This is the second part of the FeedLines article, and deals with lower freqencies and the use of open wire feeder.

 

The impedance for openwire feeder varies with construction, but is generally in the order of 450ohms. Openwire can be manufactured in the hobby environment quite easily by stretching out 2 lengths of wire (hard drawn single strand copper is the best, but any will do) parallel to each other, then placing spreaders between them at regular intervals.

The distance between the wires is dependant on the output power of the transmitter used, the higher the power, the wider the spacing. This is to prevent “flash over” on high voltage peaks on the line.

For lower output transmitters, 300ohm ribbon cable (often sold with Band II FM tuners as the antenna) is also usable. If using this, it should be noted that the output power should not be allowed to exceed 150watts, or flashover in the clear dielectric between the conductors could occur. This feeder is not recommended without some form of treatment for outdoor use, as water will collect on the dielectric when it rains increasing the risk of flashover.

Another variation on this theme is the “slotted ribbon line”. This is a “half way house” between 300ohm ribbon cable and full open wire feeder. Its impedance can be anywhere between 300 – 450 ohms, and due to its construction, is easier to deal with than full openwire feeder. The slots in the dielectric mean that it is not as susceptible to the effects of dampas ribbon feeder, however, the dielectric between conductors can harden an snap allowing the conductors to close up. This causes an impedance hump in the line and in extreme circumstances, flashover on transmitter voltage peaks. If this feeder is to be used, then it must be inspected regularly to ensure that it is in good mechanical order.

For any of the above open wire type of feeders to be used, it is imperative that matching units are used to match the impedance of the transmitter to the feeder. Most modern low power transmitters are in the order of 50ohms impedance, so direct connection to a frequency resonant dipole via openwire feeder will immediately result in an SWR of 6:1 or greater, which some valve PA’s will tolerate for a modest period of time, but can be considered to be suicide for a transistor PA. The design and use of “baluns” (balanced to unbalanced) and ATU’s (Antenna Tuning Units) is beyond the scope of this article, but both are required for the above.

Finally, for signals in the VLF (9 – 30Khz), LF(30 -300Khz) and MF (300KHz -3Mhz), any of the above systems used for HF signals can be used. The constraints about the construction of openwire feeder are even more important, due to the inability to erect efficient antenna systems (a half wave dipole or doublet antenna at 198Khz (BBC Radio 4 Long Wave frequency) is 758m long or nearly 1/2mile!!!) means that the transmitters are usually far higher output power. Also, the matching units for such antennas are substantially larger, again to reduce the possibilities of flash over during transmitter voltage peaks.

This concludes my brief look at feedlines. There is loads of information available both in books and on the internet regarding this subject. Although not the most exciting aspect of radio work, get this part wrong and you will lose that important output power, and in a transciever envirionment, this  loss is also there on recieve. So, that really large 17element 2metre beam you have just erected doesn’t work as well as joe’s 4 element quad down the road?? Did you use the right feeder and install it properly…..? Think on…!!

See you soon,

 

Alan.

 

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