>NEC models clearly show that a radial system has a rather small effect on the
>far field signal strength for ANY half-wave vertical dipole. A major advantage
>of a half wave vertical is that it raises the location of the current >maxima,
>which lowers the vertical pattern.
"Modeling" is hardly conclusive and you don't say how long your
modeled radials were. They have to be 1/2 lamda to return the induced
currents in earth from the current maxima halfway up the driven
element.
>This is also true of a loaded center-fed vertical, and of a quarter-wave base
>fed vertical. See http://k9yc.com/AntennaPlanning.pdf which ran in National
>Contest Journal several years ago. Antennas that have been >shortened by
>loading, even when the loading itself is capacitive and relatively low loss,
>produce less field strength than a full size antenna. We learned this when
>designing and building loaded 80M verticals county >expeditions and Field Day.
None of this has anything to do with my point.
>Broadcast antenna systems are part of the license, and historically were
>dictated by the Commission. Also, some of the stations currently using half
>wave radiators started out as 5/8 wave. Most were Clear Channel >stations, 50
>kW with antennas designed to cover half the country, and protected from
>interference in that protected coverage area. That all changed 30-40 years
>ago, when these stations were protected for only about 700 >miles. I suspect
>that change may be the reason that 5/8 towers were shortened -- the 5/8 tower
>has a broad, strong lobe that peaks at about 55 degrees and may fill in skip
>zones better than the narrower vertical pattern of >a half-wave radiator.
>Also, AM broadcast radial systems were historically pretty much cookie cutter
>designs. NEC modeling I've done on half-wave radiators show that radials have
>relatively little effect, but NOT zero, and >they become increasingly
>significant as the antenna gets LONGER than 180 degrees. When these stations
>were built, FM
broadcasting didn't exist, and AM was TV today, so stations were going for
every fractional dB to >increase the potential audience they could plot on
coverage map to show advertisers.
There's nothing here that's relevant including the incorrect broadcast
history lesson. WSM incorrectly attempted to operate with their Blaw
Knox at around 230 degrees initially, and quickly found that the high
angle skywave lobe came back down around 100 miles out and caused out
of phase interference with the ground wave. They solved the problem
by shortening the tower (actually shortening a mast on the top of it)
to 190 degrees.
I advise you to consult any broadcast radio engineering textbook but
also the authoritative book "ON4UN's Low-Band DXing" by John
Devoldere, ON4UN, which ironically is published by ARRL. Joel Hallas
would have done well to have consulted it, in particular, section 4 on
page 9-49 (4th ed.) Tall Verticals. May I direct your attention to
the paragraph that begins with this sentence: "In this section I will
dispel a myth that voltage-fed antennas do not require an elaborate
ground system."
73
Rob
K5UJ
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