2 Meter 3/8
Wave Low Angle Radiation Vertical by K5OE
additional notes and a "base
station modification" by
Some time ago, we
ran across these cool plans for a mobile vertical for 2 meters on
the K5OE web site which is no longer on the web, and we
realized that it had a very low angle of
radiation designed for use with the LEO satellites. Since it
already has a very low angle of radiation, why not adapt it for
"base station use" for regular 2 meter land based communications. It
seemed to us that since the pattern was much lower than many
conventional base station home brewed antennas, why not modify
it for base station use to get our signal lower to the ground where
it is needed most instead of heating up lots of air at a higher
What follows is the complete article by K5OE that he
has kindly shared with us on his mobile design with some
suggested "mods" for using it on a fixed mount instead of your
mobile for regular 2 meter simplex and repeater use....read
Do you have trouble getting
your mobile signal into the LEO satellites? Try this small 2 meter
vertical antenna with your mobile rig or HT and enjoy more success
in your uplink. I built this small vertical because I could not
uplink very well at low elevations and I just could not bring myself
to drill holes in the roof of my new truck to install a more
substantial antenna. I had been using a very common 1/4 wl mag-mount
with only marginal results.
Design: This is a high-efficiency "gain"
antenna. It is not mounted permanently: I have a magnet attached to
it and "throw" it up on the roof when I want to work a LEO
(Notice the very low
angle or radiation...about 8 degrees!)
The antenna is an atypical
vertical: instead of the common 1/4 wl vertical monopole or 1/4 wl
ground plane with 1/4 wl radials, this design employs a 3/8 wl
vertical section and short radials to complete the "ground plane."
Effectively, an off-center-fed vertical dipole that does not rely on
the earth or, in my case the truck body, to complete the bottom half
of the antenna.
This is an important point.
The fact the antenna has a fully contained lower half, i.e.,
the ground plane, makes it very efficient. This is especially
significant when compared to my mag-mount monopole antenna where the
"ground plane" had to be completed through the coax, through the
rig, then to the vehicle body. I suspect the mag-mount was not very
efficient at all. The dipole is off-center-fed to get a better match
to the feedline and the bottom "half" of the antenna is completed
with capacitive reactance from the four shortened radials.
design has an honest 3 dBi of gain at 6' elevation (2 dBi
free-space) with a suitable pattern for LEO communications--favoring the horizon. At
20' high, the same antenna exhibits almost 6 dBi of gain. The
feedpoint is a nominal 50 Ohms at 146.850 mHz.
Construction: The antenna is built using a
3/4" PVC tee, a cap for the top, and a plug (flat on the bottom) for
The tee is arranged vertically
with the top cap drilled for connection of all 5 elements (I used
6-32 stainless steel bolts/nuts/washers) and the bottom plug is
drilled for connection of a magnet (optional).
The coax is fed thru the open
side of the tee and connected directly to the elements via ring
lugs. Alternately, a lower-profile version could be constructed
substituting a coupling for the tee and drilling a hole in it for
the coax to exit. I connected the center of the coax to the mast
(the vertical section), and the shield to one of the radials
using crimp style ring terminals. I then wrapped some small gauge
wire around the outside of the cap, connecting all four radials
together, and covered the assembly with electrical tape and paint.
(The figure above shows the layout and dimensions (in
cm) of the elements.) (See
text for dimensions in inches.)
(64 cm) vertical section is combined with four 7-3/16" (20 cm)
I recommend you make the elements slightly
longer and then trim them based on SWR readings. I used 10 gauge
insulated wire for all the elements, but 1/8" or 3/16" aluminum rod
would be a suitable material--and likely more durable.
When tuning for minimum SWR, I
eventually pruned the mast to 24-3/4" and the radials to 7" to get a
1.2:1 SWR. That is much better than my mag-mount ever
showed so I stopped fine-tuning and put it on the air.
The radials are angled down at
about 30 degrees. This angle can be adjusted to get the SWR perfect
once the vertical mast is trimmed for best SWR at the desired
frequency, but I found the effect minimal. You could also "wind" a
coil of a few turns in the center of the mast to lower the profile
of the antenna without affecting feedpoint impedance drastically or
performance too much. See picture on right, below. I did that to
make it short enough to fit under my garage door header and note no
significant difference in SWR or performance.
Testing an early "straight"
prototype on the left and The final "shortened" model on
the right above. (Pictures have been
compressed in size slightly to fit
accomplished. I can now get "into" the birds at low
elevations. On it's maiden journey out of the garage, I
worked four stations (one was marine mobile) on an 8
degree AO-27 pass. If I could hear the bird, I could work
it (as long as a "big gun" station did not have the bird already
captured). At high elevations I found I could run on low power
(about 3 Watts) and capture the bird with little difficulty.
This antenna makes a nice 1/3
scaled companion to the 70 cm
Handi-Tennaor can be used for any fixed, mobile, or
portable service. Since the elements are flexible 10 gauge wire,
they can be readily folded and unfolded for backpacking.
Suggested modification for base station fixed
location: After you have built the
antenna above or sometime during the process of assembly, simply
fabricate yourself a mount for the base of the antenna to securely
rest on and mount it up as high as you can get it on a mast in the
clear. If you use the dimensions above,
the antenna will most likely have it's lowest swr at 146.850mHz, so
some adjustment of the vertical length and or the radials may be
needed for operation in your selected portion of the 2 meter
You could also use a small
metal bracket with a hole cut out that will accept an SO-239 bolted
to it and then attach the radials to the bolts and the vertical
section of the antenna to the center conductor of the SO-239,
(mounted upside down of course),..mount it to a mast up high and
enjoy...use your imagination and have fun with it! 73,
Many thanks to Jerry,
K5OE for sharing his expertise and fun with