9:1 Impedance Transformer

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.

9:1 unun. I plan to secure the toroid in place

3/16 inch brass crews, washers and nuts

Chassis mount SO-239 socket

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.

Frequency (MHz) SWR
3.700 1.5:1
7.125 2.5:1
10.125 3.9:1
14.250 1.7:1
18.000 1.0:1
21.250 3.0:1
24.750 2.5:1

27.2 metre Inverted V antenna

VSWR graph – 18.7 metre Inverted V fitted to the 9:1 impedance transformer.  Dark blue = 18.7 metres

18.7 metre Inverted V antenna

VSWR graph – 23 metre Inverted V antenna fitted to the 9:1 impedance transformer. Dark blue = 23 metres

23 metre Inverted V antenna

I will experiment further with antenna lengths at 25 and 26 metres


First published: 14 May 2017
Post Update: 15 October 2017