MARTIN - G8JNJ

I recently started looking at various wire antennas which I could fit in my 100ft long by 40ft wide garden. The plan was to try out a few different configurations to see if I could improve my signal on 160m and 80m. At one corner of the garden there is a 50ft high pine tree, which is the highest support available. The house and another 40ft pole provide other points for attaching support ropes.

My options were:-

• Increasing the size of my existing G5RV type antenna to twice size
• A 132ft inverted L
• 70m Diameter loop
• An 80m long Carolina ‘Windom’ or Off Centre Fed Dipole
• An 80m long single wire fed Windom

I made models of all the antennas on EZNEC and found that the Single Wire fed Windom seemed to meet all my criteria. However modelling can only provide a guide to actual performance. Factors such as physical construction and support, Signal to Noise performance on receive and the level of RF induced into nearby objects and property also have to be taken into account.

Twice size G5RV. This seemed like a good starting point; however by doubling the size the match on several bands became poor and my feed arrangement (coax fed remote auto-atu) resulted in a worse signal than the normal size version.

132ft inverted L. This worked quite well when fed with a 4:1 balun at the base and the remote auto-atu. Some elevated counterpoise wires running along the fence under the wire helped to improve radiation efficiency. On frequencies below 18MHz the inverted L performed better than the G5RV, but above 18 MHz it was worse.

70m Loop. This also worked quite well once the loop size became greater than twice the wavelength. However at low frequencies the results were comparable to the 132ft inverted L and the G5RV’s. It was difficult to obtain a good match on 160m and the gain seemed lower than the 132ft inverted L. I also found it difficult to support one of the sides of the loop as I could not achieve enough tension to stop the wire sagging in the middle.

Carolina Windom. 80m long. This is not really a Windom at all, but an Off Centre Fed dipole with a radiating feed line. I started constructing one of these but soon came to realise that the balun was a compromise. One of the main features of the Carolina Windom is that it is coax fed. However a 4:1 balun is inserted at the feed point on the flat top portion of the antenna where the impedance should be somewhere in the region of 200 - 400 ohms. A portion of the vertical coax feed is deliberately used as a radiating element. In order to facilitate radiation from the feeder, the balun is a 4:1 voltage balun, which provides very poor isolation and balance between the input and output ports. In order to isolate the antenna from the feed cable a further 1:1 current balun is fitted between 20 to 40 feet down the coaxial feed. The use of a voltage balun (auto-transformer) struck me as a very poor feed arrangement with a limited bandwidth, which introduced a lot of additional complexity in comparison to a standard single wire fed Windom.

Single wire fed Windom 80m long. I modelled the Carolina Windom but found that a conventional single wire fed Windom had marginally more gain. It seemed easier to feed the single wire against either an earth mat or counterpoise wires, with a 4:1 current balun at the base of the antenna and the existing coax feed back to the auto-atu in the shack. This provided the best results on 160, 80 and 40m but performance on the HF bands was degraded, especially on 28 and 50MHz, due to the high angle radiation and scattered polar plots. The performance of this antenna is generally very good, but could be further improved by use of an auto-tuner is used at the feed point, rather than the 4:1 balun and remote auto-tuner configuration I use.