Construction Details 

2Element parasitic Yagis by DK7ZB for the ShortwaveBands 10m30m
Details of a dismantable Yagi built by DL2RMC for portable use see down
Details of a 20mYagi by F4EKQ: http://enduro.civaux.free.fr/antenne.htm
No other antenna has a better relation between gain and expense than the 2ElementYagi. 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 12dB for the same bandwidth. What says the ARRLAntennaBook about this antenna? "The must bang for the buck". Right! Because the 10m, 15m and 20mBands need a greater bandwidth, the radiation resistance must be higher than for the smaller WARCBands. Therefore we take the Yagis with radiatorreflector and 50Ohms for 10m (greatest bandwidth), 28Ohms for 15 and 20m and the Yagis with radiatordirector and 28Ohms for the WARCBands. If you have a height above ground of 10m or higher for a 12mYagi, you can try a 12,5Ohmtype 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 50OhmYagi 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 28OhmYagis 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 28OhmYagis with Director
Band 
Gain 

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 12,5OhmYagi for 12m with Director
Band 
Gain 

Boom 
Radiator 
Director 
12m 
4,4dBd 
>25dB 
0,67m 
6,09m 
5,78m 
The antennas are fed with the DK7ZBmatch . A quarterwave choke of coax is grounded at the socket. 50Ohm: Single Lambda/4*Vcoax 50Ohm 28 Ohm: 2xLambda/4*Vcoax 75Ohm parallel 12,5 Ohm: 2xLambda/4*Vcoax 50Ohm parallel
Length of the cables see table below 
Lengths of the Lambda/4coax for the choke:
Band  Lambda  FullPE V=0,667  FoamPE V=0,82 (CATV) 
10 m  10,60 m  1,77 m  2,17 m 
12 m  12,03 m  2,00 m  2,47 m 
15 m  14,18 m  2,36 m  2,90 m 
17 m  17,53 m  2,76 m  3,60 m 
20 m  21,18 m  3,53 m  4,34 m 
30 m  29,62 m  4,94 m  6,07 m 
The dismantable Yagi by DL2RMC for portable use
The Yagi of Tom, DL2RMC during his expedition to Corsica island (TK/DL2RMC) He has a cheap and simple construction similiar to my 3Eleportable Yagi with aluminium tubes as a leightweight antenna. Details see in the pictures below. For each band he has an own 2x75Ohmquarterwave matching cable. 
Insulated middle part of the radiator:
The middle part of the director:
All parts ready for assembling: