KFBK's are actually two, phased for day-night directional service. Been
awhile since I saw them but the bottom insulators were maybe 10 ft above
ground level [I'm 6'2" and it was just above my reach] so the feedpoint
is a tiny bit higher than 1/4 wave. 1/2 wave at 1530 KHz is just over
300 ft and the feed system at 50 KW is mechanically complex and pretty
heavy. I think [not sure where I read it but it was moderately
recently] that KFBK's Franklin is the last AM broadcast one in NA.
NIST photos of WWV HF antennas show about the same height more or less.
WWVH uses 2-element phased elements to put a cardioid null at the CONUS
to reduce interference. I would think that getting anything [including
ground] farther out in the near field would reduce losses. The choice
is an engineering cost-benefit trade off of course.
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
On 9/10/2018 11:41 AM, Jim Brown wrote:
On 9/9/2018 7:02 PM, Wes Stewart wrote:
The WWV antennas are center-fed vertical dipoles.
Thanks for the reminder about this -- I vaguely remember reading about
their antennas years ago. Question -- from the description, is the
feedpoint higher than a quarter wave above ground? A few years ago, I
did an NEC modeling study of HF verticals that showed that doing that
improved the vertical pattern and seemed to suggest that it reduced
ground losses. Your thoughts?
The study is here. http://k9yc.com/VerticalHeight.pdf
73, Jim K9YC
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