HOMEBREW LADDER LINE

There is still a general misconception that coaxial cable is always the best option, but balanced feeders can offer much lower losses when used with balanced antennas and can handle a high SWR on the feed line. They will radiate very little RF if the spacing between the two conductor wires is small in terms of wave length. With a spacing of  6 inches the radiation loss only represents one thousandth of an S point at 14 MHz and my main operating frequencies are all below. Providing you have an atu which accepts balanced line you can take two long lengths of wire the same length of your convenience and feed with balanced line in the centre and you have a multi band antenna, for this reason ladder line is becoming more popular with the amateurs who have postage stamp size back gardens where it is impossible to erect several antennas for different bands. It can also be used to feed large full wave loops for multi band operation.

To put things into perspective a typical RG 58 (Belden 9201) coax has a loss of 2.095dB per 100 feet at 30MHz. If your transmitting power was 100 watts the power out would be 61.7 watts out. A typical 100 feet length of 600 ohm open wire line is in the order of about 0.061dB loss which is insignificant. If your power was 100 watts in at 30 MHz the power out would be 97.5 watts.

Making your own ladder line can be time consuming but is well worth the effort. Construction is Childs play as my 7 year old daughter Natalie will demonstrate as she helped me build mine. As you can see from the photo above two equal required lengths of 14 AWG multi stranded pvc coated wire were measured and cut. We made the spacers by sawing a lot of nylon tubing into 7 inch long lengths and I drilled two holes in each piece 6 inches apart. A drill bit was chosen so that when we inserted the wires through the holes it would be a snug fit.

 Natalie then threaded all the spacers onto the wires and we used a length of wood as a ruler to space them equally apart. I then used a pair of pointed nose pliers to reach into the tubes and put a kink in the wires to stop them sliding and then put drop of super glue over the points where the wires enter or exit the holes in the spacers.

                     

The characteristic impedance of a parallel open wire line is determined by the diameter of the conductor wires and spacing between them. They can be calculated but the table below gives some guide lines.

Characteristic impedance Z0 (ohms)

Conductor wire

     300

400

500

600

8  AWG

2.0cm

4.6cm

10.6cm

24.4cm

10 AWG

1.6cm

3.6cm

8.4cm

19.3cm

12 AWG

1.25cm

2.9cm

6.7cm

15.3cm

14 AWG

1.0cm

2.3cm

5.3cm

12.2cm

16 AWG

0.80cm

1.8cm

4.2cm

9.6cm

18 AWG

0.63cm

1.4cm

3.3cm

7.6cm

20 AWG

0.50cm

1.1cm

2.6cm

6.1cm

22 AWG

0.40cm

0.90cm

2.1cm

4.8cm

In use the feeder can be bent but angles of sharper than 90 degrees are best avoided. Tuned feeder should also be kept away from metal objects so I constructed some stand off insulators made from fibreglass poles. In order to maintain a good balanced the feeder is best kept parallel to nearby objects and should be used with a proper balanced line antenna tuning unit. This ladder line has worked good on my doublet and my garden loop antenna.

If nylon tubes can not be obtained it can be constructed from light weight plastic plumbers pipe available at any DIY store. Spacing between the two wires can be anywhere between 1 and 6 inches and result in impedances between 300 ohms and 600 ohms, Since a none resonant antenna can present an impedance anywhere between 10ohms and about 5000 ohms the spacing is not too critical. The only critical point is that it does not exceed more than 1% of a wave length or it will begin to radiate.

Balanced lines have been around since the dawn of radio and were very popular in the 1930s but coax cables have become a popular trend of convenience. So why not bring a bit of history back and homebrew your own ladder line. I look foreword to working you on 80M or I will see you at the local DIY store 73.

 


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