3
52 126 139 6861   52 19.2 21.4
53 126 139 7000   52 19.3 21.5
54 126 139 7140   52 19.4 21.5
55 126 139 7279   52 19.4 21.6
56 126 139 7418   52 19.5 21.7
57 126 139 7558   52 19.6 21.7
58 126 139 7697   52 19.6 21.8
59 126 139 7837   52 19.7 21.8
60 126 139 7976   52 19.7 21.9
61 126 139 8116   52 19.8 22
62 126 139 8255   52 19.9 22
63 126 139 8395   52 19.9 22.1
64 126 139 8534   52 20 22.1
65 126 139 8673   52 20 22.2
66 126 139 8813   52 20.1 22.2
67 126 139 8952   52 20.1 22.3
68 126 139 9092   52 20.2 22.3
69 126 139 9231   52 20.2 22.4
70 126 139 9371   52 20.3 22.4
71 126 139 9510   52 20.3 22.5
72 126 139 9649   52 20.4 22.5
73 126 139 9789   52 20.4 22.6
74 126 139 9928   52 20.5 22.6
75 126 139 10068   52 20.5 22.7
76 126 139 10207   52 20.6 22.7
77 126 139 10347   52 20.6 22.8
78 126 139 10486   52 20.7 22.8
79 126 139 10626   52 20.7 22.9
80 126 139 10765   52 20.8 22.9
81 126 139 10904   52 20.8 23
82 126 139 11044   52 20.9 23
83 126 139 11183   52 20.9 23
84 126 139 11323   52 20.9 23.1
85 126 139 11462   52 21 23.1
86 126 139 11602   52 21 23.2
87 126 139 11741   52 21.1 23.2
88 126 139 11880   52 21.1 23.3
89 126 139 12020   52 21.1 23.3
90 126 139 12159   52 21.2 23.3
91 126 139 12299   52 21.2 23.4
92 126 139 12438   52 21.3 23.4
93 126 139 12578   52 21.3 23.4
94 126 139 12717   52 21.3 23.5
95 126 139 12857   52 21.4 23.5
96 126 139 12996   52 21.4 23.6
97 126 139 13135   52 21.4 23.6
98 126 139 13275   52 21.5 23.6
99 126 139 13414   52 21.5 23.7
100 126 139 13554   52 21.6 23.7
101 126 139 13693   52 21.6 23.7
102 126 139 13833   52 21.6 23.8
103 126 139 13972   52 21.7 23.8
104 126 139 14112   52 21.7 23.8
105 126 139 14251   52 21.7 23.9
106 126 139 14390   52 21.8 23.9
107 126 139 14530   52 21.8 23.9
108 126 139 14669   52 21.8 24
               
COMMENTS            
The abbreviation "IT" means "Insert To", it is the construction distance from the element tip to the edge of the boom for through boom mounting
               
Spacings measured centre to centre from previous element    
Tolerance for element lengths is +/- 1 mm        
               
Boom position is the mounting point for each element as measured from the rear of the boom and includes the 30 mm overhang.The total boom length is 14699 mm including two overhangs of 30 mm
               
The beam's estimated 3dB beamwidth is 13 deg      
               
A half wave 4:1 balun uses 0.66 velocity factor RG-58 (PE) and is 115 mm long plus leads
FOLDED DIPOLE CONSTRUCTION        
Measurements are taken from the inside of bends      
Folded dipole length measured tip to tip = 178mm      
Total rod length =387mm          
Centre of rod=193mm          
Distance BC=CD=72mm          
Distance HI=GF=67mm          
Distance HA=GE=94mm          
Distance HB=GD=122mm          
Distance HC=GC=193mm          
Gap at HG=10mm            
Bend diameter BI=DF=35mm          
               

 

 

 

 

 

 

 


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