[Antennas] RE: no radial vertical antennas

[David Kelley]

I would just like to add that it IS possible to build a highly
efficient vertical antenna without radials.  All you do is turn
a dipole antenna 90 degrees from horizontal to vertical.  A
vertical dipole radiates vertical polarization, but it needs no
radials.

The primary practical disadvantage of the vertical dipole is
that it is necessary to run the feedline away from the
antenna at right angles.  An alternative is to move the
feedline from the center closer to the bottom end while
keeping the total vertical length equal to 1/2 wavelength.
The input impedance at the feedpoint will be substantially
higher than the 70 Ohms found at the center.  The actual
value will depend upon how far away from center you move
the feed.  The farther from center, the higher the impedance.

You can now "split" the short bottom section into three or
four pieces and bend them up at 90 degrees or so,
spreading the pieces out so that they have equal angles
between them.  It will look like the antenna has three or
four short radials.

For example, you could use a 3/8-wavelength piece of
aluminum tubing as a vertical radiator and feed it at the
bottom.  The center conductor of the coax would connect
to the bottom end of the tubing.  The braid would connect
to three or four equally-spaced "radials" that are each 1/8-
wavelength long.  In effect you would have a vertical half-
wave dipole fed 1/8-wavelength from the bottom end, with
the bottom end "frayed" and bent up 90 degrees.  I suspect
that many of the "no-radial" verticals have this design, or
something like it, as their foundation.  This type of antenna
should be very efficient as long as the overall length is 1/2
wavelength or close to it.

By the way, you would have to use a matching network
or transformer at the feedpoint to transform the high input
impedance of the antenna to the 50 Ohms or so of the
coax.  A choke balun might also be needed to keep
current from flowing along the outer side of the braid,
although the short radials might achieve that purpose to
a sufficient degree.

73,
Dave


>  Hi Reggie:
>
>    You have hit on a topic that causes a great deal of consternation 
> among readers in this usergroup.  In any case, it is a valid question 
> that you should have the answer to before spending alot of money on an 
> antenna that fails to meet your expectations.
>
>    The antennas hailed as "radialess" verticals are not really verticals 
> at all but are end fed windoms utilizing loading coils and, in some 
> cases, capacity hats to deliver an acceptable SWR over a number of 
> bands.  They go by the name of Cushcraft R-7, R-7000, R-8000, the MFJ 
> 1796 and others.  As a group, they do not perform as well as full size 
> arrangements, but they do deliver a system that will get you on multiple 
> bands, albeit with a less than commanding signal. This is a relative 
> comparison, but one that is supported by the many users reporting to 
> www.eham.com [reviews]...
>
>--- On Sun 04/14, Reginald Mackey  wrote:
> > I would like to install a vertical antenna and I am confused as how some
> > of
> > them are advertised to work without radials? Even the 80" radials
> > seem to
> > short according to typical vertical theory of operation. What is the
> > efficiency of these antennas? Can someone shed some light on these
> > antennas
> > for me? For 3 to 400 dollars I would not want to put 1/2 of my power into
> > the ground.
> > Thanks in Advance,
> >
> >             Reggie

--
Dave Kelley, NB4J
State College, PA
Centre County