I was enjoying a QSO on 75
meters with a
fellow who at one point proclaimed that his "full-wave loop"
antenna had
"gain" [over a dipole]. This was curious. When I asked him
in which direction the marvel achieved this gain, he replied
"all directions of course." Ignoring resistive and
ground losses, [which should be very close to the same for a loop and a
dipole] gain is achieved by directivity - reducing radiation in
one direction, "refocusing" it in another. Unless something is
very wrong, a loop won't achieve more gain in "all directions."
Many hams seem to believe that for 160 and 80 meters, a full-wave loop
is superior to a dipole. EZnec - based comparisons of
the two show
differences in the patterns and elevation angle of departure of the
main lobes, but the aggregate radiation is about the same. Height
above ground greatly influences this, and one or the other may be
optimal in certain azimuths and
elevation angles, certain polarities, - better than the other in a
particular situation, but for my money, a properly constructed and deployed
inverted vee dipole is a sure bet. A 3D pattern showing horizontal and vertical
polarity radiation over real ground is shown at right. Amazingly, according to the EZnec model,
it's almost omni.
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Although one does
not need a 1:1 SWR to achieve
good transmit efficiency, the transmitter (3cx800s in my
case) may like the load better. Particularly on 75
meters, getting a good match across the band can be impossible with a
conventional half wave dipole. For years the handbook has
shown the 130 foot center fed "doublet", using open wire line to be a
recommended multiband antenna. I think that if you can deal with
the line routing issues, it may well be the very best multiband
antenna. Balance is important for this type of
transmission line to work as intended - each side must be exactly
symmetrical to keep current the same in each conductor. My configuration is inverted-vee style with the apex
at 80 feet, the ends about 25. When I constructed it, I used a
capacitance meter from each side to ground while adjusting end heights
to match the C.
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True
open wire line
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From Terman, Radio Engineering,
2nd ed. p 700:
"The losses in a properly constructed line can be made very
small.
Thus by combining [equation references ...] it is found that a two wire
line made from No. 4 copper wire and having a characteristic impedance
of 600 ohms introduces an attenuation of 0.24 db per thousand feet at
10 mc."
He continues:
"A two wire transmission line radiates very little energy because the
close proximity of the two conductors carrying current in opposite
directions very nearly cancels the radiated field. Analysis shows
that
in the case of a perfectly balanced two-wire line the total radiation,
including that of the terminal connections, is twice the radiation that
would be obtained from an elementary antenna having a length equal to
the line spacing ... "
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Commonly
available ladder line is not exactly the same however. It's smaller -
typically 16 gauge, and has much more lossy dielectric material between
conductors. Nevertheless, for operation at 3-7 MHz, the losses
should be low.
If however, one wants the ultimate HF line (or is simply enamored with
nostalgia) one constructs ones own line. I used 10 gauge copper
wire spaced 2 inches apart (radiation is negligible, but lets minimize it with minimal spacing)
with ceramic steatite (yes, they are still made) spacers. Its
characteristic impedance is about 450 ohms - this value is not critical
- even though the line will operate with high SWR (on 75 meters the
load will be near 70 ohms), the matchbox presents a near perfect match
to the transmitter. Due to the low dielectric losses, line loss
remains low.
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The
Johnson kilowatt matchbox was made when this type of multiband antenna
was very common. It's unique in that it uses no balun to convert
a
single ended network to balanced operation, as most modern units
do. The performance of
some ferrite core baluns deteriorates as the load substantially departs
from the intended impedance - its not uncommon with this multiband
antenna system to
operate the line at 10:1 SWR with very low losses, and a perfect match
to the transmitter. From a
flexibility, efficiency, and transmitter load impedance standpoint,
this is the superior system. The tradeoffs are that
significant QSY requires some knob twisting, and unlike coax, when the line ices up,
the tuning changes significanty. In addition, the line must be
kept well away from conductive and semiconductive materials to maintain
balance. Quick settings for QSY are achieved with
a look up table of known calibrations, and icing happens maybe once
per year in Kentucky.
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