Self's building of a Shielded Two-Wire Feederline

The two-wire line (600 Ohm chicken leader, 450 Ohm Wireman, old 300 Ohm UKW ribbon conductor) is a symmetrical antenna inlet and serves in the amateur radio of the low-loss and  impedance-correct tuning of multiband antennas. It consists an impedance of two parallel wires and has depending upon distance of the wires  from approximately 200 to 600 ohms. The distance of the wires is realized to each other with measuring rods or plastic bars.

Many classical short wave antenna forms for multiband (e.g. Zepp, Dipole, extended Doppelzepp, Windom, G5RV or current sum antenna)  are adapted with a two-wire line and a antennatuner. In addition, quda.- and deltaloops can be co-ordinated with two-wire lines for multiband use. 

The advantages of the open two-wire lines are small losses, because the dielectric is air. The velocity factor is about 1.

The open two-wire line is suitable outstanding with freely developed antennas where the inlet without obstacles directly to the antenna to be advanced.

Why a shielded two-wire line? 

The main reason for the fact is the fact that each open two-wire line  is very sensitive to enviromental influences. Open two-wire lines may not be shifted being because of metallic articles, parallel to cables, directly at the mast, in the supplying pit or on the soil. 

During a shielded two-wire line (shielded pair) the kind of the transfer does not play a role. Due to the screen it has a high fail-safe characteristic and can be everywhere shifted. The characteristic to co-ordinate antennas for multiple reel enterprise remains. 

The disadvantages of the shielded two-wire line are somewhat higher losses and a velocity factor smaller 1. The losses are negligible in the high-frequency. The velocity factor depends on the used type of coaxial cable.

Structure and computation of a shielded two-wire line even built:

Shielded two-wire lines are made of two parallel led coaxial cables and represent a symmetrical antenna inlet. The two cable lengths must be equal long exactly. The cables bound in short would be away together. I use for it, approx. all 400mm, black cable straps these UV firm. One can use also tape or heat shrink sleeve.

The screen (mass network) of the coaxial cables is interconnected at both ends. The device-sided end of the line is grounded and/or comes to the mass of the radio station. The antenna-lateral end can be hung in air or also grounded. That arrives completely on the antenna. If one has e.g. a Yagi antenna with dipole exciter, then the screen comes to the boom pipe.

The two interior leaders of the coaxial cables are usually attached antenna-laterally as. Device-sided they are attached at a Balun and/or the Antennatuner. 

The Impedance results from the used type of coaxial cable. I use e.g.. RG-58 with 50 ohms, then results in that 2 x 50 ohms = 100 ohms.

The velocity factor results also from the used type of coaxial cable. With RG-58 the velocity factor is 0,66. One counts 0.66 x 0.66 = 0,4356.

I would like a given length from a building suggestion for an open two-wire line with velocity factor 1, e.g.. 13,7m, on a shielded two-wire line from RG-58 convert, then I take to 13,7m x 0.4356 = 5.97m. The shielded two-wire line is shorter.

The shielded two-wire line in practice:

Since the 100 ohms from the arithmetical example are not sufficient in practice for the adjustment of short wave antennas, I use coaxial cables of the type RG-62.  This cable has an impedance of 93 ohms and comes from the range network technology (ARCNET). It has a dielectric with air and is transferred a velocity factor of 0,75.

The computation looks here:

Impedance - 2 x 93 ohms = 186 ohms

Length - 13,7m x (0.75 x 0.75) = 7,71m

These 186 ohms (nearly 200 ohms) can be transformed now well with a 4:1 Balun transformer on the 50 ohms of the radio communication system. 

Transformation - 186 ohms : 4 = 46.5 ohms

If you using a symmetrical Antennentuner, a 1:1 transducer or a coat line trap is enough. Also a low-loss "air core" transducer can find use.

Operational experiences

With the enterprise of my shielded two-wire lines from RG-62 I stated that the length is uncritical in use with adjustment equipment. 

Thus the shielded two-wire line of my 42m "loop Skywire" left a length of approx. 10,5m. all volumes (except 160m) with antiresonant circuit for 3.5 MHz well to come to an agreement. The antenna line is led here in the supplying pit. In relation to the enterprise with "Wireman" inlet is low-noiser the receipt around 1 to 2 s-stages and I makes send-site no TVI.

The shielded two-wire line finds also recently with my 2 element "Maria Maluca" antenna with large success use.

Christian DL7JV, 2007