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     Northam, Western Australia
     QRZ Page: VK6YSF

 

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MULTI-BAND DOUBLET ANTENNA - HIGH HF AND LOW VHF

Multi-band Doublet antenna to cover the 20m, 17m, 15m, 12m, 10m and 6m bands. December 2024. 


 

Requiring an antenna to cover the 6m band (50 - 54MHz) that is horizontally polarized and also for coverage of the upper HF bands a 8.6m multi-band Doublet antenna fed with a section of coax run through the roof and building wall to a Matching tuner or potentially directly connected to the radio. The un-balanced coaxial cable is connected to the antenna with the combination of a 1:1 choking baluns and a 1:4 impedance step up balun connected in series.  The antenna, feed line, baluns 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 and 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.

The antenna will be formed by cutting insulators into two existing tower guy wires to create an Inverted 'V' dipole antenna.

 

Figure 1 Multi-band Doublet, feed-lines and balun configuration.

 

Basic Multi-band Doublet Arrangement.

(1)  Inverted 'V' Dipole. (8.6Mtr total length)

(2)  1:4 Current Balun. See Fig 4 and 5 for details 

(3)  1:1 Choking Balun. See Fig 6 for details

(4)  RG213 Coax cable (17m)

(5)  1:1 Choking Balun. See Fig 6 for details

 

Antenna modelling.

 

The antenna has been modelled with MMANA-GAL an antenna analyser application to optimise the antenna for the lower portion of the 6m band. While the modelling gives a bit of prediction of the antenna performance, the real world site will likely provide very different results.

 

Figure 2 MMANA-GAL antenna analyser modelled SWR for the 6m bands when dipole is matched at 200 Ohms.

  

 

Figure 3 MMANA-GAL antenna analyser modelled radiation plot for the 6m bands.

 

The above radiation plots were produced using MMANA-GAL Antenna Analyser software by JE3HHT, Makoto (Mako) Mori at http://hamsoft.ca/   

 

1:4 Current Balun

The 1:4 current balun is derived from two 1:1 current baluns with each consisting of a close double bifilar winding of 3.5 turns wound evenly spaced around the FT140-61 Ferrite Toroid Cores. The wire is PTFE (Polytetrafluoroethylene) silver plated copper wire, of 1.0mm diameter (AWG 18), and white for this project.

Figure 4 Schematic of the 1:4 Guanella Current balun .

 

 

Figure 5 Wiring of the 1:4 Guanella Current balun.

 

Type 

Impedance transformation

Ratio

1:4

Frequency Range

8.0 ~ 60MHz

Core Used

FT140-61 Ferrite Toroid Core x 2

Number of turns

Core 1 = 4.5 turns x 2, Core 2 = 4.5 turns x 2. PTFE silver plated copper wire, 1.0mm, AWG 18, White.

SWR

1.2:1 or less. Ref: Figure ?

 

Photo 1 1:4 balun assembled.

 

1:4 balun testing 

Photo 2 Shows the Nano VNA antenna analyser plot viewing a 200ohm resistive load through the 1:4 balun. Note the 200ohm resistor appears as 50ohms due to the 1:4 balun ratio transformation resulting in an ideal SWR of 1:1. This plot shows an SWR ranging from 1.0 MHz to 60MHz with a 1.1:1 or better SWR from 10MHz to 60MHz.

 

While the core choosing is not ideal it has tested as sufficiently efficient impedance transformation at the intended power levels of 100w or less.

For Summary of suitable ferrite cores and core types for a frequency range of 100 kHz to 50 MHz for power levels of 50W, 100W, and 500W continuous and SSB. It assumes good thermal management and proper balun design. See: Power - Ferrite Core Design  

    

1:1 Choking Balun

 

The choking balun to isolate the potential common mode RF on the coax cable and to reduce noise pick up.

 

 

Figure 6 Schematic of the 1:1 choking balun   

 

Type 

Choking Balun

Ratio

1:1

Frequency Range

14 ~ 54MHz

Choking Impedance

2k Ohms minimum.

Core Used

FT140-43 Ferrite Toroid Core

Number of turns

8 Turns.  (Coax - RG316/U 50 OHM)

SWR

1:1 Ref: Figure ?

 

Photo 3 Choking balun assembled.

 

Choking balun testing

 

Photo 4 Shows the Nano VNA antenna analyser plot of the choking attenuation in dB to mitigate common-mode RF current on the coax shield. 20dB attenuation should be considered the minimum. This plot shows a choking attenuation of -29.4dB from 40MHz to 60MHz.

 

 

Mast Head Matching Enclosure.

 

The mast head matching transformers are installed in a aluminium diecast enclosure 160mm x 115mm x 50mm deep. The feed-through insulators are cemented in place with windscreen sealant.

  

Photo 5 Mast Head Matching Enclosure 

 

Photo 6 Feed-through insulators

 

Photo 7 Feed-through insulators installed with windscreen sealant.

  

Photo 8 Mast Head Matching unit assembly.

 

Photo 9 Mast Head Matching unit installed on the mast with the dipole antenna cut into the guy wires.

 

Photo 10 Mast Head Matching unit installed on the mast with the dipole antenna cut into the guy wires.

  

 

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Page initiated 01 December, 2024 

Page last revised 04 January, 2025

 

 

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VK6YSF JS8Call and Olivia 8/250 operations.

Current activity is generally, however not restricted to operations with JS8Call and Olivia 8/250 on the 20m and 40m band.

Proposed band and mode activity is often communicated on HamSpots: https://hamspots.net/js8/ or my profile on 

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VK6MJM LF (136 kHz) and MF (474 kHz) Beacon

VK6MJM is a  LF/MF station located in Manjimup, Western Australia.

Mode: Mode: FST4W 300 (Similar to WSPR)

Station details were presented by Peter Hall (VK6HP) at PerthTech 2024.Presentation PDF: Peter Hall VK6HP at PerthTech

QRZ Page: VK6MJM on QRZ.com

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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