My 160m antenna is unique in that I have never run into anyone else with
anything exactly like it. It consists of 127' of Rohn 25, insulated at the
base, with 120 buried radials each 133'6" long. Probably overkill, but I got a
good deal on a 16,000 ft spool of #12 bare soft-drawn back in 1974 when the
economy had just tanked and the price of copper dipped precipitously. A few
years later I constructed a homebrew radial plough that allowed me to bury the
whole roll in about 4 days.
The tower also holds up a 137' long doublet, attached at the top guy bracket at
the 119' level. The legs of the doublet slope slightly downwards, at 18 degrees
from horizontal, so that the ends are approximately 20' lower than the mid
point. The doublet is fed with a balanced open wire feedline, made of #10 wires
spaced 2" apart; Zo calculates to 438 ohms. The open wire line runs down
through the interior of the tower, with the two conductors spaced at the
geometric centre of the triangle, exiting the tower near the base. There is no
direct electrical connection between the doublet or feedline to the tower
structure at any point. A knife switch at the base of the tower allows both
conductors of the OWL to be shorted directly to the tower for lightning
protection, and there is a lightning ball gap across the base insulator.
Another knife switch in the ATU housing, about 8' from the tower, allows the
OWL to be disconnected from the dipole matching network and float free.
I have never tried a thorough analysis of this system, which would appear to me
to be very complex, but the antenna has been very effective ever since it was
completed in 1983. This antenna would probably be more accurately described as
a vertical tee, rather than a simple quarter wave vertical, since the close
proximity of the feed line along nearly the entire length of the tower appears
to couple the doublet very closely to the tower. The top-loading wires, formed
by each leg of the doublet, are each 68' 6" long. If the top loading wires were
bonded directly to the top of the tower in true vertical tee configuration, the
maximum current point should be near the mid-section of the tower, but I am
uncertain as to what actually happens with the present configuration.
I normally feed the base of the tower with both knife switches open and thus
the OWL to the dipole floating. The matching network consists of a parallel
tuned circuit with one end grounded and the tower fed via a tap on the coil.
The transmission line from the shack to the base of the tower is another
438-ohm OWL, coupled to the main coil through a 14-turn primary winding.
Designed largely by trial-and-error, the ATU allows the transmission line back
to the shack to be adjusted to a near-perfect match with no standing waves,
anywhere in the band by simply adjusting the variable capacitor in the parallel
tuned circuit that comprises the secondary winding.
Here are the measurements.
Usual operation, with the OWL floating:
1800 kHz, Z = 142 ohms + j286
1900 kHz, Z = 235 ohms + j378
2000 kHz, Z = 392 ohms + j477
Shorting the OWL to the base of the tower by closing the lightning switch,
radically changes the measurements:
1800 kHz, Z = 66 ohms + j59
1900 kHz, Z = 112 ohms + j145
2000 kHz, Z = 162 ohms + j213
These measurements were taken with the OWL grounded directly to earth via the
radial system:
1800 kHz, Z = 12 ohms + j52
1900 kHz, Z = 22 ohms + j62
2000 kHz, Z = 24 ohms + j69
I have never run any kind of test to compare distant signal strengths with the
OWL floating, shorted to the tower, and grounded to the radials, perhaps
something I should do.
Don k4kyv
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