On 160 meters, for dxing I use a T antenna with a 252 foot flat top
(broadside NW-SE), a 75 foot vertical section, a second 55 foot section
(linear load) and one #12 1/4 wave radial on the ground surface. Per the
EZNEC 3.0 modeling program my T has a near omnidirectional pattern (2 db
gain towards Europe and Australia) with a takeoff angle of 25 degrees. I
load the antenna up through a parallel network. My 252 foot flat top slopes
on both ends down to 15 feet. Modeling shows that you can get similar
results by using a 180 foot flat top, a length more easily erected on 1/4
acre-ville.
I also have a 192 foot 3/8 wave L that is used for ragchewing. It has a
vertical section of 45 feet, a flat top of 147 feet (broadside N-S) and and
one #12 stranded 1/4 wave on the ground surface. Also per the EZNEC 3.0
modeling program, this 3/8's wave L has a near omnidirectional pattern (1 db
gain north and south) with a takeoff angle of 85 degrees. This antenna was
designed for close in (400 miles or less) ragchewing and henceforth the high
takeoff angle. However the 3/8 wave L can be turned into a low angle
vertical signal takeoff antenna by linear loading the vertical section. I
use a T network for matching.
I have used 450 ohm ladderline and 50 ohm coax as feedlines for both
antenna's and a balanced network with the 450 ohm twinlead . The twinlead is
kind of difficult to support but of course can be lower loss on real long
runs. Some people get tweaked by matching networks but I see no difference
in changing feedpoint impedance up or down by using high quality variable
condensers and rotary inductors or using distributed capacitance and
inductance by trimming the antenna and or feedline lengths.
This next statement will make some big gun dx'er antenna experts bristle but
my antenna's don't need an extensive ground radial or counterpoise system,
as both antenna's are much longer then a quarterwave and have the highest
current points elevated well above the ground surface in the near field,
which reduces capacitive coupling ground losses, as modeled on EZNEC 3.0.
Basically the other half of the antenna is present to be driven against,
much like a dipole. As the Fresnel zone is approximately 52,000 feet away
with a full vertical 1/2 wave T antenna and can be 100's of feet away with a
3/8's wave L, there is not much that can be done to reduce losses with an
extensive near field radial system, henceforth I drive my matching networks
against single 1/4 wave radials on the ground.
A four wire 1/4 wave counterpoise up at 15 feet will also work fine for a
3/8's or 1/2 wave L/T but you lose 15 feet of your effective vertical
section. Note, the elevated counterpoise is not a good idea for a simple 1/4
wave vertical or L, as you will see 5-6 db of loss in the near field ground,
as verified by W8JI. Use a ground mounted radial system and make your ground
system radials approximately the same length as the vertical section of your
1/4 wave vertical or L. You basically see rapidly diminishing returns after
12-24 radials but if you have time, money and room, then 120 1/4 or 1/2
wavelength radials under your 1/4 wave are best, as verified by W8JI.
You can read more about these antenna's by going to W4RNL's website or at
mine at http://kn4lf.tripod.com/index-6.html . If you have the EZNEC 3.0
modeling program, I can import complete antenna modeling files to you.
73,
Thomas Giella, KN4LF
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