Vertical, off-center-fed, half-wave antenna features single support, minimum footprint november 15, 2018 by dan kb6nu

The other day I worked Charlie, KM4ZZ. As I normally do, I loaded up his QRZ.Com page. What I found was this little gem: a description of the vertical, off-center-fed, half-wave dipole antenna that he uses for portable operations:

I use a variety of antenna systems. At home I have a 40M dipole up 35 feet. In the field I have horizontal dipoles, end fed verticals but usually I’m using an off center fed vertical half wave. I also use the 10 foot vertical whip on the PRC-104s on occasion.

Amateur radio for me means operating portable.  A high RF noise level in and around my house forces me into a nomadic existence. My first antenna was an inverted “V”, but setting that up in a public park was a real hassle. I was 30 plus feet under the feed point, but the guidelines were 60 plus feet out in two directions. This took up too much real estate in a place with kids, bikes, dogs and Frisbees all running around just looking for a way to stumble into a stake or guideline. I needed an antenna with a single support that minimized my footprint. I had great success with an end fed antenna following Steven Yates, AA5TB excellent web pages, but I wondered if I was losing some efficiency without a defined counterpoise. A center fed half wave would have the feed point too far off the ground, so the result is this: a VERTICAL, OFF CENTER FED HALF WAVE.

The feed point was chosen to give an impedance of 450 ohms. This would be only about 4 ½ feet off the ground and would allow a 9:1: Balan to be used to bring the impedance down to 50 ohms. Add in a 1:1 Balun to make the system a 9:1 Unun and the system is complete. The 9:1 Unun was designed with the assistance of Jerry Sevick’s excellent tests. This is a critical statement. Numerous eperiments with a 9:1 Balun failed until I realized I needed a 9:1 Unun!

I started with 18 gage automotive hookup wire, cut longer than the design lengths.

X=(150 Meters)/(frequency in MHz)

X1=0.131 X

X2= X-X1

X1 comes from the following equation that given us the feed point impedance of an off center fed half wave antenna.

Z=(72 Ohms)/(Sin(pi*X1/X)^2

Setting Z equal to 50 ohms and solving for X1 gives the correct feed point location

Using an antenna analyzer, I tuned the antenna by carefully shortening the lengths of the short and long arms until the resonate frequency was correct and the SWR was very close to 1:1. If you start with wires that are two long, it is important to remember that any cut on either arm raises the resonant frequency. A cut on the long arm also decrease the feed point impedance, and a cut on the short arm raises the feed point impedance. With these rules in mind I was able to achieve an SWR of 1.05 and a resonate frequency right in the middle of the SSB potion of the 20 Meter band. The whole band has an SWR of less than 1.3.

The antenna works very well. I use 15 feet of R-8X coax to hook up my trusty Yaesu FT-817. With five watts I get great reports from all over Europe and South America from Virginia.  Because it is a half wave antenna, I don’t need any counterpoise which makes set up easy and minimizes the footprint.

When I asked for permission to publish this description, he stressed, “I think my description of the antenna is a bit incomplete without a more careful description of the 50:450 unun from Jerry Sevick’s book, Buildling and Using Baluns and Ununs. I found that only a Guanella type transformer works here, the Ruthroff type does not. ” If you don’t have the book, KH6GRT has a great post on his website on how to build one.

I think that this design is rather creative. It’s a resonant antenna, so it should be quite effiecient, and you don’t have to muck around with counterpoises. You could probably even make a fan version of this for multi-band operation, or simply have a set of elements for each of the different bands that you want to operate.