HF ladder feed-line to coaxial cable choke and 1:4 balun. (0.1MHz - 30MHz).

The main HF antenna is a 42m multi-band Doublet antenna fed with open wire ladder-line and enters the radio room with 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. (7.5Mtr)

(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 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.

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Page last revised 25 September, 2019