I have constructed a 9:1 unbalanced to unbalanced (unun) impedance transformer based on a published design by Peter VK6YSF. Over the next six months or so my plan is to experiment with an Inverted V end fed antenna for 80 to 17m, I’m not aiming for transmission on 12 and 10m.
My current 80m SOTA antenna is a home-brew half wave inverted V dipole, which will be useful as a baseline to compare an end fed arrangement on a hilltop. BTW there is nothing wrong with my 80m half wave dipole, I just like to experiment with different antenna options, after all our hobby amateur radio encourages experimentation and home-brew construction methods. Brand name wire antennas and baluns can be constructed at home.
Post update 15 October 2017. I have combined the 9:1 impedance transformer with three different Inverted V antenna lengths, 18.7 metres, 23 metres and 27.2 metres with the Inverted V appex at 9 metres above ground level. The feed line is a 3.5 metre length of RG58CU Mil Spec. My test instrument is a RigExpert AA-600 analyser.
Jaycar L15 ferrite toroid
Jaycar waterproof plastic case
1 mm enamelled copper wire 18 gauge
2 3/16 inch brass screws
4 3/16 inch brass nuts and washers
2 3/16 inch solder tabs
1 SO239 flange socket
4 3 mm screws, nuts and washers
Testing of the input impedance between 1.8 MHz and 30 MHz, as seen by a radio at 50 ohms, produces a good VSWR match with the antenna terminals terminated in a 450 ohm resistive load.
VSWR graph (0 to 30 MHz) of the 9:1 impedance transformer combined with a 27.2 metre Inverted V antenna. Antenna appex is 9 metres above ground level while the far end is 2 metres above ground. The feedpoint 9:1 impedance transformer is about 50 centimetres above ground.
VSWR graph – 18.7 metre Inverted V fitted to the 9:1 impedance transformer. Dark blue = 18.7 metres
VSWR graph – 23 metre Inverted V antenna fitted to the 9:1 impedance transformer. Dark blue = 23 metres
I will experiment further with antenna lengths at 25 and 26 metres.
Post update 1 Nov 17. I have worked ZL on 20m and VK2 on 80m using a wire length of 25.9 metres.
Post update 26 Dec 17. I have update this page to include an updated VSWR plot below. Compare the results of the 25.9 long wire below versus 27.2 metre long wire antenna above. I think I will revert back to 27 metres.
VSWR graph – 25.9 metre Inverted V antenna fitted to the 9:1 impedance transformer
VSWR graph – 25.9 metre Inverted V fitted to the 9:1 impedance transformer. Dark blue = 25.9 metres, light blue = 27.2 metres
First published: 14 May 2017
Post Update: 26 December 2017