| 2-El.-6-17m-V. Mk Vl |
2-El.-10-17m-25 Ohm |
2-Element-Portable-Yagi with 25 Ohm,
dismantable, for 10 m-20 m (by DK7ZB)
This Yagi has a constant element-distance of 1,50 m. You must only correct the lengths of the elements for QSY, see table down. For the bands 10 m and 12 m the Yagi is working as a reflector-radiator-Yagi, for 15 m and 17 m as a beam with radiator and director. All parts are not longer than 1,55 m and fit in a fishing bag for the transportation. The Yagis are a high-gain beams with small bandwidth. The dipole must be mounted insulated with two halfs, spacing 20 mm.
For tuning you can shift the end pieces of the radiator, the other element should not be changed.
Short data for the 4 bands (free space)
|
|
10m |
12m |
15m |
17m |
|
Gain |
4,8dBd |
4,6dBd |
4,6dBd |
4,5dBd |
|
F/B |
6dB |
9dB |
12dB |
16dB |
Needed aluminium parts for the beam
|
Dimension |
Use |
|
1 piece 30x30x2mm-square-Alu, 155cm |
Boom |
|
2 pieces
round tube 25x2mm, 150cm |
inner
pieces rad + ref/dir |
|
4 pieces round tube 20x1,5mm, 150cm |
middle pieces for the elements |
|
4 pieces round tube 16x1,5mm, 150cm |
|
|
2 pieces round tube 12x1,0mm, 85cm+60cm |
outer
pieces for the elements |
The schematic picture of the beam (one element half)
The Balun The impedance for all bands is 25Ohm, so we use a homemade balun 1:2 for the matching 25Ohm balanced to 50Ohm unbalanced. The balun has two sections on the same core and the principle is shown in the picture right. |
 |
|
|
The first part is a trifilar Ruthtroff-Un-Un 50 Ohm to 28 Ohm with the windings 1-6 (1,0 mm isolated copper-wire). The second Guanella-transformer is a 25-Ohm-Balun Un-Bal, made with two parallel coax-cables RG174U. A-B is the outer braid of the two cables, C-D are the inner conductors. The mismatch 25/28 Ohm does not cause any problems. Both parts are wound on the same core, an Amidon FT240-77. With RG174 the max. power is 200 Watts, with Teflon-coax RG188 about 500 Watts. Peter, HB9PJT, has tested the FT240-61 as a better choice. He found that 3 trifilar windings for the balun 2:1 gave the best results of nearly 1:1 with this core. Look for the pictures of HB9PJT below! |
The table shows that the balun has an inductive part +j which can be compensated by shortening the radiator halves.
|
Band |
6m |
10m |
12m |
15m |
17m |
20m |
30m |
|
Impedance |
38+j15 Ohm |
51+j20 Ohm |
58+j19
Ohm |
66+j13
Ohm |
68+j9
Ohm |
67+j3
Ohm |
64+j5
Ohm |
|
SWR
50 Ohm |
1,5 |
1,4 |
1,4 |
1,4 |
1,4 |
1,3 |
1,2 |
The director/reflector is mounted with two 5-mm-screws and nuts (Inox steel)
The middle part of the radiator
The box with the balun
The Beam, built by Peter, HB9PJT. Click on the thumbnails for greater pictures!
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Table 1: Lengths for the
10-m-Band (Radiator-Reflector), one
half
|
Tubes |
25x2mm |
20x1,5mm |
16x1,5mm
(CW) |
16x1,5mm
(SSB) |
|
Reflector |
75cm |
135cm |
51cm |
47cm |
|
Radiator |
75cm |
135cm |
40cm |
37,5cm |
Table 2: Lengths for the
12-m-Band (Radiator-Reflector), one
half
|
Tubes |
25x2
mm |
20x1,5
mm |
16x1,5
mm |
|
Reflector |
75
cm |
135
cm |
92
cm |
|
Radiator |
75
cm |
135
cm |
73,5
cm |
Table 3: Lengths for the
15-m-Band (Radiator-Director), one
half
|
Tubes |
25x2
mm |
20x1,5
mm |
16x1,5
mm
|
12x1
mm
|
|
Radiator |
75
cm |
135
cm |
135
cm |
9
cm |
|
Director |
75
cm |
135
cm |
123,5
cm |
- |
Table 4: Lengths for the
17-m-Band (Radiator-Director), one
half
|
Tubes |
25x2
mm |
20x1,5mm |
16x1,5
mm
|
12x1
mm |
|
Radiator |
75
cm |
135
cm |
135
cm |
74
cm |
|
Director |
75
cm |
135
cm |
135
cm |
50
cm |
It is possible to use the Yagi for 20m with reduced gain of 3,6 dBd. We must decide if we use the CW- or the SSB-Band. Try it with the following tubes:
Table 5: Lengths for the
20-m-Band (Radiator-Director), one
half
|
Rohre |
25x2
mm |
20x1,5
mm |
16x1,5
mm
|
12x1
mm |
10x1
mm
(CW) |
10x1mm (SSB) |
|
Radiator |
75
cm |
135
cm |
135
cm |
135
cm |
73
cm |
65 cm |
|
Director |
75
cm |
135
cm |
135
cm |
135
cm |
34
cm |
29 cm |
