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
Photo 4. Twin-lead
feed terminated at the apex of the All Band Doublet.
Photo 5. Twin-lead
Photo 6. Twin-lead
feed terminated at Balun.
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
ZoCharacteristic Impedance of the feedline in ohms.
SSeperation from centre to centre of the conducting wire = 60mm.
dDiameter of the conducting wire = 1.77mm.
PVC wire insulation used on a twin-lead antenna feed line will affect
the characteristic impedance.