Bob wrote, in part:
> The GAP is really a vertically-polarized 1/2-wave antenna with
> tuning rods to provide the multi-band (and RG8X coax inside the
> main tubing for 80/75 meter operation) capability. Radials, per se,
> aren't required because there's no "missing" 1/4-wavelength to be
> made up for, which is the case in the Marconi.
> Both my GAPs were ground
> mounted......
> I personally do not consider these antennas dummy loads...
They are not dummy loads; however, electric field lines from
the high RF voltage near and at the end of the dipole, very
close to the ground mounting point, do couple to the ground,
and do induce current into the ground resistance: which current
should be radiating as displacement current in space!!
Here on Kauai, WWVH uses Rohn 1/2 wave vertical dipole
towers with BIG center, feedpoint insulators! Upper bands
use phased vertical dipole towers to generate cardiod pattern
propagation West. And, there is a HUGE radial field
installed to prevent exactly the E-field coupling to earth
as described above ! Estimate about half the radiated power
(3 dB) is now radiated, and saved from loss because of
these extensive radial fields.
The on-site engineer is now challenged to replace everything
in about 3 years; all is near end-of life since the entire
station moved from Maui and came on line here on Kauai
with continuous operation beginning in July, 1971, nearly
30 years ago now. Those antennas are "on the beach",
so have seen a lot of salt air, and many hurricanes blow
by, with Iniki right on them in 1992.
Problem the engineer now has is, Rohn is not quoting new
1/2 wave vertical, center insulated towers ! Evidently now
have a different view of liabilities. Rohn prefers steel all the
way up these days it seems, hi.
Beautiful station set up, multiple 10 kW transmitters, with
hot backups at every frequency of operation. Huge Caterpillar
tractor engine to drive the emergency 500 kW generator
plant when the local island power plant goes down, as it
did for weeks following Iniki. But WWVH stayed on line,
thanks to lots of diesel in tanks on station.
Anyway, it appears from the Gov's experience, that radials
under 1/2 wave vertical antennas, with the lower end near
ground, can result in a much higher efficiency radiating
system as judged from field strength measurements
way out in the Western Pacific.
More about the WWV / WWVH antennas at:
http://www.boulder.nist.gov/timefreq/wwv/quests/trans.html
73, Jim, KH7M
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