When I visited the 50,000W AM station WRAL some 35 years ago, their
coaxial feedline was home brewed. There were a set metal pipes about 1.5
inch diameter cemented into the ground and sticking up 3 feet or so. A 2x4
8ft long was connected between the pipes on top of pipe flanges. About
every 4ft was a ceramic standoff holding a 1 inch diameter copper pipe
(center conductor). Wrapped around the pipe and spaced about 3 inches away
from it was copper sheathing nailed to the 2x4. The whole thing looked like
a 200ft long mail box! I asked the chief engineer what the impedance was
and he said he had never measured it but the original data taken at the time
the station was built, said it was 63 Ohms. He said it was about 40-45
years old and stated that the antenna current was still the same as day one.
It was obviously very well built coax....
From: firstname.lastname@example.org [mailto:email@example.com]
On Behalf Of Jim Brown
Sent: Friday, January 27, 2012 3:09 PM
Subject: Re: [TenTec] Re. [Ten Tec] Grounds and balanced fed verticals
On 1/27/2012 11:41 AM, Steve Hunt wrote:
> What "bugs" me most about this issue is that it isn't Rocket Science -
> simple application of Ohm's Law will give you the answers.
> Some may think that's an insignificant difference and that we are
> "nit-picking"; but now imagine you are running a long length of that
> ladderline at high VSWR based on an Antenna Book loss prediction of 2dB
> - would you be happy if you discovered that the real loss was at least
> 4.8dB? Or that what you thought was a 4dB loss was actually nearer 10dB?
Exactly my point as well. One option that smart engineers always
consider for very long runs are open wire lines. Anyone who has had the
opportunity to visit one of the WW!! vintage HF broadcast and
communications stations or a high power AM broadcast transmitter has
seen LOTS of open wire lines feeding rhombics, Sterba curtains, vertical
radiators. Some are/were balanced, and some are/were in a coaxial
configuration. They are typically supported on rigid poles with a
U-shaped bracket and insulators at the top. In a typical coaxial
configuration, there might be four closely spaced parallel wires
functioning in parallel as the center conductor and four or more wires
at much wider spacing surrounding the center conductor as the shield.
73, Jim Brown K9YC
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