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The
main HF antenna is a 42m multi-band Doublet antenna fed with open wire
ladder-line and enters the radio room wiyh a sort length of coax run
through the roof and building wall to a T-Match tuner. The
un-balanced coaxial cable is connected to the balanced feed line
with the combination of a 1:1 choking balun and a 1:4 impedance step up
balun connected in series.
The antenna, feed lines, balun and antenna set-up is shown in below in
Fig 1.
The
1:1 choking balun is to mitigate common mode RF currents on the coax
cable, reduces noise pickup on the coax from within the building
entering the antenna system and also produce a balanced antenna system
that will have a more predictable radiation pattern .
The 1:4 impedance step up balun is
included to more broadly match the range of impedance at the
antenna/feed line with the nominal 50-ohm impedance of the coax.
Figure
1 Multi-band Doublet, feed-lines and balun configuration
Basic Multi-band
Doublet Arrangement
(1)
Inverted 'V' Dipole. (42Mtr total length)
(2)
450 Ohm Ladder Line. (8Mtr)
(3)
1:4 Current Balun. See Balun Guanella Current 1:4 Single core
(4)
1:1 Choking Balun. See Balun Choke HF
(5)
LDF4-50 Heliax
(3.5Mtr)
(6)
T-Match Tuner. See T Match Tuner
(7)
VSWR Meter.
(8)
HF Transceiver.
Construction
The
1:1 choking balun is a length of RG58 50ohm coax wound with 14 turns
on a single FT240-43
ferrite toroid core. See
Fig 2 for the connection details and Balun Choke HF
for the
1:1 choking balun details.
The
1:4 current balun has two ideally 100ohm transmission lines
wound evenly spaced in the same direction around a single FT240-43
ferrite toroid core. See
Fig 2 for the connection details and Balun Guanella Current 1:4 Single core
for balun details.
Figure
2 Detail Balun configuration
Photo
1 Balun housing for coax to ladder line interface.
Photo
2 Balun assembly.
Testing
The AIM 4170C antenna analyser recorded the balun impedance
transformation efficiency and common mode choking for a frequency
range from less than 100kHz to 30MHz.
Figure
3 AIM
4170C antenna analyser
plot viewing a 200ohm resistive load through the feed-line
balun. Note the 200ohm resistor appears as
50ohms due to the 1:4 balun ratio resulting in an ideal SWR of 1:1. This
plot shows an SWR of no greater than 1.7:1 with from 100kHz to 30MHz.
Figure
4 AIM
4170C antenna analyser
plot viewing a 200ohm resistive load through the feed-line
balun. Note the 200ohm resistor appears as
50ohms due to the 1:4 balun ratio. This plot shows an 50 ~ 60
ohms from 100kHz to 30MHz.
Figure
5 The evaluation of the choking impedance of the balun over a bandwidth
from 0.5MHz- 30MHz.
Figure
6 Choking impedance to dB of choking. 20dB attenuation should be considered the minimum.
Also
see other baluns and ununs:
1:1
Choking balun
Choking balun
for lower HF and MF bands. (1.8MHz - 10MHz) T250-26
Powdered Iron Toroid Core. 1:1
Choking balun low band VHF
Choking balun
for lower band VHF. (14 ~ 54MHz) FT140-43
Ferrite Toroid Core. 1:1
Guanella
current
balun
1:1
Guanella
current
balun (1.8 - 30MHz) L15
ferrite toroid core. 1:4
Guanella
current
balun
1:4 Guanella
current
balun (1.8 - 30MHz) L15
ferrite toroid core.
1:1 Ruthroff voltage
balun, 1:1 Ruthroff voltage balun (1.8 - 30MHz) T-200-2 powdered
iron toroid core. 4:1 Ruthroff voltage balun
4:1 Ruthroff voltage balun (1.8 - 30MHz) T-200-2 powdered iron toroid
core. 6:1 Ruthroff voltage balun
6:1 Ruthroff voltage balun (1.8 - 30MHz) L15
ferrite toroid core. 1:9
voltage unun_v1 9:1 voltage unun (1.8 - 30MHz) T-200-2
powdered iron toroid core. Version 1 1:9
voltage unun_v2 9:1 voltage unun (1.8 - 30MHz) L15
ferrite toroid core. Version 2
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