"Ugly Duck" coaxial vertical dipoles for FM
easy, inexpensive, and better than expected performance
Inverted low profile vertical installation
By Steve Culp W5SDC
With the advent of linking amateur radio to the internet via Voice over IP, more and more HAMs are discovering and using the power
of this technology to link low power, limited distance radios and antennas to repeater systems and conferences all over the world. Good amateur practice and FCC rules
dictate that our first responsibility is to always establish communications using the lowest and least disruptive resources possible. A small, vertical FM dipole antenna
serves this purpose very well not only for link radios, but everyday general purpose use also. The simple coaxial dipole is certainly not a new invention; however, it is an antenna that always
seems to get the job done in a elementary and effective way. The dipole has been and always will be in the roots of amateur radio.
I pondered some ideas that I had found online from a HAM that had home brewed a 1/4 wave FM antenna using a metal
tube from an old floor lamp with the coax passing through the middle with the shield soldered to the tube. I also saw a design using copper tubing on smaller coax with
the shield tied to the tubing. I decided to try to emulate these designs using a piece of LMR 400 coax that I had in stock. I cut a portion of LMR to 1.3m 1/4 wave proportion and tuned it for the 220 band
since I had already stuck a 144/70 cm mobile antenna to a large square vent up on the roof as a temporary measure. I was impressed by how well the first Ugly Duck performed on 220 so I decided to build one for 2m and 70 cm to diplex together on the only coax run that I had left to the back corner of the shack. More on how the Ugly Duck got it's nickname later.
|Band||1/4 wave radiator
||2 m||49 cm
||70 cm||16 cm
Transmission pattern illustrates why inverted operation works just fine. A perfect specimen will produce 1.76 dBi of gain.
To get started, simply cut a piece of LMR 400 to 2x the quarter wavelength of the desired band2 plus a few inches to make sure that you have
enough distance on the bottom to slide the shield up and down for tuning and the addition of a Pl 258 or 259 connector if desired.
Without damaging the shield, carefully strip off the outer insulation of the coax to approximately 1/2 inch longer than
the length of the radiator that you require. Using both hands, gently push the shield toward the remaining coax sheath
so that it will compress and start to bulge to a diameter larger than the full outer size of the coax. Carefully roll the shield
inside out over the outer insulation leaving the center insulation and radiator exposed. If you desire, you can snug the
shield down with a tie wrap at this time. Note: Not trimming the tie wrap will leave you a "handle" to use while tuning as touching braid while tuning will
throw your readings off. A few "wild hairs" in the braid after fold back are OK as long as they are not shorted to the center conductor. Just trim them back with a pair of small scissors.
Next, strip the center conductor insulation off leaving approx 1/8 to 1/4 in above from where the shield folds back. If the center
conductor insulation is not shielded with foil as mine was, then this step is probably not required. Now the radiator
can be cut to match your desired 1/4 wavelength. Cut the exposed portion of the center conductor to exactly 1/4 wave measuring from the
stub to the end of the radiator. Install your PL-258 or 259 at the bottom and you will be ready to check it for shorts with an Ohm meter and then to tune her up. Taping the braid
at the bottom of the tie wrap to prevent sliding during handling and installation is highly recommended.
The assembly slides easily in to 3/4 inch Schedule 40 PVC which did not affect the tuning on the three flavors that I built. I don't see
any reason why just putting this on the end of a piece of coax and clamping or stapling it in place a few inches below the radiative shield wouldn't make a dandy
attic antenna that would be easy to horizontally polarize for SSB operation if desired. Note: My tuning experience revealed that this antenna can be tuned for optimal SWR on either but not both polarizations. Or, at least
it couldn't be on my test bench.
The antennas that I built were both the necessity of economics and invention using the LMR 400 that I already had on hand. My original intent was only to create a 220 antenna
for my Alinco DR-235 220 Mhz AllStar Link radio, but during testing I discovered many full quieting signals from local repeaters up to 20 miles away. Good signal reports
on 5 watt transmissions as well. The 2m and 440 Mhz versions have similar performance as well. I even worked a little vertically polarized SSB on 144.220 the other day with K5WET
who was testing a new homebrew beam.
When I was playing around with the 440 version shown in the picture above, I screwed the PL-259 onto a BNC adapter and popped it onto my Alinco
DJ-596 dual band talkie to compare 440 performance to the factory duck. At sixteen inches high it blew the factory antenna away but it
sure made the HT look ugly. Hence the name: Ugly Duck. But in its defense I think that a little heat shrink or PVC can transform it into a beautiful
swan for your particular application and budget.