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Figure-8 flex used as RF transmission line

In my early days of hamming, I can recall hearing that common Figure-8 flex wasn’t a bad open wire RF transmission line for HF, and that it had a characteristic impedance in the 70 to 80 ohms region, just right for a dipole. Like many others, I tried it, and at a time when resources were scarce and coax expensive, it provided service.

You will still hear recommendations to grab 25m of the stuff, split down 5m and tie a knot in it, and there you have it, an instant 20m half wave dipole with 20m of feedline. A great antenna for portable work… well how good or bad is it?

Firstly, there is no such thing as “common” figure 8 flex, the stuff used in the US (known as ZIP Cord) is for a 110V supply system and has a higher current rating (ie thicker conductor) that the stuff used here in Australia.

I have characterised the RF performance of Australian 7.5A rated twin 0.5mm2 flex (24/0.2), which is near identical to the earlier imperial stranding of 23/0.0076. It’s characteristic impedance is 137 ohms, it has a velocity factor of 0.68, and it is relatively lossy.

Considering the dipole example above, the loss in the 20m of feedline, loaded with 65+j0 ohms, is about 2.9dB, so that 49% of the transmitter power is lost in the feedline. The line is not flat, and the impedance presented to the transmitter depends on the length of the feedline, and could be as high as 200 ohms, and may require an ATU to develop rated power from the transmitter.

By way of a comparison, the same load on the same length of RG6 coax (common inexpensive TV distribution coax) is about 0.4dB, less than 10% of the transmitter power is lost in the feedline.

It is a myth that Figure-8 flex is a good open wire RF transmission line with a characteristic impedance around 70 to 80 ohms, Australian 0.5mm2 Figure-8 flex (24/0.2) is relatively lossy and has a characteristic impedance around 137 ohms.

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Changes

Version Date Description
1.01 12/05/2005 Initial.
1.02    
1.03    

 


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