Requiring a choking balun to isolate the potential feed-line common mode RF on the coax
cable for HF/MF bands
1.5MHz - 30MHz. A simple coax cable wound on to a ferrite toroidal core was
chosen.
The antenna is a 40m band half sloper
that is it is a quarter wavelength on the primary band of 7 MHz and
with a third harmonic band near 21MHz. This antenna can also be quite
useful on other bands apart from 40m and 15m.
The antenna is an antenna of
convenience as it is made by cutting two insulators into a tower guy
wire and despite this simple approach the antenna has some good
performance characteristics that have been confirmed by measurement
with WSPR.
The antenna is simply fed with coax
cable to the apex of the antenna and while not a good design it is not
an uncommon amateur radio antenna.
The problem with the antenna is due to
the direct coax feed connection the antenna, the antenna exhibits some
annoying multi-mode RF currents on the coax that manifest themselves
as RF feedback, such as crackling in unconnected speakers, that is
speakers not connected to the radio in question.
- Prevents
unwanted RFI by eliminating feed-line common mode currents and
radiation
- All
power goes to the antenna, improving efficiency
- Reduces
noise or EMI picked-up by your coax shield
- Power
is balanced between driven elements of antenna

Figure 1 Schematic of the 1:1 choking balun
Type |
Choking Balun |
Ratio |
1:1 |
Frequency Range |
1.5 ~ 30MHz |
Choking Impedance |
3k Ohms (-30dB) min to
>10k Ohms (-40dB) Ref:
Figure 3 |
Core Used |
FT140-43
Ferrite Toroid Core |
Number of turns |
19 (9.5 + 9.5). Ref:
Figure 2 |
SWR |
1:1 Ref: Figure 5 |

Figure
2 Winding details of the 1:1 choking balun
Construction
The
construction was simply to wind 9.5 turns of RG316 coax with a cross
over and another 9.5 turns onto a FT140-43 Ferrite Toroid Core. The
result of 19 turns achieved an average of 40uH lumped value inductive
reactance to common mode RF currents from approximately 1.0MHz to
30MHz.
Parts
list.
- FT140-43
Ferrite Toroid Core
- About
1.0mtr of RG316 coax.

Photo
1 Choking balun
assembled.

Photo
2 Choking balun
assembled.
Testing
The mini-VNA antenna analyser data points recorded and
graphed in a spreadsheet in Figure 3 show the attenuation as measured in the coax
shield presents a minimum of -28dB at 1.0MHz to -33dB at 30MHz and much greater
attenuation of better than -40dB from 4MHz to 18MHz.

Figure
3 The evaluation of the choking attenuation of the balun over a bandwidth
from 1.0MHz- 30MHz.

Figure
4 Choking impedance to dB of choking. 20dB attenuation should be considered the minimum.

Figure
5 SWR.from 0.1MHz- 30MHz
Also
see other baluns and ununs:
BALUN
1:1 CHOKE & 1:4 BALUN HF
ladder feed-line to coaxial cable combination choke and 1:4 balun.
(0.1MHz - 30MHz).
BALUN
1:1 CHOKING Choking balun
for lower HF and MF bands. (200kHz - 10MHz).
CHOKING
1:1 BALUN - HF BANDS Reisert
choking balun.
(1.0MHz - 30MHz). FT240-43 Ferrite Toroid Core.
CHOKING
1:1 BALUN - HF BANDS Reisert
choking balun
(1.5MHz - 30MHz). FT140-43 Ferrite Toroid Core.
CHOKING
1:1 BALUN - LOW VHF BAND Choking balun.
(10MHz - 60MHz). FT140-43 Ferrite Toroid Core.
BALUN
1:1 CURRENT 1:1
Guanella
Current
balun
using a L15 ferrite core (1.8 - 30MHz).
BALUN
1:4 CURRENT 1:4
Guanella
Current
balun
using a L15 ferrite core (1.8 - 30MHz).
BALUN
1:4 SINGLE CORE CURRENT 1:4 Guanella
Current Balun, single FT240-43
ferrite toroid cores.
(0.3MHz - 30MHz).
BALUN
1:1 VOLTAGE 1:1 Ruthroff
voltage balun using a T-200-2
powdered iron toroid core (1.8 -
30MHz).
BALUN
4:1 VOLTAGE 4:1 Ruthroff
voltage balun using a T-200-2
powdered iron toroid core (1.8 -
30MHz).
BALUN
6:1 VOLTAGE - VERSION 1 6:1
Voltage balun using a L15
ferrite toroid core (1.8 - 30MHz).
BALUN
6:1 VOLTAGE - VERSION 2 6:1
Voltage balun using a FT140-43
Ferrite Toroid Core (1.8 - 30MHz)
BALUN
9:1 VOLTAGE - VERSION 1 9:1
Voltage balun using a L15
ferrite toroid core (1.8 - 30MHz).
BALUN
9:1 VOLTAGE - VERSION 2 9:1
Voltage balun using a FT140-43
Ferrite Toroid Core (0.5 - 60MHz).
UNUN
9:1 VOLTAGE 9:1 voltage unun
using a
T-200-2 powdered iron toroid
core
(1.8 -
30MHz).
UNUN
9:1 VOLTAGE VERSION 2 9:1 voltage unun
using a L15 ferrite core (1.8 -
30MHz).
UNUN
9:1 VOLTAGE VERSION 3 9:1 voltage unun
using a FT140-43 ferrite core (0.5 -
60MHz).
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