More on Open Wire Feeder August 19th, 2013
I have had quite a while to evaluate my pretty basic antenna system which comprises a top length of around 45ft fed in the centre with 450 Ohm ladder line. It’s a modern take on what was widely used years ago and is still used commercially today for HfF transmission. Open wire feeder has some very nice properties, the best of which is the exceptionally low loss, when there is a high VSWR. So why would you even want to operate an antenna with a high VSWR?
One of the easiest multiband antennas to make is one with a random top section that is fed at the centre with open wire feeder, or in my case ladder line. The 450 Ohm ladder line has the same natural impedance as its title suggests. And like open wire feeder it can carry high VSWR with minimal loss. In fact, for normal domestic lengths right up to 30Mz, the loss can be discounted.
Now at some frequency this top section is going to be a resonant half wave, but everywhere else it will have reactance and a VSWR that will go from fairly low to very high. We can deal with that VSWR at the transmitter end by employing a matching unit, more commonly known as an ATU. This does not change the VSWR on the feeder, but simply compensates for this awful state of affairs and presents the transceiver with a value that looks right and enables the full power to be sent into the ATU and up to the antenna.
Most ATUs, if correctly designed and set up, will have an insertion loss of less than 1dB, and as we have already discussed, there is virtually no loss on the open wire feeder. The end result is that all the power goes to the antenna, with minimal loss on any band we like to operate on. Of course the downside is that you have to set up your ATU for each band. But many of the modern auto ATUs can work with balanced line and make matching almost instant.
There are limits to what can be matched and I have found that if the top section is around a minimum of 3/8th wave long, then it will work on that band and everything above. Its a great antenna that often solves a lot of installation problems in small gardens where all band coverage is needed. Peter G3OJV