Actually Jack, you can achieve a near perfect azimuthal radiation pattern
with just one single radial.
The trick is to run it in one direction for about 1/3 of its length, bend it
180 degrees and back on itself, spaced about 3 to 5 inches apart, past the
vertical and extend in the other direction.
2 advantages:
- achieve nearly symmetrical radiation pattern in all directions
- the space occupied by the radial is less than a single radial would take
up.
The creator of this idea was Les Moxon, G6XN (SK).
I have built and used this on 40m.
The only other things to note is that the length of the radiator had to be
extended about 10%.
I will guess that the folded radial resulted in a capacitance, which
compensated for the additional inductive reactance, and has the added
advantage of raising the feedpoint impedance to very close to 50 Ohms.
I had to play with the overall length of the single radial, as well as how
far it extended in each direction.
This will vary depending on how much spacing you have in the folded radial.
For starters, you can make it 1/3 of its total length in each direction.
Somewhere I have a write-up of this antenna.
I call it the "One Arm Bandit Antenna".
I gleaned it from RSGB's RADCOM about 7 or 8 years ago.
73
Rick
-----Original Message-----
From: tentec-bounces@contesting.com [mailto:tentec-bounces@contesting.com]
On Behalf Of Jack Mandelman
Sent: Thursday, January 06, 2011 1:37 PM
To: tentec@contesting.com
Subject: Re: [TenTec] New and Improved Terminology (NVIS origins)
And, as the base of the ground plane vertical is further elevated above
ground
a smaller portion of the near field fringes through the ground, and the
efficiency
increases. In the infinite height limit (free space) all of the near
field is between
the vertical element and the radials. At about a quarter wave above
ground a single
radial is highly efficient, although the azimuthal radiation pattern is
asymmetric.
Such an approach may be used to provide some directivity in a direction
of interest.
Keep in mind that as the centroid (the base of a standard ground plane)
of the
vertically oriented current distribution increases above a quarter wave
above
ground, a larger fraction of the energy is radiated at higher angles.
I have obtained good results with the base at a quarter wave above
ground and 2
radials. In this configuration the input impedance of the antenna is
negligibly
affected by seasonal changes in ground moisture content (wet vs dry),
since coupling
to ground is weak.
Jack K1VT
> That's true. But the best solution is to use raised radials a few feet
> above the ground (I used 10 feet). Just 3 raised radials has been
> show to
> make the antenna as efficient as 10 to 30 ground radials. And you don't
> have all the work of burying the radials.
> Carl Moreschi N4PY
> 121 Little Bell Drive
> Hays, NC 28635
> www.n4py.com
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