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 ?

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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!

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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