>" 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."
The change happened long before that. By 1935, any thought as to 5/8 wave
radiators on medium wave was completely abandoned. See G.H. Brown, " A
Critical Study of the Characteristics of Broadcast Antennas as Affected by
Antenna Current Distribution," IRE, Vol. 24, No. 1, p 52 (bottom).
Brown's 1936 IRE paper compares 230 degree and 190 vertical radiators and
the effect of the longer radiator on fading caused by the 230 degree
radiator's secondary lobe. One important aspect of the 1/2 wave radiator's
need for a radial field is that the surface (ground ) wave increases, an
important attribute of daytime MW broadcasting. An increase in the surface
wave field may also assist with lower launch angles on 160m.
Paul, W9AC
-----Original Message-----
From: TowerTalk <towertalk-bounces@contesting.com> On Behalf Of Rob Atkinson
Sent: Wednesday, October 28, 2020 8:06 AM
To: towertalk <towertalk@contesting.com>
Subject: Re: [TowerTalk] Vertical dipole other choices?
>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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