This (Sept/Oct 2020) issue of QEX has our (N7QT & me) article about our
H40TT suitcase DXpedition vertical with a single coil loaded pair of
75ft elevated radials for a top loaded helix wound vertical on (but
unfortunately in the H40 case only near) the beach. Not my idea, N6LF
discusses it in detail in Antenna Compendium 6 for over/near salt water.
Our coil was 22uH for 2x 75ft radials to resonate the pair. N6LF also at
https://rudys.typepad.com/ant/files/antenna_vertical_short_radials.pdf
The modeling correlated well with the over the air performance
comparison vs my T.
re the Weber article, read it, & don't get it. N6LF in his QEX series
shows the performance as elevated radial length is changed and shorter
than 1/4wl is slightly better. My modeling showed the asymmetry is
small for 8 very unbalanced radials. Even the single radial CrankIR
shows about 3 db asymmetry, which we have used to favor a desired path.
Of course the more the earth is part of the current path, the lower the
efficiency of the antenna. With available space or transport
constraints there are ways to get on the air on 160 at reasonable
performance.
Grant KZ1W
On 8/29/2020 20:05, Robert Harmon wrote:
I am very interested in the shortened radials hypothesis. Has there been any
documented testing of full length vs shortened radials done ?
Bob
K6UJ
On Aug 29, 2020, at 7:16 PM, Richard (Rick) Karlquist <richard@karlquist.com>
wrote:
On 8/29/2020 5:44 PM, Dennis W0JX via TowerTalk wrote:
Unequal currents in 1/4 wave radials is to be expected primarily due to three
factors: 1.) varying quality of the ground under each radial; 2.) the effect of
close by objects such as buildings and other towers; and 3.) the variation in
the length of the radial wires depending on type of wire, insulation, and
construction methods. Effect on the actual radiation pattern depends, of
course, on the severity of the current variation and the number of radials
utilized.
Dick Weber, K5IU, wrote a very informative article in the late 90's which was
published in Communications Quarterly. He argued that 1/4 wave long radials
were the poorest length on could use. He strongly recommended radial lengths of
either 60-70 degrees (shortened) or 120 degrees long. These radials are tied
together at a common point and fed either through a coil or a capacitor.
W5UN had a very successful 4 square for 160 meters which used four, 96 foot
long radials fed through a common coil. The radials were about 15 feet off the
ground.
Here at W0JX, I have four, 48 foot radials on my elevated 80 meter ground
plane. The four radials come to a common point below the radiator and are tied
to the coax shield through an inductor of approximately 3.3 uh. A big advantage
of this arrangement is that I can resonate the antenna system by moving the tap
on the radial loading coil. After many years of using full-sized radials, my on
the air results tell me that the shortened radials are working better than the
full size radials did.
73 Dennis W0JXMilan OH
I'm not following the logic here. If various radials have
unequal impedances, I don't see now tying them in parallel
though a coil or capacitor with magically make them have
equal currents. Did Dick actually compare the currents in
the various configurations as built? All I see are anecdotes,
not AB comparisons or measurements.
Now if someone told me to use shortened or lengthened radials
and put individual coils or capacitors in series with each radial, then
I might believe the currents would be forced into equality.
Are you sure you got the story straight about Dick's article?
(I don't have access to it).
73
Rick N6RK
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