Summer 2008 Issue
2500 Miles with JUST a
Paul Lieb, KH6HME, the tropo voice of
Count the horizontal omnidirectional loop antenna in for your VHF/ UHF mobile/portable station. The tiny loop might also bridge the Pacific (or Gulf, Midwest, or East Coast) from a hidden attic installation.
“Every July, like Pacific Coast clockwork, a Pacific high builds in between California and Hawaii,” comments Julian Frost, N3JF. “Our first California-Hawaii opening this year developed on June 18th, and signals were so strong that I could easily hear the KH6HME beacon coming in on my attic loop, spanning 2500 miles!”
Often homeowner associations ban any sort of visible outside ham antenna, but a single loop might double as an over-the-air outside digital TV antenna, and can easily hide inside an attic for some exciting horizontally-polarized VHF/ UHF contacts:
50.125 MHz calling SSB
144.200 MHz calling SSB
432.100 MHz calling SSB
1296.100 MHz calling SSB
Tropospheric ducting is a weather phenomenon. Loop antennas, horizontally polarized, eliminate 10- to 20-dB cross-polarization loss if you are trying to work VHF/UHF SSB and CW from a monster collinear vertical or that big vertically polarized beam.
When weather conditions associated with a
stationary high-pressure system form, “tropo” signal enhancement can be
so pronounced that a 0-dB gain horizontally-polarized loop can span over
the water to a distance greater than 2500 miles!
The horizontal loop antenna is a half-wave dipole formed in a circle to offer no-null performance in all directions. There are multiple commercial manufacturers of single-band VHF/UHF loops (see References), including one unique loop that offers 2-meter and 432-MHz performance—one loop, two bands. This is handy when operating multi-band HF equipment that outputs 2 meters and 440 MHz to a single antenna connector. This dual-band loop allows you to jump from 2 meters to 70 cm without any coax switching.
The popular single-band 2-meter horizontal loop may incorporate a capacitive end-element termination, compensating for its relatively small size, for compact, sturdy mobile operation. The feedpoint may include a gamma match and/or series capacitance, usually encased within a protective cover. The loop antenna offers high-Q performance with an extremely sharp SWR drop at the bottom of the 2-meter band. Rain or icing may dramatically de-tune these small loops, so each manufacturer has developed its own unique capacitive end-loading scheme—some using a gap in the loop, and others terminating to a solid insulator at the loop ends.
Of great importance to the construction of
these loop antennas, including home-brew, is zero vibration of loop
elements where they join capacitive matching assemblies. During the 20
years of testing very-high-frequency loops, noted VHF/UHF SSB DXer
Frank, AA2DR, concluded that only the most rigid loop element structures
will withstand typical mobile operation. And when ice storms hit Long
Island, Frank reports the immediate demise of both mobile loops as well
as mobile loop performance over the air.
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