Minggu, 14 Februari 2010

No other antenna has a better relation between gain and expense than the 2-Element-Yagi. One element additional to the dipole will give you 4dB gain. Each further element will give much more mechanical problems and the increase in gain is only 1-2dB for the same bandwidth. What says the ARRL-Antenna-Book about this antenna? "The must bang for the buck". Right!
Because the 10m, 15m and 20m-Bands need a greater bandwidth, the radiation resistance must be higher than for the smaller WARC-Bands. Therefore we take the Yagis with radiator-reflector and 50Ohms for 10m (greatest bandwidth), 28Ohms for 15 and 20m and the Yagis with radiator-director and 28Ohms for the WARC-Bands.
If you have a height above ground of 10m or higher for a 12m-Yagi, you can try a 12,5Ohm-type for that band. The antenna needs a very clear surrounding! With a very short boom and small bandwidth you get very high F/B.
 Gain, F/B, Boomlength and Elements of the 50-Ohm-Yagi with Reflector
Band
Impedance
Gain
F/B
Boom
Radiator
Reflector
10m
50Ohm
4,0dBd
10,5dB
1,76m
4,93m
5,36m
 Gain, F/B, Boomlength and Elements of the 28-Ohm-Yagis with Reflector
Band
Impedance
Gain
F/B
Boom
Radiator
Reflector
15m
28Ohm
4,2dBd
10dB
1,61m
6,87m
7,22m
20m
28Ohm
4,0dBd
10dB
2,30m
10,26m
11,08m
Gain, F/B, Boomlength and Elements of the 28-Ohm-Yagis with Director
Band
Gain
F/B
Boom
Radiator
Director
12m
4,3dBd
14,5dB
1,18m
6,05m
5,60m
17m
4,5dBd
13,5dB
1,70m
8,32m
7,80m
30m
4,4dBd
14dB
3,00m
14,92m
14,04m
  Gain, F/B, Boomlength and Elements of the 12,5-Ohm-Yagi for 12m with Director
Band
Gain
F/B
Boom
Radiator
Director
12m
4,4dBd
>25dB
0,67m
6,09m
5,78m

 A quarter-wave choke of coax is grounded at the socket.
50Ohm: Single Lambda/4*V-coax 50Ohm
28 Ohm: 2xLambda/4*V-coax 75Ohm parallel
12,5 Ohm: 2xLambda/4*V-coax 50Ohm parallel
 
Length of the cables see table below
Lengths of the Lambda/4-coax for the choke:
Band Lambda Full-PE  V=0,667 Foam-PE  V=0,82 (CATV)
10m 10,60m 1,77m 2,17m
12m 12,03m 2,00m 2,47m
15m 14,18m 2,36m 2,90m
17m 17,53m 2,76m 3,60m
20m 21,18m 3,53m 4,34m
30m 29,62m 4,94m 6,07m