KB6NU's Ham Radio Blog

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Vertical, off-center-fed, half-wave antenna features single support,
minimum footprint

Posted: 15 Nov 2018 10:36 AM PST
http://feedproxy.google.com/~r/kb6nu...m_medium=email


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.Â* 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 andÂ*solving forÂ*X1 givesÂ*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 mindÂ*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.Â* Because it is a half wave antenna,
I dont 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 dont have the book, KH6GRT
has a great post on his website on how to build one.

I think that this design is rather creative. Its a resonant antenna, so it
should be quite effiecient, and you dont 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.

The post Vertical, off-center-fed, half-wave antenna features single
support, minimum footprint appeared first on KB6NUs Ham Radio Blog.