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TWIN-LEAD (LADDER LINE) FEED

Twin-lead feed ladder line replacement - September 2023


My old 450-ohm ladder line, which had been in service for more than 20 years, was showing signs of deterioration—mostly in the form of cracking in the insulation due to the harsh Western Australian sun—and was due for replacement. At Perth Tech 2023, Donald Howarth, VK6JDM, presented a novel approach to constructing high-quality ladder lines. This innovative method involves using cheap Vine Ties as feed-line spacers, offering a very affordable yet effective solution.

Photo 1. Vine Ties

Using the very inexpensive Vine Ties that appear perfect for ladder line spacers and are also useful to tie grape vines to trainer wires. The Vine Ties are very similar in appearance and functionality to more expensive products specifically designed for the purpose of constructing radio antenna twin wire transmission ladder line. As the Vine Ties are produced for commercial vineyards, it is reasonable to expect that they will have reasonable longevity, as commercial operators will not want costly ongoing maintenance despite the low initial cost.

Requiring at least 7.5m of ladder line, I purchased 20m of general purpose automotive PVC covered stranded tinned copper wire. The wire is 26/0.3mm (16 AWG) including PVC insulation has a diameter of 3.3mm that is a perfect tight fit for the Vine Ties clips. The spacer ties (Vine Ties) were install with a separation of 400mm.  

  

 Photo 2. Vine Ties  

 

 Photo 3. Twin-lead feed

  

Photo 4. Twin-lead feed terminated at the apex of the All Band Doublet. 

 

Photo 5. Twin-lead feed lead.

 

Photo 6. Twin-lead feed terminated at Balun.

 

While the characteristic Impedance of the feed-line is not critical providing it is in the hundreds of ohms range the below calculation shows that the new feed-line is approximately 500 ohms and similar to the 450ohm commercial ladder feed-line that it replaces.

 

Zo           Characteristic Impedance of the feedline in ohms.

S            Seperation from centre to centre of the conducting wire = 60mm.

d             Diameter of the conducting wire = 1.77mm.

The PVC wire insulation used on a twin-lead antenna feed line will affect the characteristic impedance.

 

Basic Multi-band Doublet Arrangement

Fig 1 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

       For details of the feed-lines and balun configuration. See HF feedline Choke & Balun

(5)  LDF4-50 Heliax (3.5Mtr)

(6)  T-Match Tuner. See T Match Tuner

(7)  VSWR Meter.

(8)  HF Transceiver.

 

 

Video of making the ladder line feeder and the final installation

 
 
 

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Page last revised 13 June 2024  

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