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2-El.-10-20m-25Ohm new |
2-Element-Portable-Yagi with 25Ohm,
dismantable, for 10m-20m (by DK7ZB)
This Yagi has a constant element-distance of 1,50m. You must only correct the lengths of the elements for QSY, see table down. For the bands 10m and 12m the Yagi is working as a reflector-radiator-Yagi, for 15m and 17m as a beam with radiator and director. All parts are not longer than 1,55m 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 20mm.
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 |
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4 pieces round tube 16x1,5mm, 150cm |
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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. |
 |
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The first part is a trifilar Ruthtroff-Un-Un 50Ohm to 28Ohm with the windings 1-6 (1,0mm isolated copper-wire). The second Guanella-transformer is a 25Ohm-Balun Un-Bal, made with two parallel coax-cables RG-174U. A-B is the outer braid of the two cables, C-D are the inner conductors. The mismatch 25/28Ohm does not cause any problems. Both parts are wound on the same core, an Amidon FT240-77. With RG-174 the max. power is 200Watts, with Teflon-coax RG-188 about 500Watts. |
The director/reflector is mounted with two 5mm-screws and nuts (Inox steel)
The middle part of the radiator
The box with the balun
Table 1: Lengths for the
10m-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
12m-Band (Radiator-Reflector), one
half
|
Tubes |
25x2mm |
20x1,5mm |
16x1,5mm |
|
Reflector |
75cm |
135cm |
92cm |
|
Radiator |
75cm |
135cm |
73,5cm |
Table 3: Lengths for the
15m-Band (Radiator-Director), one
half
|
Tubes |
25x2mm |
20x1,5mm |
16x1,5mm
|
12x1mm
|
|
Radiator |
75cm |
135cm |
135cm |
9cm |
|
Director |
75cm |
135cm |
123,5cm |
- |
Table 4: Lengths for the
17m-Band (Radiator-Director), one
half
|
Tubes |
25x2mm |
20x1,5mm |
16x1,5mm
|
12x1mm |
|
Radiator |
75cm |
135cm |
135cm |
74cm |
|
Director |
75cm |
135cm |
135cm |
50cm |
It is possible to use the Yagi for 20m with reduced gain of 3,6dBd. We must decide if we use the CW- or the SSB-Band. Try it with the following tubes:
Table 5: Lengths for the
20m-Band (Radiator-Director), one
half
|
Rohre |
25x2mm |
20x1,5mm |
16x1,5mm
|
12x1mm |
10x1mm
(CW) |
10x1mm (SSB) |
|
Strahler |
75cm |
135cm |
135cm |
135cm |
73cm |
65cm |
|
Direktor |
75cm |
135cm |
135cm |
135cm |
34cm |
29cm |
