Amateur Radio (G3TXQ) - Twin Feed experiment

The HexBeam pages on this web site recommend that low impedance feedline be used for interconnecting the various wire elements of the antenna. 50 Ohm coax is the usual choice, although coaxial cable can be difficult for the novice constructor to terminate neatly. Twin Feed would make construction a little easier, but it is only available commercially with characteristic impedances (Zo) of 300 Ohms or 450 Ohms - too high for the HexBeam application. Hence my interest in home-constructed low-impedance Twin Feed transmission line.

The usually-quoted approximation for the characteristic impedance of twin-feed with air as the dielectric is:

Zo = 120 Ln( 2*b/a)

where b is the centre-to-centre spacing of the wires and a is their diameter. However this expression is only valid for Zo > 200 Ohms; for lower values of Zo the more-complete formula is required:

Zo = 120*ArcCosh(b/a)

or if you wish to avoid the inverse hyperbolic function:

Zo = 120*(2*Ln(Sqrt(b/a-1)+Sqrt(b/a+1))-Ln(2))

Either formula shows that decreasing the ratio b/a will reduce Zo. This suggests that low impedance Twin Feed could be constructed by placing side-by-side 2 large diameter wires which have relatively thin insulation. Stripping the insulation from one of the wires would reduce the spacing even further. This approach is shown in the two photographs on the right, and is often quoted as a simple way of producing a low impedance line.

I searched the electrical section of my local DIY store for suitable cable. Standard 6mm² UK earth wire seemed to have a high ratio of conductor diameter (3mm) to insulation thickness (0.9mm), so I purchased a short length with which to experiment. One wire was stripped of insulation and tied closely to the insulated wire with plastic wraps every 100mm.

The following RF characteristics were measured:

Characteristic Impedance - Zo: 88 Ohms
Velocity factor - Vf: 0.73
Matched loss (14 MHz): 0.2dB/m
Matched loss (28 MHz): 0.4dB/m
VSWR 2:1 loss (45 MHz): 0.6dB/m

The formula shown earlier predicts a characteristic impedance of 90 Ohms for this cable geometry - very close to the measured value of 88 Ohms. Interestingly, the presence of the insulation has had little effect on Zo because it forms only a small fraction of the total volume of dielectric containing the field between the two conductors.

For many applications the high losses would make this cable a "non-starter", but in the HexBeam application the losses are probably acceptable. However, it is clear that producing a Zo as low as the recommended 50 Ohms is challenging (b/a needs to be 1.088) and can only be achieved by very careful choice of cable type.